Publications by Charles M. Marcus

  • 2020
    • Quantum-Dot Parity Effects in Trivial and Topological Josephson Junctions - Abstract
      • An odd-occupied quantum dot in a Josephson junction can flip transmission phase, creating a {\pi}-junction. When the junction couples topological superconductors, no phase flip is expected. We investigate this and related effects in a full-shell hybrid interferometer, using gate voltage to control dot-junction parity and axial magnetic flux to control the transition from trivial to topological superconductivity. Enhanced zero-bias conductance and critical current for odd parity in the topological phase reflects hybridization of the confined spin with zero-energy modes in the leads.
    • D. Razmadze, E. C. T. O'Farrell, P. Krogstrup, C. M. Marcus
      2005.11848v2 [pdf]

    • Destructive Little-Parks Effect in a Full-Shell Nanowire-based Transmon - Abstract
      • A semiconductor transmon with an epitaxial Al shell fully surrounding an InAs nanowire core is investigated in the low $E_J/E_C$ regime. Little-Parks oscillations as a function of flux along the hybrid wire axis are destructive, creating lobes of reentrant superconductivity separated by a metallic state at a half-quantum of applied flux. In the first lobe, phase winding around the shell can induce topological superconductivity in the core. Coherent qubit operation is observed in both the zeroth and first lobes. Splitting of parity bands by coherent single-electron coupling across the junction is not resolved beyond line broadening, placing a bound on Majorana coupling, $E_M/h$ < 10 MHz, much smaller than the Josephson coupling $E_J/h$ ~ 4.7 GHz.
    • Deividas Sabonis, Oscar Erlandsson, Anders Kringhøj, Bernard van Heck, Thorvald W. Larsen, Ivana Petkovic, Peter Krogstrup, Karl D. Petersson, Charles M. Marcus
      2005.01748v1 [pdf]

    • Nonlocality of Majorana modes in hybrid nanowires - Abstract
      • Spatial separation of Majorana zero modes distinguishes trivial from topological midgap states and is key to topological protection in quantum computing applications. Although signatures of Majorana zero modes in tunneling spectroscopy have been reported in numerous studies, a quantitative measure of the degree of separation, or nonlocality, of the emergent zero modes has not been reported. Here, we present results of an experimental study of nonlocality of emergent zero modes in superconductor-semiconductor hybrid nanowire devices. The approach takes advantage of recent theory showing that nonlocality can be measured from splitting due to hybridization of the zero mode in resonance with a quantum dot state at one end of the nanowire. From these splittings as well as anticrossing of the dot states, measured for even and odd occupied quantum dot states, we extract both the degree of nonlocality of the emergent zero mode, as well as the spin canting angles of the nonlocal zero mode. Depending on the device measured, we obtain either a moderate degree of nonlocality, suggesting a partially separated Andreev subgap state, or a highly nonlocal state consistent with a well-developed Majorana mode.
    • M. T. Deng, S. Vaitiekénas, E. Prada, P. San-Jose, J. Nygård, P. Krogstrup, R. Aguado, C. M. Marcus
      Journal reference: Phys. Rev. B 98, 085125 (2018) [ 1712.03536v2 ]
      DOI: 10.1103/PhysRevB.98.085125

    • A Parity-Protected Superconductor-Semiconductor Qubit - Abstract
      • Coherence of superconducting qubits can be improved by implementing designs that protect the parity of Cooper pairs on superconducting islands. Here, we introduce a parity-protected qubit based on voltage-controlled semiconductor nanowire Josephson junctions, taking advantage of the higher harmonic content in the energy-phase relation of few-channel junctions. A symmetric interferometer formed by two such junctions, gate-tuned into balance and frustrated by a half-quantum of applied flux, yields a cos(2{\phi}) Josephson element, reflecting coherent transport of pairs of Cooper pairs. We demonstrate that relaxation of the qubit can be suppressed ten-fold by tuning into the protected regime.
    • T. W. Larsen, M. E. Gershenson, L. Casparis, A. Kringhøj, N. J. Pearson, R. P. G. McNeil, F. Kuemmeth, P. Krogstrup, K. D. Petersson, C. M. Marcus
      2004.03975v1 [pdf]

    • Zero-field Topological Superconductivity in Ferromagnetic Hybrid Nanowires - Abstract
      • We report transport measurements and tunneling spectroscopy in hybrid nanowires with epitaxial layers of superconducting Al and the ferromagnetic insulator EuS, grown on semiconducting InAs nanowires. In devices where the Al and EuS covered facets overlap, we infer a remanent effective Zeeman field of order 1 T, and observe stable zero-bias conductance peaks in tunneling spectroscopy into the end of the nanowire, consistent with topological superconductivity at zero applied field. Hysteretic features in critical current and tunneling spectra as a function of applied magnetic field support this picture. Nanowires with non-overlapping Al and EuS covered facets do not show comparable features. Topological superconductivity in zero applied field allows new device geometries and types of control.
    • S. Vaitiekėnas, Y. Liu, P. Krogstrup, C. M. Marcus
      2004.02226v1 [pdf]

    • Flux-induced topological superconductivity in full-shell nanowires - Abstract
      • We present a novel route to realizing topological superconductivity using magnetic flux applied to a full superconducting shell surrounding a semiconducting nanowire core. In the destructive Little-Parks regime, reentrant regions of superconductivity are associated with integer number of phase windings in the shell. Tunneling into the core reveals a hard induced gap near zero applied flux, corresponding to zero phase winding, and a gapped region with a discrete zero-energy state around one applied flux quantum, {\Phi}_0 = h/2e, corresponding to 2{\pi} phase winding. Theoretical analysis indicates that in the presence of radial spin-orbit coupling in the semiconductor, the winding of the superconducting phase can induce a transition to a topological phase supporting Majorana zero modes. Realistic modeling shows a topological phase persisting over a wide range of parameters, and reproduces experimental tunneling conductance data. Further measurements of Coulomb blockade peak spacing around one flux quantum in full-shell nanowire islands shows exponentially decreasing deviation from 1e periodicity with device length, consistent with Majorana modes at the ends of the nanowire.
    • S. Vaitiekėnas, G. W. Winkler, B. van Heck, T. Karzig, M. -T. Deng, K. Flensberg, L. I. Glazman, C. Nayak, P. Krogstrup, R. M. Lutchyn, C. M. Marcus
      Journal reference: Science 367, eaav3392 (2020) [ 2003.13177v1 ]
      DOI: 10.1126/science.aav3392

    • Anomalous metallic phase in tunable destructive superconductors - Abstract
      • Multiply connected superconductors smaller than the coherence length show destructive superconductivity, characterized by reentrant quantum phase transitions driven by magnetic flux. We investigate the dependence of destructive superconductivity on flux, transverse magnetic field, temperature, and current in InAs nanowires with a surrounding epitaxial Al shell, finding excellent agreement with mean-field theory across multiple reentrant transitions. Near the crossover between destructive and nondestructive regimes, an anomalous metal phase is observed with temperature-independent resistance, controlled over two orders of magnitude by a millitesla-scale transverse magnetic field.
    • S. Vaitiekėnas, P. Krogstrup, C. M. Marcus
      Journal reference: Phys. Rev. B 101, 060507 (2020) [ 1909.10654v2 ]
      DOI: 10.1103/PhysRevB.101.060507

  • 2019
    • Relating Andreev Bound States and Supercurrents in Hybrid Josephson Junctions - Abstract
      • We investigate superconducting quantum interference devices consisting of two highly transmissive Josephson junctions coupled by a superconducting loop, all defined in an epitaxial InAs/Al heterostructure. A novel device design allows for independent measurements of the Andreev bound state spectrum within the normal region of a junction and the resulting current-phase relation. We show that knowledge of the Andreev bound state spectrum alone is enough to derive the independently measured phase dependent supercurrent. On the other hand, the opposite relation does not generally hold true as details of the energy spectrum are averaged out in a critical current measurement. Finally, quantitative understanding of field dependent spectrum and supercurrent require taking into account the second junction in the loop and the kinetic inductance of the epitaxial Al film.
    • F. Nichele, E. Portolés, A. Fornieri, A. M. Whiticar, A. C. C. Drachmann, T. Wang, G. C. Gardner, C. Thomas, A. T. Hatke, M. J. Manfra, C. M. Marcus
      1912.08893v1 [pdf]

    • Suppressed Charge Dispersion via Resonant Tunneling in a Single-Channel Transmon - Abstract
      • We demonstrate strong suppression of charge dispersion in a semiconductor-based transmon qubit across Josephson resonances associated with a quantum dot in the junction. On resonance, dispersion is drastically reduced compared to conventional transmons with corresponding Josephson and charging energies. We develop a model of qubit dispersion for a single-channel resonance, which is in quantitative agreement with experimental data.
    • A. Kringhøj, B. van Heck, T. W. Larsen, O. Erlandsson, D. Sabonis, P. Krogstrup, L. Casparis, K. D. Petersson, C. M. Marcus
      1911.10011v2 [pdf]

    • Fast Charge Sensing of Si/SiGe Quantum Dots via a High-Frequency Accumulation GateFast Charge Sensing of Si/SiGe Quantum Dots via a High-Frequency Accumulation Gate - Abstract
      • Quantum dot arrays are a versatile platform for the implementation of spin qubits, as high-bandwidth sensor dots can be integrated with single-, double- and triple-dot qubits yielding fast and high-fidelity qubit readout. However, for undoped silicon devices, reflectometry off sensor ohmics suffers from the finite resistivity of the two-dimensional electron gas (2DEG), and alternative readout methods are limited to measuring qubit capacitance, rather than qubit charge. By coupling a surface-mount resonant circuit to the plunger gate of a high-impedance sensor, we realized a fast charge sensing technique that is compatible with resistive 2DEGs. We demonstrate this by acquiring at high speed charge stability diagrams of double- and triple-dot arrays in Si/SiGe heterostructures as well as pulsed-gate single-shot charge and spin readout with integration times as low as 2.4 $\mu$s.
    • Christian Volk, Anasua Chatterjee, Fabio Ansaloni, Charles M. Marcus, Ferdinand Kuemmeth
      Journal reference: Nano Letters 19, 5628-5633 (2019) [ 1906.10584v2 ]
      DOI: 10.1021/acs.nanolett.9b02149

    • Controlled dc Monitoring of a Superconducting Qubit - Abstract
      • Creating a transmon qubit using semiconductor-superconductor hybrid materials not only provides electrostatic control of the qubit frequency, it also allows parts of the circuit to be electrically connected and disconnected in situ by operating a semiconductor region of the device as a field-effect transistor (FET). Here, we exploit this feature to compare in the same device characteristics of the qubit, such as frequency and relaxation time, with related transport properties such as critical supercurrent and normal-state resistance. Gradually opening the FET to the monitoring circuit allows the influence of weak-to-strong DC monitoring of a live qubit to be measured. A model of this influence yields excellent agreement with experiment, demonstrating a relaxation rate mediated by a gate-controlled environmental coupling.
    • A. Kringhøj, T. W. Larsen, B. van Heck, D. Sabonis, O. Erlandsson, I. Petkovic, D. I. Pikulin, P. Krogstrup, K. D. Petersson, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 124, 056801 (2020) [ 1910.08200v2 ]
      DOI: 10.1103/PhysRevLett.124.056801

    • Semiconductor - Ferromagnetic Insulator - Superconductor Nanowires: Stray Field and Exchange Field - Abstract
      • Nanowires can serve as flexible substrates for hybrid epitaxial growth on selected facets, allowing for design of heterostructures with complex material combinations and geometries. In this work we report on hybrid epitaxy of semiconductor - ferromagnetic insulator - superconductor (InAs/EuS/Al) nanowire heterostructures. We study the crystal growth and complex epitaxial matching of wurtzite InAs / rock-salt EuS interfaces as well as rock-salt EuS / face-centered cubic Al interfaces. Because of the magnetic anisotropy originating from the nanowire shape, the magnetic structure of the EuS phase are easily tuned into single magnetic domains. This effect efficiently ejects the stray field lines along the nanowires. With tunnel spectroscopy measurements of the density of states, we show the material has a hard induced superconducting gap, and magnetic hysteretic evolution which indicates that the magnetic exchange fields are not negligible. These hybrid nanowires fulfil key material requirements for serving as a platform for spin-based quantum applications, such as scalable topological quantum computing.
    • Yu Liu, Saulius Vaitiekenas, Sara Marti-Sanchez, Christian Koch, Sean Hart, Zheng Cui, Thomas Kanne, Sabbir A. Khan, Rawa Tanta, Shivendra Upadhyay, Martin Espineira Cachaza, Charles M. Marcus, Jordi Arbiol, Kathryn A. Moler, Peter Krogstrup
      DOI: 10.1021/acs.nanolett.9b04187
      1910.03364v1 [pdf]

    • End-to-end correlated subgap states in hybrid nanowires - Abstract
      • End-to-end correlated bound states are investigated in superconductor-semiconductor hybrid nanowires at zero magnetic field. Peaks in subgap conductance are independently identified from each wire end, and a cross-correlation function is computed that counts end-to-end coincidences, averaging over thousands of subgap features. Strong correlations in a short, $300~\mathrm{nm}$ device are reduced by a factor of four in a long, $900~\mathrm{nm}$ device. In addition, subgap conductance distributions are investigated, and correlations between the left and right distributions are identified based on their mutual information.
    • G. L. R. Anselmetti, E. A. Martinez, G. C. Ménard, D. Puglia, F. K. Malinowski, J. S. Lee, S. Choi, M. Pendharkar, C. J. Palmstrøm, C. M. Marcus, L. Casparis, A. P. Higginbotham
      Journal reference: Phys. Rev. B 100, 205412 (2019) [ 1908.05549v2 ]
      DOI: 10.1103/PhysRevB.100.205412

    • Detecting parity effect in a superconducting device in the presence of parity switches - Abstract
      • We present a superconducting device showing a clear parity effect in the number of electrons, even when there is, on average, a single nonequilibrium quasiparticle present and the parity of the island switches due to quasiparticles tunneling in and out of the device at rates on the order of 100 Hz. We detect the switching by monitoring in real time the charge state of a superconducting island connected to normal leads by tunnel junctions. The quasiparticles are created by Cooper pairs breaking on the island at a rate of a few kHz. We demonstrate that the pair breaking is caused by the backaction of the single-electron transistor used as a charge detector. With sufficiently low probing currents, our superconducting island is free of quasiparticles 97% of the time.
    • E. T. Mannila, V. F. Maisi, H. Q. Nguyen, C. M. Marcus, J. P. Pekola
      Journal reference: Phys. Rev. B 100, 020502 (2019) [ 1807.01733v2 ]
      DOI: 10.1103/PhysRevB.100.020502

    • Dispersive sensing in hybrid InAs/Al nanowires - Abstract
      • Dispersive charge sensing is realized in hybrid semiconductor-superconductor nanowires in gate-defined single- and double-island device geometries. Signal-to-noise ratios (SNRs) were measured both in the frequency and time domain. Frequency-domain measurements were carried out as a function of frequency and power and yield a charge sensitivity of $1 \times 10^{-3} e/\sqrt{\rm Hz}$ for an 11 MHz measurement bandwidth. Time-domain measurements yield SNR > 1 for 20 $\mu$s integration time. At zero magnetic field, photon-assisted tunneling was detected dispersively in a double-island geometry, indicating coherent hybridization of the two superconducting islands. At an axial magnetic field of 0.6 T, subgap states are detected dispersively, demonstrating the suitability of the method for sensing in the topological regime.
    • Deividas Sabonis, Eoin C. T. O'Farrell, Davydas Razmadze, David M. T. van Zanten, Judith Suter, Peter Krogstrup, Charles M. Marcus
      Journal reference: Appl. Phys. Lett. 115, 102601 (2019) [ 1906.10659v1 ]
      DOI: 10.1063/1.5116377

    • Conductance-Matrix Symmetries of a Three-Terminal Hybrid Device - Abstract
      • We present conductance-matrix measurements of a three-terminal superconductor-semiconductor hybrid device consisting of two normal leads and one superconducting lead. Using a symmetry decomposition of the conductance, we find that the antisymmetric components of pairs of local and nonlocal conductances match at energies below the superconducting gap, consistent with expectations based on a non-interacting scattering matrix approach. Further, the local charge character of Andreev bound states is extracted from the symmetry-decomposed conductance data and is found to be similar at both ends of the device and tunable with gate voltage. Finally, we measure the conductance matrix as a function of magnetic field and identify correlated splittings in low-energy features, demonstrating how conductance-matrix measurements can complement traditional tunneling-probe measurements in the search for Majorana zero modes.
    • G. C. Ménard, G. L. R. Anselmetti, E. A. Martinez, D. Puglia, F. K. Malinowski, J. S. Lee, S. Choi, M. Pendharkar, C. J. Palmstrøm, K. Flensberg, C. M. Marcus, L. Casparis, A. P. Higginbotham
      Journal reference: Phys. Rev. Lett. 124, 036802 (2020) [ 1905.05505v1 ]
      DOI: 10.1103/PhysRevLett.124.036802

    • Selective-area chemical beam epitaxy of in-plane InAs one-dimensional channels grown on InP(001), InP(111)B, and InP(011) surfaces - Abstract
      • We report on the selective-area chemical beam epitaxial growth of InAs in-plane, one-dimensional (1-D) channels using patterned SiO$_{2}$-coated InP(001), InP(111)B, and InP(110) substrates to establish a scalable platform for topological superconductor networks. Top-view scanning electron micrographs show excellent surface selectivity and dependence of major facet planes on the substrate orientations and ridge directions, and the ratios of the surface energies of the major facet planes were estimated. Detailed structural properties and defects in the InAs nanowires (NWs) were characterized by transmission electron microscopic analysis of cross-sections perpendicular to the NW ridge direction and along the NW ridge direction. Electrical transport properties of the InAs NWs were investigated using Hall bars, a field effect mobility device, a quantum dot, and an Aharonov-Bohm loop device, which reflect the strong spin-orbit interaction and phase-coherent transport characteristic in the selectively grown InAs systems. This study demonstrates that selective-area chemical beam epitaxy is a scalable approach to realize semiconductor 1-D channel networks with the excellent surface selectivity and this material system is suitable for quantum transport studies.
    • Joon Sue Lee, Sukgeun Choi, Mihir Pendharkar, Dan J. Pennachio, Brian Markman, Micheal Seas, Sebastian Koelling, Marcel A. Verheijen, Lucas Casparis, Karl D. Petersson, Ivana Petkovic, Vanessa Schaller, Mark J. W. Rodwell, Charles M. Marcus, Peter Krogstrup, Leo P. Kouwenhoven, Erik P. A. M. Bakkers, Chris J. Palmstrøm
      Journal reference: Phys. Rev. Materials 3, 084606 (2019) [ 1808.04563v2 ]
      DOI: 10.1103/PhysRevMaterials.3.084606

    • Current-phase relations of InAs nanowire Josephson junctions: From interacting to multimode regimes - Abstract
      • Gate-tunable semiconductor-superconductor nanowires with superconducting leads form exotic Josephson junctions that are a highly desirable platform for two types of qubits: those with topological superconductivity (Majorana qubits) and those based on tunable anharmonicity (gatemon qubits). Controlling their behavior, however, requires understanding their electrostatic environment and electronic structure. Here we study gated InAs nanowires with epitaxial aluminum shells. By measuring current-phase relations (CPR) and comparing them with analytical and numerical calculations, we show that we can tune the number of modes, determine the transparency of each mode, and tune into regimes in which electron-electron interactions are apparent, indicating the presence of a quantum dot. To take into account electrostatic and geometrical effects, we perform microscopic self-consistent Schrodinger-Poisson numerical simulations, revealing the energy spectrum of Andreev states in the junction as well as their spatial distribution. Our work systematically demonstrates the effect of device geometry, gate voltage and phase bias on mode behavior, providing new insights into ongoing experimental efforts and predictive device design.
    • Sean Hart, Zheng Cui, Gerbold Menard, Mingtang Deng, Andrey Antipov, Roman M. Lutchyn, Peter Krogstrup, Charles M. Marcus, Kathryn A. Moler
      Journal reference: Phys. Rev. B 100, 064523 (2019) [ 1902.07804v1 ]
      DOI: 10.1103/PhysRevB.100.064523

    • In-Plane Magnetoconductance Mapping of InSb Quantum Wells - Abstract
      • In-plane magnetoconductance of InSb quantum wells (QW) containing a two dimensional electron gas (2DEG) is presented. Using a vector magnet, we created a magnetoconductance map which shows the suppression of weak antilocalization (WAL) as a function of applied field. By fitting the in-plane field response of the 2DEG, we estimate material disorder and g-factor as a function of crystal direction. The in-plane WAL suppression is found to be dominated by the Zeeman effect and to show a small crystal-orientation-dependent anistropy in disorder and g-factor. These measurements show the utility of multi-directional measurement of magnetoconductance in analyzing material properties.
    • J. T. Mlack, K. S. Wickramasinghe, T. D. Mishima, M. B. Santos, C. M. Marcus
      1902.07570v1 [pdf]

    • Interferometry and coherent single-electron transport through hybrid superconductor-semiconductor Coulomb islands - Abstract
      • Majorana zero modes are leading candidates for topological quantum computation due to their non-local character and non-abelian exchange statistics. Among their attributes, spatially separated Majorana modes are expected to allow coherent single-electron transport through one-dimensional topological superconductors in the Coulomb blockade (CB) regime. We have investigated this feature by patterning an elongated epitaxial InAs-Al Coulomb island embedded in an Aharonov-Bohm interferometer. Using a parallel magnetic field to lower the energy of a discrete sub-gap state in the island below its charging energy, conductance oscillations in the ring were observed with a flux period of h/e (h is Planck's constant and e is the elementary charge), indicating coherent single-electron transport through the interferometer. Oscillation amplitude was largest when CB conductance peaks in the island were 1e periodic, and suppressed when CB was 2e periodic or when superconductivity was suppressed. Oscillation phase shifts of {\pi} were observed when the charge occupancy of the island was changed by 1e, indicating that the interferometer can detect island parity. Magnetic fields oriented orthogonal to the island reduced the field at which 2e periodic peaks split and where coherent transport could also be observed, suggesting additional non-Majorana mechanisms for 1e transport through these moderately short wires.
    • A. M. Whiticar, A. Fornieri, E. C. T. O'Farrell, A. C. C. Drachmann, T. Wang, C. Thomas, S. Gronin, R. Kallaher, G. C. Gardner, M. J. Manfra, C. M. Marcus, F. Nichele
      1902.07085v1 [pdf]

    • Suppressing quasiparticle poisoning with a voltage-controlled filter - Abstract
      • We study single-electron charging events in an Al/InAs nanowire hybrid system with deliberately introduced gapless regions. The occupancy of a Coulomb island is detected using a nearby radio-frequency quantum dot as a charge sensor. We demonstrate that a 1 micron gapped segment of the wire can be used to efficiently suppress single electron poisoning of the gapless region and therefore protect the parity of the island while maintaining good electrical contact with a normal lead. In the absence of protection by charging energy, the 1e switching rate can be reduced below 200 per second. In the same configuration, we observe strong quantum charge fluctuations due to exchange of electron pairs between the island and the lead. The magnetic field dependence of the poisoning rate yields a zero-field superconducting coherence length of ~ 90 nm.
    • Gerbold C. Ménard, Filip K. Malinowski, Denise Puglia, Dmitry I. Pikulin, Torsten Karzig, Bela Bauer, Peter Krogstrup, Charles M. Marcus
      Journal reference: Phys. Rev. B 100, 165307 (2019) [ 1902.02689v1 ]
      DOI: 10.1103/PhysRevB.100.165307

    • Photon-assisted tunnelling of zero modes in a Majorana wire - Abstract
      • Hybrid nanowires with proximity-induced superconductivity in the topological regime host Majorana zero modes (MZMs) at their ends, and networks of such structures can produce topologically protected qubits. In a double-island geometry where each segment hosts a pair of MZMs, inter-pair coupling mixes the charge parity of the islands and opens an energy gap between the even and odd charge states at the inter-island charge degeneracy. Here, we report on the spectroscopic measurement of such an energy gap in an InAs/Al double-island device by tracking the position of the microwave-induced quasiparticle (qp) transitions using a radio-frequency (rf) charge sensor. In zero magnetic field, photon assisted tunneling (PAT) of Cooper pairs gives rise to resonant lines in the 2e-2e periodic charge stability diagram. In the presence of a magnetic field aligned along the nanowire, resonance lines are observed parallel to the inter-island charge degeneracy of the 1e-1e periodic charge stability diagram, where the 1e periodicity results from a zero-energy sub-gap state that emerges in magnetic field. Resonant lines in the charge stability diagram indicate coherent photon assisted tunneling of single-electron states, changing the parity of the two islands. The dependence of resonant frequency on detuning indicates a sizable (GHz-scale) hybridization of zero modes across the junction separating islands.
    • David M. T. van Zanten, Deividas Sabonis, Judith Suter, Jukka I. Väyrynen, Torsten Karzig, Dmitry I. Pikulin, Eoin C. T. O'Farrell, Davydas Razmadze, Karl D. Petersson, Peter Krogstrup, Charles M. Marcus
      Journal reference: Nature Physics (2020) [ 1902.00797v1 ]
      DOI: 10.1038/s41567-020-0858-0

    • Radio-Frequency Methods for Majorana-Based Quantum Devices: Fast Charge Sensing and Phase-Diagram Mapping - Abstract
      • Radio-frequency (RF) reflectometry is implemented in hybrid semiconductor-superconductor nanowire systems designed to probe Majorana zero modes. Two approaches are presented. In the first, hybrid nanowire-based devices are part of a resonant circuit, allowing conductance to be measured as a function of several gate voltages ~40 times faster than using conventional low-frequency lock-in methods. In the second, nanowire devices are capacitively coupled to a nearby RF single-electron transistor made from a separate nanowire, allowing RF detection of charge, including charge-only measurement of the crossover from 2e inter-island charge transitions at zero magnetic field to 1e transitions at axial magnetic fields above 0.6 T, where a topological state is expected. Single-electron sensing yields signal-to-noise exceeding 3 and visibility 99.8% for a measurement time of 1 {\mu}s.
    • Davydas Razmadze, Deividas Sabonis, Filip K. Malinowski, Gerbold C. Menard, Sebastian Pauka, Hung Nguyen, David M. T. van Zanten, Eoin C. T. O'Farrell, Judith Suter, Peter Krogstrup, Ferdinand Kuemmeth, Charles M. Marcus
      Journal reference: Phys. Rev. Applied 11, 064011 (2019) [ 1902.00789v1 ]
      DOI: 10.1103/PhysRevApplied.11.064011

  • 2018
    • Effective g-factor in Majorana Wires - Abstract
      • We use the effective g-factor of subgap states, g*, in hybrid InAs nanowires with an epitaxial Al shell to investigate how the superconducting density of states is distributed between the semiconductor core and the metallic shell. We find a step-like reduction of g* and improved hard gap with reduced carrier density in the nanowire, controlled by gate voltage. These observations are relevant for Majorana devices, which require tunable carrier density and g* exceeding the g-factor of the proximitizing superconductor. Additionally, we observe the closing and reopening of a gap in the subgap spectrum coincident with the appearance of a zero-bias conductance peak.
    • S. Vaitiekėnas, M. T. Deng, J. Nygård, P. Krogstrup, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 121, 037703 (2018) [pdf]
      DOI: 10.1103/PhysRevLett.121.037703

    • Flux-induced topological superconductivity in full-shell nanowires - Abstract
      • We demonstrate a novel means of creating Majorana zero modes using magnetic flux applied to a full superconducting shell surrounding a semiconducting nanowire core, unifying approaches based on proximitized nanowires and vortices in topological superconductors. In the destructive Little-Parks regime, reentrant regions of superconductivity are associated with integer number of phase windings in the shell. Tunneling into the core reveals a hard induced gap near zero applied flux, corresponding to zero phase winding, and a gapped region with a discrete zero-energy state for flux around {\Phi}_0 = h/2e, corresponding to 2{\pi} phase winding. Coulomb peak spacing in full-shell islands around one applied flux shows exponentially decreasing deviation from 1e periodicity with device length, consistent with the picture of Majorana modes located at the ends of the wire.
    • S. Vaitiekėnas, M. -T. Deng, P. Krogstrup, C. M. Marcus
      Journal reference: Science 367, eaav3392 (2020) [ 1809.05513v1 ]
      DOI: 10.1126/science.aav3392

    • Selective-Area-Grown Semiconductor-Superconductor Hybrids: A Basis for Topological Networks - Abstract
      • We introduce selective area grown hybrid InAs/Al nanowires based on molecular beam epitaxy, allowing arbitrary semiconductor-superconductor networks containing loops and branches. Transport reveals a hard induced gap and unpoisoned 2e-periodic Coulomb blockade, with temperature dependent 1e features in agreement with theory. Coulomb peak spacing in parallel magnetic field displays overshoot, indicating an oscillating discrete near-zero subgap state consistent with device length. Finally, we investigate a loop network, finding strong spin-orbit coupling and a coherence length of several microns. These results demonstrate the potential of this platform for scalable topological networks among other applications.
    • S. Vaitiekėnas, A. M. Whiticar, M. T. Deng, F. Krizek, J. E. Sestoft, C. J. Palmstrøm, S. Marti-Sanchez, J. Arbiol, P. Krogstrup, L. Casparis, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 121, 147701 (2018) [ 1802.04210v3 ]
      DOI: 10.1103/PhysRevLett.121.147701

    • Evidence of topological superconductivity in planar Josephson junctions - Abstract
      • Majorana zero modes are quasiparticle states localized at the boundaries of topological superconductors that are expected to be ideal building blocks for fault-tolerant quantum computing. Several observations of zero-bias conductance peaks measured in tunneling spectroscopy above a critical magnetic field have been reported as experimental indications of Majorana zero modes in superconductor/semiconductor nanowires. On the other hand, two dimensional systems offer the alternative approach to confine Ma jorana channels within planar Josephson junctions, in which the phase difference {\phi} between the superconducting leads represents an additional tuning knob predicted to drive the system into the topological phase at lower magnetic fields. Here, we report the observation of phase-dependent zero-bias conductance peaks measured by tunneling spectroscopy at the end of Josephson junctions realized on a InAs/Al heterostructure. Biasing the junction to {\phi} ~ {\pi} significantly reduces the critical field at which the zero-bias peak appears, with respect to {\phi} = 0. The phase and magnetic field dependence of the zero-energy states is consistent with a model of Majorana zero modes in finite-size Josephson junctions. Besides providing experimental evidence of phase-tuned topological superconductivity, our devices are compatible with superconducting quantum electrodynamics architectures and scalable to complex geometries needed for topological quantum computing.
    • Antonio Fornieri, Alexander M. Whiticar, F. Setiawan, Elías Portolés Marín, Asbjørn C. C. Drachmann, Anna Keselman, Sergei Gronin, Candice Thomas, Tian Wang, Ray Kallaher, Geoffrey C. Gardner, Erez Berg, Michael J. Manfra, Ady Stern, Charles M. Marcus, Fabrizio Nichele
      Journal reference: Nature 569, 89-92 (2019) [ 1809.03037v1 ]
      DOI: 10.1038/s41586-019-1068-8

    • Fast spin exchange across a multielectron mediator - Abstract
      • The Heisenberg exchange interaction between neighboring quantum dots allows precise voltage control over spin dynamics, due to the ability to precisely control the overlap of orbital wavefunctions by gate electrodes. This allows the study of fundamental electronic phenomena and finds applications in quantum information processing. Although spin-based quantum circuits based on short-range exchange interactions are possible, the development of scalable, longer-range coupling schemes constitutes a critical challenge within the spin-qubit community. Approaches based on capacitative coupling and cavity-mediated interactions effectively couple spin qubits to the charge degree of freedom, making them susceptible to electrically-induced decoherence. The alternative is to extend the range of the Heisenberg exchange interaction by means of a quantum mediator. Here, we show that a multielectron quantum dot with 50-100 electrons serves as an excellent mediator, preserving speed and coherence of the resulting spin-spin coupling while providing several functionalities that are of practical importance. These include speed (mediated two-qubit rates up to several gigahertz), distance (of order of a micrometer), voltage control, possibility of sweet spot operation (reducing susceptibility to charge noise), and reversal of the interaction sign (useful for dynamical decoupling from noise).
    • Filip K. Malinowski, Frederico Martins, Thomas B. Smith, Stephen D. Bartlett, Andrew C. Doherty, Peter D. Nissen, Saeed Fallahi, Geoffrey C. Gardner, Michael J. Manfra, Charles M. Marcus, Ferdinand Kuemmeth
      Journal reference: Nature Communications 10, 1196 (2019) [ 1808.09736v1 ]
      DOI: 10.1038/s41467-019-09194-x

    • Hybridization of Subgap States in One-Dimensional Superconductor-Semiconductor Coulomb Islands - Abstract
      • We present measurements of one-dimensional superconductor-semiconductor Coulomb islands, fabricated by gate confinement of a two-dimensional InAs heterostructure with an epitaxial Al layer. When tuned via electrostatic side gates to regimes without sub-gap states, Coulomb blockade reveals Cooper-pair mediated transport. When sub-gap states are present, Coulomb peak positions and heights oscillate in a correlated way with magnetic field and gate voltage, as predicted theoretically, with (anti) crossings in (parallel) transverse magnetic field indicating Rashba-type spin-orbit coupling. Overall results are consistent with a picture of overlapping Majorana zero modes in finite wires.
    • E. C. T. O'Farrell, A. C. C. Drachmann, M. Hell, A. Fornieri, A. M. Whiticar, E. B. Hansen, S. Gronin, G. C. Gardener, C. Thomas, M. J. Manfra, K. Flensberg, C. M. Marcus, F. Nichele
      Journal reference: Phys. Rev. Lett. 121, 256803 (2018) [ 1804.09676v1 ]
      DOI: 10.1103/PhysRevLett.121.256803

    • Field effect enhancement in buffered quantum nanowire networks - Abstract
      • III-V semiconductor nanowires have shown great potential in various quantum transport experiments. However, realizing a scalable high-quality nanowire-based platform that could lead to quantum information applications has been challenging. Here, we study the potential of selective area growth by molecular beam epitaxy of InAs nanowire networks grown on GaAs-based buffer layers. The buffered geometry allows for substantial elastic strain relaxation and a strong enhancement of field effect mobility. We show that the networks possess strong spin-orbit interaction and long phase coherence lengths with a temperature dependence indicating ballistic transport. With these findings, and the compatibility of the growth method with hybrid epitaxy, we conclude that the material platform fulfills the requirements for a wide range of quantum experiments and applications.
    • Filip Krizek, Joachim E. Sestoft, Pavel Aseev, Sara Marti-Sanchez, Saulius Vaitiekenas, Lucas Casparis, Sabbir A. Khan, Yu Liu, Tomas Stankevic, Alexander M. Whiticar, Alexandra Fursina, Frenk Boekhout, Rene Koops, Emanuele Uccelli, Leo P. Kouwenhoven, Charles M. Marcus, Jordi Arbiol, Peter Krogstrup
      Journal reference: Phys. Rev. Materials 2, 093401 (2018) [ 1802.07808v2 ]
      DOI: 10.1103/PhysRevMaterials.2.093401

    • Majorana zero modes in superconductor–semiconductor heterostructures - Abstract
      • Realizing topological superconductivity and Majorana zero modes in the laboratory is one of the major goals in condensed matter physics. We review the current status of this rapidly-developing field, focusing on semiconductor-superconductor proposals for topological superconductivity. Material science progress and robust signatures of Majorana zero modes in recent experiments are discussed. After a brief introduction to the subject, we outline several next-generation experiments probing exotic properties of Majorana zero modes, including fusion rules and non-Abelian exchange statistics. Finally, we discuss prospects for implementing Majorana-based topological quantum computation in these systems.
    • R. M. Lutchyn, E. P. A. M. Bakkers, L. P. Kouwenhoven, P. Krogstrup, C. M. Marcus, Y. Oreg
      Journal reference: Nat Rev Mater 3, 52 - 68 (2018) [ 1707.04899v2 ]
      DOI: 10.1038/s41578-018-0003-1

    • Voltage-controlled superconducting quantum bus - Abstract
      • We demonstrate the ability of an epitaxial semiconductor-superconductor nanowire to serve as a field-effect switch to tune a superconducting cavity. Two superconducting gatemon qubits are coupled to the cavity, which acts as a quantum bus. Using a gate voltage to control the superconducting switch yields up to a factor of 8 change in qubit-qubit coupling between the on and off states without detrimental effect on qubit coherence. High-bandwidth operation of the coupling switch on nanosecond timescales degrades qubit coherence.
    • L. Casparis, N. J. Pearson, A. Kringhøj, T. W. Larsen, F. Kuemmeth, J. Nygård, P. Krogstrup, K. D. Petersson, C. M. Marcus
      Journal reference: Phys. Rev. B 99, 085434 (2019) [ 1802.01327v1 ]
      DOI: 10.1103/PhysRevB.99.085434

  • 2017
    • Engineering hybrid epitaxial InAsSb/Al nanowires for stronger topological protection - Abstract
      • The combination of strong spin-orbit coupling, large $g$-factors, and the coupling to a superconductor can be used to create a topologically protected state in a semiconductor nanowire. Here we report on growth and characterization of hybrid epitaxial InAsSb/Al nanowires, with varying composition and crystal structure. We find the strongest spin-orbit interaction at intermediate compositions in zincblende InAs$_{1-x}$Sb$_{x}$ nanowires, exceeding that of both InAs and InSb materials, confirming recent theoretical studies \cite{winkler2016topological}. We show that the epitaxial InAsSb/Al interfaces allows for a hard induced superconducting gap and 2$e$ transport in Coulomb charging experiments, similar to experiments on InAs/Al and InSb/Al materials, and find measurements consistent with topological phase transitions at low magnetic fields due to large effective $g$-factors. Finally we present a method to grow pure wurtzite InAsSb nanowires which are predicted to exhibit even stronger spin-orbit coupling than the zincblende structure.
    • Joachim E. Sestoft, Thomas Kanne, Aske Nørskov Gejl, Merlin von Soosten, Jeremy S. Yodh, Daniel Sherman, Brian Tarasinski, Michael Wimmer, Erik Johnson, Mingtang Deng, Jesper Nygård, Thomas Sand Jespersen, Charles M. Marcus, Peter Krogstrup
      Journal reference: Phys. Rev. Materials 2, 044202 (2018) [ 1711.06864v2 ]
      DOI: 10.1103/PhysRevMaterials.2.044202

    • Superconducting, insulating and anomalous metallic regimes in a gated two-dimensional semiconductor–superconductor array - Abstract
      • The superconductor-insulator transition in two dimensions has been widely investigated as a paradigmatic quantum phase transition. The topic remains controversial, however, because many experiments exhibit a metallic regime with saturating low-temperature resistance, at odds with conventional theory. Here, we explore this transition in a novel, highly controllable system, a semiconductor heterostructure with epitaxial Al, patterned to form a regular array of superconducting islands connected by a gateable quantum well. Spanning nine orders of magnitude in resistance, the system exhibits regimes of superconducting, metallic, and insulating behavior, along with signatures of flux commensurability and vortex penetration. An in-plane magnetic field eliminates the metallic regime, restoring the direct superconductor-insulator transition, and improves scaling, while strongly altering the scaling exponent.
    • C. G. L. Bøttcher, F. Nichele, M. Kjaergaard, H. J. Suominen, J. Shabani, C. J. Palmstrøm, C. M. Marcus
      Journal reference: Nature Physics 14, 1138 (2018) [ 1711.01451v2 ]
      DOI: 10.1038/s41567-018-0259-9

    • Superconducting gatemon qubit based on a proximitized two-dimensional electron gas - Abstract
      • The coherent tunnelling of Cooper pairs across Josephson junctions (JJs) generates a nonlinear inductance that is used extensively in quantum information processors based on superconducting circuits, from setting qubit transition frequencies and interqubit coupling strengths, to the gain of parametric amplifiers for quantum-limited readout. The inductance is either set by tailoring the metal-oxide dimensions of single JJs, or magnetically tuned by parallelizing multiple JJs in superconducting quantum interference devices (SQUIDs) with local current-biased flux lines. JJs based on superconductor-semiconductor hybrids represent a tantalizing all-electric alternative. The gatemon is a recently developed transmon variant which employs locally gated nanowire (NW) superconductor-semiconductor JJs for qubit control. Here, we go beyond proof-of-concept and demonstrate that semiconducting channels etched from a wafer-scale two-dimensional electron gas (2DEG) are a suitable platform for building a scalable gatemon-based quantum computer. We show 2DEG gatemons meet the requirements by performing voltage-controlled single qubit rotations and two-qubit swap operations. We measure qubit coherence times up to ~2 us, limited by dielectric loss in the 2DEG host substrate.
    • Lucas Casparis, Malcolm R. Connolly, Morten Kjaergaard, Natalie J. Pearson, Anders Kringhøj, Thorvald W. Larsen, Ferdinand Kuemmeth, Tiantian Wang, Candice Thomas, Sergei Gronin, Geoffrey C. Gardner, Michael J. Manfra, Charles M. Marcus, Karl D. Petersson
      Journal reference: Nature Nanotechnology 13, 915 (2018) [ 1711.07665v3 ]
      DOI: 10.1038/s41565-018-0207-y

    • Zero-Energy Modes from Coalescing Andreev States in a Two-Dimensional Semiconductor-Superconductor Hybrid Platform - Abstract
      • We investigate zero-bias conductance peaks that arise from coalescing subgap Andreev states, consistent with emerging Majorana zero modes, in hybrid semiconductor-superconductor wires defined in a two-dimensional InAs/Al heterostructure using top-down lithography and gating. The measurements indicate a hard superconducting gap, ballistic tunneling contact, and in-plane critical fields up to $3$~T. Top-down lithography allows complex geometries, branched structures, and straightforward scaling to multicomponent devices compared to structures made from assembled nanowires.
    • Henri J. Suominen, Morten Kjaergaard, Alexander R. Hamilton, Javad Shabani, Chris J. Palmstrøm, Charles M. Marcus, Fabrizio Nichele
      Journal reference: Phys. Rev. Lett. 119, 176805 (2017) [ 1703.03699v2 ]
      DOI: 10.1103/PhysRevLett.119.176805

    • Spin of a Multielectron Quantum Dot and Its Interaction with a Neighboring Electron - Abstract
      • We investigate the spin of a multielectron GaAs quantum dot in a sequence of nine charge occupancies, by exchange coupling the multielectron dot to a neighboring two-electron double quantum dot. For all nine occupancies, we make use of a leakage spectroscopy technique to reconstruct the spectrum of spin states in the vicinity of the interdot charge transition between a single- and a multielectron quantum dot. In the same regime we also perform time-resolved measurements of coherent exchange oscillations between the single- and multielectron quantum dot. With these measurements, we identify distinct characteristics of the multielectron spin state, depending on whether the dot's occupancy is even or odd. For three out of four even occupancies we do not observe any exchange interaction with the single quantum dot, indicating a spin-0 ground state. For the one remaining even occupancy, we observe an exchange interaction that we associate with a spin-1 multielectron quantum dot ground state. For all five of the odd occupancies, we observe an exchange interaction associated with a spin-1/2 ground state. For three of these odd occupancies, we clearly demonstrate that the exchange interaction changes sign in the vicinity of the charge transition. For one of these, the exchange interaction is negative (i.e. triplet-preferring) beyond the interdot charge transition, consistent with the observed spin-1 for the next (even) occupancy. Our experimental results are interpreted through the use of a Hubbard model involving two orbitals of the multielectron quantum dot. Allowing for the spin correlation energy (i.e. including a term favoring Hund's rules) and different tunnel coupling to different orbitals, we qualitatively reproduce the measured exchange profiles for all occupancies.
    • Filip K. Malinowski, Frederico Martins, Thomas B. Smith, Stephen D. Bartlett, Andrew C. Doherty, Peter D. Nissen, Saeed Fallahi, Geoffrey C. Gardner, Michael J. Manfra, Charles M. Marcus, Ferdinand Kuemmeth
      Journal reference: Phys. Rev. X 8, 011045 (2018) [ 1710.10012v1 ]
      DOI: 10.1103/PhysRevX.8.011045

    • Conduction channels of an InAs-Al nanowire Josephson weak link - Abstract
      • We present a quantitative characterization of an electrically tunable Josephson junction defined in an InAs nanowire proximitized by an epitax-ially-grown superconducting Al shell. The gate-dependence of the number of conduction channels and of the set of transmission coefficients are extracted from the highly nonlinear current-voltage characteristics. Although the transmissions evolve non-monotonically, the number of independent channels can be tuned, and configurations with a single quasi-ballistic channel achieved.
    • M. F. Goffman, C. Urbina, H. Pothier, J. Nygård, C. M. Marcus, P. Krogstrup
      Journal reference: New Journal of Physics, Institute of Physics: Open Access Journals, 2017, 19, pp.092002 [ 1706.09150v2 ]
      DOI: 10.1088/1367-2630/aa7641

    • $h/e$ superconducting quantum interference through trivial edge states in InAs - Abstract
      • Josephson junctions defined in strong spin orbit semiconductors are highly interesting for the search for topological systems. However, next to topological edge states that emerge in a sufficient magnetic field, trivial edge states can also occur. We study the trivial edge states with superconducting quantum interference measurements on non-topological InAs Josephson junctions. We observe a SQUID pattern, an indication of superconducting edge transport. Also, a remarkable $h/e$ SQUID signal is observed that, as we find, stems from crossed Andreev states.
    • Folkert K. de Vries, Tom Timmerman, Viacheslav P. Ostroukh, Jasper van Veen, Arjan J. A. Beukman, Fanming Qu, Michael Wimmer, Binh-Minh Nguyen, Andrey A. Kiselev, Wei Yi, Marko Sokolich, Michael J. Manfra, Charles M. Marcus, Leo P. Kouwenhoven
      Journal reference: Phys. Rev. Lett. 120, 047702 (2018) [ 1709.03727v2 ]
      DOI: 10.1103/PhysRevLett.120.047702

    • Scaling of Majorana Zero-Bias Conductance Peaks - Abstract
      • We report an experimental study of the scaling of zero-bias conductance peaks compatible with Majorana zero modes as a function of magnetic field, tunnel coupling, and temperature in one-dimensional structures fabricated from an epitaxial semiconductor-superconductor heterostructure. Results are consistent with theory, including a peak conductance that is proportional to tunnel coupling, saturates at $2e^2/h$, decreases as expected with field-dependent gap, and collapses onto a simple scaling function in the dimensionless ratio of temperature and tunnel coupling.
    • Fabrizio Nichele, Asbjorn C. C. Drachmann, Alexander M. Whiticar, Eoin C. T. O'Farrell, Henri J. Suominen, Antonio Fornieri, Tian Wang, Geoffrey C. Gardner, Candice Thomas, Anthony T. Hatke, Peter Krogstrup, Michael J. Manfra, Karsten Flensberg, Charles M. Marcus
      Journal reference: Phys. Rev. Lett. 119, 136803 (2017) [ 1706.07033v2 ]
      DOI: 10.1103/PhysRevLett.119.136803

    • Scalable designs for quasiparticle-poisoning-protected topological quantum computation with Majorana zero modes - Abstract
      • We present designs for scalable quantum computers composed of qubits encoded in aggregates of four or more Majorana zero modes, realized at the ends of topological superconducting wire segments that are assembled into superconducting islands with significant charging energy. Quantum information can be manipulated according to a measurement-only protocol, which is facilitated by tunable couplings between Majorana zero modes and nearby semiconductor quantum dots. Our proposed architecture designs have the following principal virtues: (1) the magnetic field can be aligned in the direction of all of the topological superconducting wires since they are all parallel; (2) topological $T$-junctions are not used, obviating possible difficulties in their fabrication and utilization; (3) quasiparticle poisoning is abated by the charging energy; (4) Clifford operations are executed by a relatively standard measurement: detection of corrections to quantum dot energy, charge, or differential capacitance induced by quantum fluctuations; (5) it is compatible with strategies for producing good approximate magic states.
    • Torsten Karzig, Christina Knapp, Roman M. Lutchyn, Parsa Bonderson, Matthew B. Hastings, Chetan Nayak, Jason Alicea, Karsten Flensberg, Stephan Plugge, Yuval Oreg, Charles M. Marcus, Michael H. Freedman
      Journal reference: Phys. Rev. B 95, 235305 (2017) [ 1610.05289v4 ]
      DOI: 10.1103/PhysRevB.95.235305

    • Negative Spin Exchange in a Multielectron Quantum Dot - Abstract
      • By operating a one-electron quantum dot (fabricated between a multielectron dot and a one-electron reference dot) as a spectroscopic probe, we study the spin properties of a gate-controlled multielectron GaAs quantum dot at the transition between odd and even occupation number. We observe that the multielectron groundstate transitions from spin-1/2-like to singlet-like to triplet-like as we increase the detuning towards the next higher charge state. The sign reversal in the inferred exchange energy persists at zero magnetic field, and the exchange strength is tunable by gate voltages and in-plane magnetic fields. Complementing spin leakage spectroscopy data, the inspection of coherent multielectron spin exchange oscillations provides further evidence for the sign reversal and, inferentially, for the importance of non-trivial multielectron spin exchange correlations.
    • Frederico Martins, Filip K. Malinowski, Peter D. Nissen, Saeed Fallahi, Geoffrey C. Gardner, Michael J. Manfra, Charles M. Marcus, Ferdinand Kuemmeth
      Journal reference: Phys. Rev. Lett. 119, 227701 (2017) [ 1706.01007v1 ]
      DOI: 10.1103/PhysRevLett.119.227701

    • Transport Studies of Epi-Al/InAs Two-Dimensional Electron Gas Systems for Required Building-Blocks in Topological Superconductor Networks - Abstract
      • One-dimensional (1D) electronic transport and induced superconductivity in semiconductor nano-structures are crucial ingredients to realize topological superconductivity. Our approach for topological superconductivity employs a two-dimensional electron gas (2DEG) formed by an InAs quantum well, cleanly interfaced with a superconductor (epitaxial Al). This epi-Al/InAs quantum well heterostructure is advantageous for fabricating large-scale nano-structures consisting of multiple Majorana zero modes. Here, we demonstrate building-block transport studies using a high-quality epi-Al/InAs 2DEG heterostructure, which could be put together to realize the proposed 1D nanowire-based nano-structures and 2DEG-based networks that could host multiple Majorana zero modes: 1D transport using 1) quantum point contacts and 2) gate-defined quasi-1D channels in the InAs 2DEG as well as induced superconductivity in 3) a ballistic Al-InAs 2DEG-Al Josephson junction. From 1D transport, systematic evolution of conductance plateaus in half-integer conductance quanta are observed as a result of strong spin-orbit coupling in the InAs 2DEG. Large IcRn, a product of critical current and normal state resistance from the Josephson junction, indicates that the interface between the epitaxial Al and the InAs 2DEG is highly transparent. Our results of electronic transport studies based on the 2D approach suggest that the epitaxial superconductor/2D semiconductor system is suitable for realizing large-scale nano-structures for quantum computing applications.
    • Joon Sue Lee, Borzoyeh Shojaei, Mihir Pendharkar, Anthony P. McFadden, Younghyun Kim, Henri J. Suominen, Morten Kjaergaard, Fabrizio Nichele, Charles M. Marcus, Chris J. Palmstrøm
      Journal reference: Nano Lett. 19, 3083 (2019) [ 1705.05049v1 ]
      DOI: 10.1021/acs.nanolett.9b00494

    • Spectrum of the Nuclear Environment for GaAs Spin Qubits - Abstract
      • Using a singlet-triplet spin qubit as a sensitive spectrometer of the GaAs nuclear spin bath, we demonstrate that the spectrum of Overhauser noise agrees with a classical spin diffusion model over six orders of magnitude in frequency, from 1 mHz to 1 kHz, is flat below 10 mHz, and falls as $1/f^2$ for frequency $f \! \gtrsim \! 1$ Hz. Increasing the applied magnetic field from 0.1 T to 0.75 T suppresses electron-mediated spin diffusion, which decreases spectral content in the $1/f^2$ region and lowers the saturation frequency, each by an order of magnitude, consistent with a numerical model. Spectral content at megahertz frequencies is accessed using dynamical decoupling, which shows a crossover from the few-pulse regime ($\lesssim \! 16$ $\pi$-pulses), where transverse Overhauser fluctuations dominate dephasing, to the many-pulse regime ($\gtrsim \! 32$ $\pi$-pulses), where longitudinal Overhauser fluctuations with a $1/f$ spectrum dominate.
    • Filip K. Malinowski, Frederico Martins, Łukasz Cywiński, Mark S. Rudner, Peter D. Nissen, Saeed Fallahi, Geoffrey C. Gardner, Michael J. Manfra, Charles M. Marcus, Ferdinand Kuemmeth
      Journal reference: Phys. Rev. Lett. 118, 177702 (2017) [ 1701.01855v2 ]
      DOI: 10.1103/PhysRevLett.118.177702

    • Spin-orbit interaction in a dual gated InAs/GaSb quantum well - Abstract
      • Spin-orbit interaction is investigated in a dual gated InAs/GaSb quantum well. Using an electric field the quantum well can be tuned between a single carrier regime with exclusively electrons as carriers and a two-carriers regime where electrons and holes coexist. Spin-orbit interaction in both regimes manifests itself as a beating in the Shubnikov-de Haas oscillations. In the single carrier regime the linear Dresselhaus strength is characterized by $\beta =$ 28.5 meV$\AA$ and the Rashba coefficient $\alpha$ is tuned from 75 to 53 meV$\AA$ by changing the electric field. In the two-carriers regime the spin splitting shows a nonmonotonic behavior with gate voltage, which is consistent with our band structure calculations.
    • Arjan J. A. Beukman, Folkert K. de Vries, Jasper van Veen, Rafal Skolasinski, Michael Wimmer, Fanming Qu, David T. de Vries, Binh-Minh Nguyen, Wei Yi, Andrey A. Kiselev, Marko Sokolich, Michael J. Manfra, Fabrizio Nichele, Charles M. Marcus, Leo P. Kouwenhoven
      Journal reference: Phys. Rev. B 96, 241401 (2017) [ 1704.03482v1 ]
      DOI: 10.1103/PhysRevB.96.241401

    • Symmetric operation of the resonant exchange qubit - Abstract
      • We operate a resonant exchange qubit in a highly symmetric triple-dot configuration using IQ-modulated RF pulses. At the resulting three-dimensional sweet spot the qubit splitting is an order of magnitude less sensitive to all relevant control voltages, compared to the conventional operating point, but we observe no significant improvement in the quality of Rabi oscillations. For weak driving this is consistent with Overhauser field fluctuations modulating the qubit splitting. For strong driving we infer that effective voltage noise modulates the coupling strength between RF drive and the qubit, thereby quickening Rabi decay. Application of CPMG dynamical decoupling sequences consisting of up to n = 32 {\pi} pulses significantly prolongs qubit coherence, leading to marginally longer dephasing times in the symmetric configuration. This is consistent with dynamical decoupling from low frequency noise, but quantitatively cannot be explained by effective gate voltage noise and Overhauser field fluctuations alone. Our results inform recent strategies for the utilization of partial sweet spots in the operation and long-distance coupling of triple-dot qubits.
    • Filip K. Malinowski, Frederico Martins, Peter D. Nissen, Saeed Fallahi, Geoffrey C. Gardner, Michael J. Manfra, Charles M. Marcus, Ferdinand Kuemmeth
      Journal reference: Phys. Rev. B 96, 045443 (2017) [ 1704.01298v1 ]
      DOI: 10.1103/PhysRevB.96.045443

    • Anharmonicity of a superconducting qubit with a few-mode Josephson junction - Abstract
      • Coherent operation of gate-voltage-controlled hybrid transmon qubits (gatemons) based on semiconductor nanowires was recently demonstrated. Here we experimentally investigate the anharmonicity in epitaxial InAs-Al Josephson junctions, a key parameter for their use as a qubit. Anharmonicity is found to be reduced by roughly a factor of two compared to conventional metallic junctions, and dependent on gate voltage. Experimental results are consistent with a theoretical model, indicating that Josephson coupling is mediated by a small number of highly transmitting modes in the semiconductor junction.
    • A. Kringhøj, L. Casparis, M. Hell, T. W. Larsen, F. Kuemmeth, M. Leijnse, K. Flensberg, P. Krogstrup, J. Nygård, K. D. Petersson, C. M. Marcus
      Journal reference: Phys. Rev. B 97, 060508 (2018) [ 1703.05643v1 ]
      DOI: 10.1103/PhysRevB.97.060508

    • Notch filtering the nuclear environment of a spin qubit - Abstract
      • Electron spins in gate-defined quantum dots provide a promising platform for quantum computation. In particular, spin-based quantum computing in gallium arsenide takes advantage of the high quality of semiconducting materials, reliability in fabricating arrays of quantum dots, and accurate qubit operations. However, the effective magnetic noise arising from the hyperfine interaction with uncontrolled nuclear spins in the host lattice constitutes a major source of decoherence. Low frequency nuclear noise, responsible for fast (10 ns) inhomogeneous dephasing, can be removed by echo techniques. High frequency nuclear noise, recently studied via echo revivals, occurs in narrow frequency bands related to differences in Larmor precession of the three isotopes $\mathbf{^{69}Ga}$, $\mathbf{^{71}Ga}$, and $\mathbf{^{75}As}$. Here we show that both low and high frequency nuclear noise can be filtered by appropriate dynamical decoupling sequences, resulting in a substantial enhancement of spin qubit coherence times. Using nuclear notch filtering, we demonstrate a spin coherence time ($\mathbf{T_{2}}$) of 0.87 ms, five orders of magnitude longer than typical exchange gate times, and exceeding the longest coherence times reported to date in Si/SiGe gate-defined quantum dots.
    • F. K. Malinowski, F. Martins, P. D. Nissen, E. Barnes, Ł. Cywiński, M. S. Rudner, S. Fallahi, G. C. Gardner, M. J. Manfra, C. M. Marcus, F. Kuemmeth
      Journal reference: Nat. Nanotechnol. 12, 16-20 (2017) [ 1601.06677v3 ]
      DOI: 10.1038/nnano.2016.170

    • Current–phase relations of few-mode InAs nanowire Josephson junctions - Abstract
      • Gate-tunable semiconductor nanowires with superconducting leads have great potential for quantum computation and as model systems for mesoscopic Josephson junctions. The supercurrent, $I$, versus the phase, $\phi$, across the junction is called the current-phase relation (CPR). It can reveal not only the amplitude of the critical current, but also the number of modes and their transmission. We measured the CPR of many individual InAs nanowire Josephson junctions, one junction at a time. Both the amplitude and shape of the CPR varied between junctions, with small critical currents and skewed CPRs indicating few-mode junctions with high transmissions. In a gate-tunable junction, we found that the CPR varied with gate voltage: Near the onset of supercurrent, we observed behavior consistent with resonant tunneling through a single, highly transmitting mode. The gate dependence is consistent with modeled subband structure that includes an effective tunneling barrier due to an abrupt change in the Fermi level at the boundary of the gate-tuned region. These measurements of skewed, tunable, few-mode CPRs are promising both for applications that require anharmonic junctions and for Majorana readout proposals.
    • Eric M. Spanton, Mingtang Deng, Saulius Vaitiekėnas, Peter Krogstrup, Jesper Nygård, Charles M. Marcus, Kathryn A. Moler
      Journal reference: Nature Physics (2017) [ 1701.01188v1 ]
      DOI: 10.1038/nphys4224

    • Majorana bound state in a coupled quantum-dot hybrid-nanowire system - Abstract
      • Hybrid nanowires combining semiconductor and superconductor materials appear well suited for the creation, detection, and control of Majorana bound states (MBSs). We demonstrate the emergence of MBSs from coalescing Andreev bound states (ABSs) in a hybrid InAs nanowire with epitaxial Al, using a quantum dot at the end of the nanowire as a spectrometer. Electrostatic gating tuned the nanowire density to a regime of one or a few ABSs. In an applied axial magnetic field, a topological phase emerges in which ABSs move to zero energy and remain there, forming MBSs. We observed hybridization of the MBS with the end-dot bound state, which is in agreement with a numerical model. The ABS/MBS spectra provide parameters that are useful for understanding topological superconductivity in this system.
    • M. T. Deng, S. Vaitiekenas, E. B. Hansen, J. Danon, M. Leijnse, K. Flensberg, J. Nygård, P. Krogstrup, C. M. Marcus
      Journal reference: Science 354, 1557-1562 (2016) [ 1612.07989v2 ]
      DOI: 10.1126/science.aaf3961

  • 2016
    • Transport Signatures of Quasiparticle Poisoning in a Majorana Island - Abstract
      • We investigate effects of quasiparticle poisoning in a Majorana island with strong tunnel coupling to normal-metal leads. In addition to the main Coulomb blockade diamonds, "shadow" diamonds appear, shifted by 1e in gate voltage, consistent with transport through an excited (poisoned) state of the island. Comparison to a simple model yields an estimate of parity lifetime for the strongly coupled island (~ 1 {\mu}s) and sets a bound for a weakly coupled island (> 10 {\mu}s). Fluctuations in the gate-voltage spacing of Coulomb peaks at high field, reflecting Majorana hybridization, are enhanced by the reduced lever arm at strong coupling. In energy units, fluctuations are consistent with previous measurements.
    • S. M. Albrecht, E. B. Hansen, A. P. Higginbotham, F. Kuemmeth, T. S. Jespersen, J. Nygård, P. Krogstrup, J. Danon, K. Flensberg, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 118, 137701 (2017) [ 1612.05748v1 ]
      DOI: 10.1103/PhysRevLett.118.137701

    • Proximity Effect Transfer from NbTi into a Semiconductor Heterostructure via Epitaxial AluminumProximity Effect Transfer from NbTi into a Semiconductor Heterostructure via Epitaxial Aluminum - Abstract
      • We demonstrate the transfer of the superconducting properties of NbTi---a large-gap high-critical-field superconductor---into an InAs heterostructure via a thin intermediate layer of epitaxial Al. Two device geometries, a Josephson junction and a gate-defined quantum point contact, are used to characterize interface transparency and the two-step proximity effect. In the Josephson junction, multiple Andreev reflection reveal near-unity transparency, with an induced gap $\Delta^*=0.50~\mathrm{meV}$ and a critical temperature of $7.8~\mathrm{K}$. Tunneling spectroscopy yields a hard induced gap in the InAs adjacent to the superconductor of $\Delta^*=0.43~\mathrm{meV}$ with substructure characteristic of both Al and NbTi.
    • A. C. C. Drachmann, H. J. Suominen, M. Kjaergaard, B. Shojaei, C. J. Palmstrøm, C. M. Marcus, F. Nichele
      Journal reference: Nano Lett. 17, 1200 (2017) [ 1611.10166v1 ]
      DOI: 10.1021/acs.nanolett.6b04964

    • Quantized conductance doubling and hard gap in a two-dimensional semiconductor–superconductor heterostructure - Abstract
      • The prospect of coupling a two-dimensional (2D) semiconductor heterostructure to a superconductor opens new research and technology opportunities, including fundamental problems in mesoscopic superconductivity, scalable superconducting electronics, and new topological states of matter. For instance, one route toward realizing topological matter is by coupling a 2D electron gas (2DEG) with strong spin-orbit interaction to an s-wave superconductor. Previous efforts along these lines have been hindered by interface disorder and unstable gating. Here, we report measurements on a gateable InGaAs/InAs 2DEG with patterned epitaxial Al, yielding multilayer devices with atomically pristine interfaces between semiconductor and superconductor. Using surface gates to form a quantum point contact (QPC), we find a hard superconducting gap in the tunneling regime, overcoming the soft-gap problem in 2D superconductor-semiconductor hybrid systems. With the QPC in the open regime, we observe a first conductance plateau at 4e^2/h, as expected theoretically for a normal-QPC-superconductor structure. The realization of a hard-gap semiconductor-superconductor system that is amenable to top-down processing provides a means of fabricating scalable multicomponent hybrid systems for applications in low-dissipation electronics and topological quantum information.
    • M. Kjaergaard, F. Nichele, H. J. Suominen, M. P. Nowak, M. Wimmer, A. R. Akhmerov, J. A. Folk, K. Flensberg, J. Shabani, C. J. Palmstrom, C. M. Marcus
      Journal reference: Nat. Commun. 7, 12841 (2016) [ 1603.01852v2 ]
      DOI: 10.1038/ncomms12841

    • InAs Nanowire with Epitaxial Aluminum as a Single-Electron Transistor with Fixed Tunnel Barriers - Abstract
      • We report on fabrication of single-electron transistors using InAs nanowires with epitaxial aluminium with fixed tunnel barriers made of aluminium oxide. The devices exhibit a hard superconducting gap induced by the proximized aluminium cover shell and they behave as metallic single-electron transistors. In contrast to the typical few channel contacts in semiconducting devices, our approach forms opaque multichannel contacts to a semiconducting wire and thus provides a complementary way to study them. In addition, we confirm that unwanted extra quantum dots can appear at the surface of the nanowire. Their presence is prevented in our devices, and also by inserting a protective layer of GaAs between the InAs and Al, the latter being suitable for standard measurement methods.
    • M. Taupin, E. Mannila, P. Krogstrup, V. F. Maisi, H. Nguyen, S. M. Albrecht, J. Nygard, C. M. Marcus, J. P. Pekola
      Journal reference: Phys. Rev. Applied 6, 054017 (2016) [ 1601.01149v4 ]
      DOI: 10.1103/PhysRevApplied.6.054017

    • Giant spin-orbit splitting in inverted InAs/GaSb double quantum wells - Abstract
      • Transport measurements in inverted InAs/GaSb quantum wells reveal a giant spin-orbit splitting of the energy bands close to the hybridization gap. The splitting results from the interplay of electron-hole mixing and spin-orbit coupling, and can exceed the hybridization gap. We experimentally investigate the band splitting as a function of top gate voltage for both electron-like and hole-like states. Unlike conventional, noninverted two-dimensional electron gases, the Fermi energy in InAs/GaSb can cross a single spin-resolved band, resulting in full spin-orbit polarization. In the fully polarized regime we observe exotic transport phenomena such as quantum Hall plateaus evolving in $e^2/h$ steps and a non-trivial Berry phase.
    • Fabrizio Nichele, Morten Kjaergaard, Henri J. Suominen, Rafal Skolasinski, Michael Wimmer, Binh-Minh Nguyen, Andrey A. Kiselev, Wei Yi, Marko Sokolich, Michael J. Manfra, Fanming Qu, Arjan J. A. Beukman, Leo P. Kouwenhoven, Charles M. Marcus
      Journal reference: Phys. Rev. Lett. 118, 016801 (2017) [ 1605.01241v2 ]
      DOI: 10.1103/PhysRevLett.118.016801

    • Anomalous Fraunhofer interference in epitaxial superconductor-semiconductor Josephson junctions - Abstract
      • We investigate patterns of critical current as a function of perpendicular and in-plane magnetic fields in superconductor-semiconductor-superconductor (SNS) junctions based on InAs/InGaAs heterostructures with an epitaxial Al layer. This material system is of interest due to its exceptionally good superconductor-semiconductor coupling, as well as large spin-orbit interaction and g-factor in the semiconductor. Thin epitaxial Al allows the application of large in-plane field without destroying superconductivity. For fields perpendicular to the junction, flux focusing results in aperiodic node spacings in the pattern of critical currents known as Fraunhofer patterns by analogy to the related interference effect in optics. Adding an in-plane field yields two further anomalies in the pattern. First, higher order nodes are systematically strengthened, indicating current flow along the edges of the device, as a result of confinement of Andreev states driven by an induced flux dipole; second, asymmetries in the interference appear that depend on the field direction and magnitude. A model is presented, showing good agreement with experiment, elucidating the roles of flux focusing, Zeeman and spin-orbit coupling, and disorder in producing these effects.
    • H. J. Suominen, J. Danon, M. Kjaergaard, K. Flensberg, J. Shabani, C. J. Palmstrøm, F. Nichele, C. M. Marcus
      Journal reference: Phys. Rev. B 95, 035307 (2017) [ 1611.00190v1 ]
      DOI: 10.1103/PhysRevB.95.035307

    • Limits to mobility in InAs quantum wells with nearly lattice-matched barriers - Abstract
      • The growth and the density dependence of the low temperature mobility of a series of two-dimensional electron systems confined to un-intentionally doped, low extended defect density InAs quantum wells with Al$_{1-x}$Ga$_{x}$Sb barriers are reported. The electron mobility limiting scattering mechanisms were determined by utilizing dual-gated devices to study the dependence of mobility on carrier density and electric field independently. Analysis of the possible scattering mechanisms indicate the mobility was limited primarily by rough interfaces in narrow quantum wells and a combination of alloy disorder and interface roughness in wide wells at high carrier density within the first occupied electronic sub-band. At low carrier density the functional dependence of the mobility on carrier density provided evidence of coulombic scattering from charged defects. A gate-tuned electron mobility exceeding 750,000 cm$^{2}$/Vs was achieved at a sample temperature of 2 K.
    • B. Shojaei, A. C. C. Drachmann, M. Pendharkar, D. J. Pennachio, M. P. Echlin, P. G. Callahan, S. Kraemer, T. M. Pollock, C. M. Marcus, C. J. Palmstrøm
      Journal reference: Phys. Rev. B 94, 245306 (2016) [ 1610.03785v2 ]
      DOI: 10.1103/PhysRevB.94.245306

    • Quantized conductance and large g-factor anisotropy in InSb quantum point contactsQuantized Conductance and Large gFactor Anisotropy in InSb Quantum Point Contacts - Abstract
      • Due to a strong spin-orbit interaction and a large Land\'e g-factor, InSb plays an important role in research on Majorana fermions. To further explore novel properties of Majorana fermions, hybrid devices based on quantum wells are conceived as an alternative approach to nanowires. In this work, we report a pronounced conductance quantization of quantum point contact devices in InSb/InAlSb quantum wells. Using a rotating magnetic field, we observe a large in-plane (|g1|=26) and out-of-plane (|g1|=52) g-factor anisotropy. Additionally, we investigate crossings of subbands with opposite spins and extract the electron effective mass from magnetic depopulation of one-dimensional subbands.
    • Fanming Qu, Jasper van Veen, Folkert K. de Vries, Arjan J. A. Beukman, Michael Wimmer, Wei Yi, Andrey A. Kiselev, Binh-Minh Nguyen, Marko Sokolich, Michael J. Manfra, Fabrizio Nichele, Charles M. Marcus, Leo P. Kouwenhoven
      Journal reference: Nano Lett., 2016, 16, 7509 [ 1608.05478v1 ]
      DOI: 10.1021/acs.nanolett.6b03297

    • Edge transport in the trivial phase of InAs/GaSb - Abstract
      • We present transport and scanning SQUID measurements on InAs/GaSb double quantum wells, a system predicted to be a two-dimensional topological insulator. Top and back gates allow independent control of density and band offset, allowing tuning from the trivial to the topological regime. In the trivial regime, bulk conductivity is quenched but transport persists along the edges, superficially resembling the predicted helical edge-channels in the topological regime. We characterize edge conduction in the trivial regime in a wide variety of sample geometries and measurement configurations, as a function of temperature, magnetic field, and edge length. Despite similarities to studies claiming measurements of helical edge channels, our characterization points to a non-topological origin for these observations.
    • Fabrizio Nichele, Henri J. Suominen, Morten Kjaergaard, Charles M. Marcus, Ebrahim Sajadi, Joshua A. Folk, Fanming Qu, Arjan J. A. Beukman, Folkert K. de Vries, Jasper van Veen, Stevan Nadj-Perge, Leo P. Kouwenhoven, Binh-Minh Nguyen, Andrey A. Kiselev, Wei Yi, Marko Sokolich, Michael J. Manfra, Eric M. Spanton, Kathryn A. Moler
      Journal reference: New J. Phys. 18, 083005 (2016) [ 1511.01728v4 ]
      DOI: 10.1088/1367-2630/18/8/083005

    • Transparent Semiconductor-Superconductor Interface and Induced Gap in an Epitaxial Heterostructure Josephson Junction - Abstract
      • Measurement of multiple Andreev reflection (MAR) in a Josephson junction made from an InAs heterostructure with epitaxial aluminum is used to quantify the highly transparent semiconductor-superconductor interface, indicating near-unity transmission. The observed temperature dependence of MAR does not follow a conventional BCS form, but instead agrees with a model in which the density of states in the quantum well acquires an effective induced gap, in our case 180 {\mu}eV, close to that of the epitaxial superconductor. Carrier density dependence of MAR is investigated using a depletion gate, revealing the subband structure of the semiconductor quantum well, consistent with magnetotransport experiment of the bare InAs performed on the same wafer.
    • M. Kjaergaard, H. J. Suominen, M. P. Nowak, A. R. Akhmerov, J. Shabani, C. J. Palmstrøm, F. Nichele, C. M. Marcus
      Journal reference: Phys. Rev. Applied 7, 034029 (2017) [ 1607.04164v2 ]
      DOI: 10.1103/PhysRevApplied.7.034029

    • Decoupling edge versus bulk conductance in the trivial regime of an InAs/GaSb double quantum well using Corbino ring geometry - Abstract
      • A Corbino ring geometry is utilized to analyze edge and bulk conductance of InAs/GaSb quantum well structures. We show that edge conductance exists in the trivial regime of this theoretically-predicted topological system with a temperature insensitive linear resistivity per unit length in the range of 2 kOhm/um. A resistor network model of the device is developed to decouple the edge conductance from the bulk conductance, providing a quantitative technique to further investigate the nature of this trivial edge conductance, conclusively identified here as being of n-type.
    • Binh-Minh Nguyen, Andrey A. Kiselev, Ramsey Noah, Wei Yi, Fanming Qu, Arjan J. A. Beukman, Folkert K. de Vries, Jasper van Veen, Stevan Nadj-Perge, Leo P. Kouwenhoven, Morten Kjaergaard, Henri J. Suominen, Fabrizio Nichele, Charles M. Marcus, Michael J. Manfra, Marko Sokolich
      Journal reference: Phys. Rev. Lett. 117, 077701 (2016) [ 1605.04818v1 ]
      DOI: 10.1103/PhysRevLett.117.077701

    • Normal, superconducting and topological regimes of hybrid double quantum dots - Abstract
      • Epitaxial semiconductor-superconductor hybrid materials are an excellent basis for studying mesoscopic and topological superconductivity, as the semiconductor inherits a hard superconducting gap while retaining tunable carrier density. Here, we investigate double-quantum-dot devices made from InAs nanowires with a patterned epitaxial Al two-facet shell that proximitizes two gate-defined segments along the nanowire. We follow the evolution of mesoscopic superconductivity and charging energy in this system as a function of magnetic field and voltage-tuned barriers. Inter-dot coupling is varied from strong to weak using side gates, and the ground state is varied between normal, superconducting, and topological regimes by applying a magnetic field. We identify the topological transition by tracking the spacing between successive cotunneling peaks as a function of axial magnetic field and show that the individual dots host weakly hybridized Majorana modes.
    • D. Sherman, J. S. Yodh, S. M. Albrecht, J. Nygård, P. Krogstrup, C. M. Marcus
      Journal reference: Nature Nanotechnology 12, 212 (2017) [ 1605.01865v1 ]
      DOI: 10.1038/nnano.2016.227

    • Milestones Toward Majorana-Based Quantum Computing - Abstract
      • We introduce a scheme for preparation, manipulation, and readout of Majorana zero modes in semiconducting wires with mesoscopic superconducting islands. Our approach synthesizes recent advances in materials growth with tools commonly used in quantum-dot experiments, including gate-control of tunnel barriers and Coulomb effects, charge sensing, and charge pumping. We outline a sequence of milestones interpolating between zero-mode detection and quantum computing that includes (1) detection of fusion rules for non-Abelian anyons using either proximal charge sensors or pumped current; (2) validation of a prototype topological qubit; and (3) demonstration of non-Abelian statistics by braiding in a branched geometry. The first two milestones require only a single wire with two islands, and additionally enable sensitive measurements of the system's excitation gap, quasiparticle poisoning rates, residual Majorana zero-mode splittings, and topological-qubit coherence times. These pre-braiding experiments can be adapted to other manipulation and readout schemes as well.
    • David Aasen, Michael Hell, Ryan V. Mishmash, Andrew Higginbotham, Jeroen Danon, Martin Leijnse, Thomas S. Jespersen, Joshua A. Folk, Charles M. Marcus, Karsten Flensberg, Jason Alicea
      Journal reference: Phys. Rev. X 6, 031016 (2016) [ 1511.05153v2 ]
      DOI: 10.1103/PhysRevX.6.031016

    • Filter function formalism beyond pure dephasing and non-Markovian noise in singlet-triplet qubits - Abstract
      • The filter function formalism quantitatively describes the dephasing of a qubit by a bath that causes Gaussian fluctuations in the qubit energies with an arbitrary noise power spectrum. Here, we extend this formalism to account for more general types of noise that couple to the qubit through terms that do not commute with the qubit's bare Hamiltonian. Our approach applies to any power spectrum that generates slow noise fluctuations in the qubit's evolution. We demonstrate our formalism in the case of singlet-triplet qubits subject to both quasistatic nuclear noise and $1/\omega^\alpha$ charge noise and find good agreement with recent experimental findings. This comparison shows the efficacy of our approach in describing real systems and additionally highlights the challenges with distinguishing different types of noise in free induction decay experiments.
    • Edwin Barnes, Mark S. Rudner, Frederico Martins, Filip K. Malinowski, Charles M. Marcus, Ferdinand Kuemmeth
      Journal reference: Phys. Rev. B 93, 121407 (2016) [pdf]
      DOI: 10.1103/PhysRevB.93.121407

    • Exponential protection of zero modes in Majorana islands - Abstract
      • Majorana zero modes are quasiparticle excitations in condensed matter systems that have been proposed as building blocks of fault-tolerant quantum computers [1]. They are expected to exhibit non-Abelian particle statistics, in contrast to the usual statistics of fermions and bosons, enabling quantum operations to be performed by braiding isolated modes around one another. Quantum braiding operations are topologically protected insofar as these modes are pinned near zero energy, and the pinning is predicted to be exponential as the modes become spatially separated. Following theoretical proposals, several experiments have identified signatures of Majorana modes in proximitized nanowires and atomic chains, with small mode-splitting potentially explained by hybridization of Majoranas. Here, we use Coulomb-blockade spectroscopy in an InAs nanowire segment with epitaxial aluminum, which forms a proximity-induced superconducting Coulomb island (a Majorana island) that is isolated from normal-metal leads by tunnel barriers, to measure the splitting of near-zero-energy Majorana modes. We observe exponential suppression of energy splitting with increasing wire length. For short devices of a few hundred nanometers, sub-gap state energies oscillate as the magnetic field is varied, as is expected for hybridized Majorana modes. Splitting decreases by a factor of about ten for each half micrometer of increased wire length. For devices longer than about one micrometer, transport in strong magnetic fields occurs through a zero-energy state that is energetically isolated from a continuum, yielding uniformly spaced Coulomb-blockade conductance peaks, consistent with teleportation via Majorana modes. Our results help explain the trivial-to-topological transition in finite systems and to quantify the scaling of topological protection with end-mode separation.
    • S. M. Albrecht, A. P. Higginbotham, M. Madsen, F. Kuemmeth, T. S. Jespersen, J. Nygård, P. Krogstrup, C. M. Marcus
      Journal reference: Nature 531, 206 (2016) [pdf]
      DOI: 10.1038/nature17162

    • Noise Suppression Using Symmetric Exchange Gates in Spin Qubits - Abstract
      • We demonstrate a substantial improvement in the spin-exchange gate using symmetric control instead of conventional detuning in GaAs spin qubits, up to a factor-of-six increase in the quality factor of the gate. For symmetric operation, nanosecond voltage pulses are applied to the barrier that controls the interdot potential between quantum dots, modulating the exchange interaction while maintaining symmetry between the dots. Excellent agreement is found with a model that separately includes electrical and nuclear noise sources for both detuning and symmetric gating schemes. Unlike exchange control via detuning, the decoherence of symmetric exchange rotations is dominated by rotation-axis fluctuations due to nuclear field noise rather than direct exchange noise.
    • Frederico Martins, Filip K. Malinowski, Peter D. Nissen, Edwin Barnes, Saeed Fallahi, Geoffrey C. Gardner, Michael J. Manfra, Charles M. Marcus, Ferdinand Kuemmeth
      Journal reference: Phys. Rev. Lett. 116, 116801 (2016) [pdf]
      DOI: 10.1103/PhysRevLett.116.116801

  • 2015
    • Gatemon Benchmarking and Two-Qubit Operations - Abstract
      • Recent experiments have demonstrated superconducting transmon qubits with semiconductor nanowire Josephson junctions. These hybrid gatemon qubits utilize field effect tunability characteristic for semiconductors to allow complete qubit control using gate voltages, potentially a technological advantage over conventional flux-controlled transmons. Here, we present experiments with a two-qubit gatemon circuit. We characterize qubit coherence and stability and use randomized benchmarking to demonstrate single-qubit gate errors below 0.7% for all gates, including voltage-controlled $Z$ rotations. We show coherent capacitive coupling between two gatemons and coherent swap operations. Finally, we perform a two-qubit controlled-phase gate with an estimated fidelity of 91%, demonstrating the potential of gatemon qubits for building scalable quantum processors.
    • L. Casparis, T. W. Larsen, M. S. Olsen, F. Kuemmeth, P. Krogstrup, J. Nygård, K. D. Petersson, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 116, 150505 (2016) [ 1512.09195v1 ]
      DOI: 10.1103/PhysRevLett.116.150505

    • Two-dimensional epitaxial superconductor-semiconductor heterostructures: A platform for topological superconducting networks - Abstract
      • Progress in the emergent field of topological superconductivity relies on synthesis of new material combinations, combining superconductivity, low density, and spin-orbit coupling (SOC). For example, theory [1-4] indicates that the interface between a one-dimensional (1D) semiconductor (Sm) with strong SOC and a superconductor (S) hosts Majorana modes with nontrivial topological properties [5-8]. Recently, epitaxial growth of Al on InAs nanowires was shown to yield a high quality S-Sm system with uniformly transparent interfaces [9] and a hard induced gap, indicted by strongly suppressed sub gap tunneling conductance [10]. Here we report the realization of a two-dimensional (2D) InAs/InGaAs heterostructure with epitaxial Al, yielding a planar S-Sm system with structural and transport characteristics as good as the epitaxial wires. The realization of 2D epitaxial S-Sm systems represent a significant advance over wires, allowing extended networks via top-down processing. Among numerous potential applications, this new material system can serve as a platform for complex networks of topological superconductors with gate-controlled Majorana zero modes [1-4]. We demonstrate gateable Josephson junctions and a highly transparent 2D S-Sm interface based on the product of excess current and normal state resistance.
    • J. Shabani, M. Kjaergaard, H. J. Suominen, Younghyun Kim, F. Nichele, K. Pakrouski, T. Stankevic, R. M. Lutchyn, P. Krogstrup, R. Feidenhans'l, S. Kraemer, C. Nayak, M. Troyer, C. M. Marcus, C. J. Palmstrøm
      Journal reference: Phys. Rev. B 93, 155402 (2016) [pdf]
      DOI: 10.1103/PhysRevB.93.155402

    • Semiconductor-Nanowire-Based Superconducting Qubit - Abstract
      • We introduce a hybrid qubit based on a semiconductor nanowire with an epitaxially grown superconductor layer. Josephson energy of the transmon-like device ("gatemon") is controlled by an electrostatic gate that depletes carriers in a semiconducting weak link region. Strong coupling to an on-chip microwave cavity and coherent qubit control via gate voltage pulses is demonstrated, yielding reasonably long relaxation times (0.8 {\mu}s) and dephasing times (1 {\mu}s), exceeding gate operation times by two orders of magnitude, in these first-generation devices. Because qubit control relies on voltages rather than fluxes, dissipation in resistive control lines is reduced, screening reduces crosstalk, and the absence of flux control allows operation in a magnetic field, relevant for topological quantum information.
    • T. W. Larsen, K. D. Petersson, F. Kuemmeth, T. S. Jespersen, P. Krogstrup, J. Nygard, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 115, 127001 (2015) [pdf]
      DOI: 10.1103/PhysRevLett.115.127001

    • Gate-tunable high mobility remote-doped InSb/In_{1-x}Al_{x}Sb quantum well heterostructures - Abstract
      • Gate-tunable high-mobility InSb/In_{1-x}Al_{x}Sb quantum wells (QWs) grown on GaAs substrates are reported. The QW two-dimensional electron gas (2DEG) channel mobility in excess of 200,000 cm^{2}/Vs is measured at T=1.8K. In asymmetrically remote-doped samples with an HfO_{2} gate dielectric formed by atomic layer deposition, parallel conduction is eliminated and complete 2DEG channel depletion is reached with minimal hysteresis in gate bias response of the 2DEG electron density. The integer quantum Hall effect with Landau level filling factor down to 1 is observed. A high-transparency non-alloyed Ohmic contact to the 2DEG with contact resistance below 1{\Omega} \cdot mm is achieved at 1.8K.
    • Wei Yi, Andrey A. Kiselev, Jacob Thorp, Ramsey Noah, Binh-Minh Nguyen, Steven Bui, Rajesh D. Rajavel, Tahir Hussain, Mark Gyure, Philip Kratz, Qi Qian, Michael J. Manfra, Vlad S. Pribiag, Leo P. Kouwenhoven, Charles M. Marcus, Marko Sokolich
      Journal reference: Appl. Phys. Lett. 106, 142103 (2015) [pdf]
      DOI: 10.1063/1.4917027

    • Electric and Magnetic Tuning Between the Trivial and Topological Phases in InAs/GaSb Double Quantum Wells - Abstract
      • Among the theoretically predicted two-dimensional topological insulators, InAs/GaSb double quantum wells (DQWs) have a unique double-layered structure with electron and hole gases separated in two layers, which enables tuning of the band alignment via electric and magnetic fields. However, the rich trivial-topological phase diagram has yet to be experimentally explored. We present an in situ and continuous tuning between the trivial and topological insulating phases in InAs/GaSb DQWs through electrical dual-gating. Furthermore, we show that an in-plane magnetic field shifts the electron and hole bands relatively to each other in momentum space, functioning as a powerful tool to discriminate between the topologically distinct states.
    • Fanming Qu, Arjan J. A. Beukman, Stevan Nadj-Perge, Michael Wimmer, Binh-Minh Nguyen, Wei Yi, Jacob Thorp, Marko Sokolich, Andrey A. Kiselev, Michael J. Manfra, Charles M. Marcus, Leo P. Kouwenhoven
      Journal reference: Phys. Rev. Lett. 115, 036803 (2015) [pdf]
      DOI: 10.1103/PhysRevLett.115.036803

    • Hard gap in epitaxial semiconductor–superconductor nanowires - Abstract
      • Many present and future applications of superconductivity would benefit from electrostatic control of carrier density and tunneling rates, the hallmark of semiconductor devices. One particularly exciting application is the realization of topological superconductivity as a basis for quantum information processing. Proposals in this direction based on proximity effect in semiconductor nanowires are appealing because the key ingredients are currently in hand. However, previous instances of proximitized semiconductors show significant tunneling conductance below the superconducting gap, suggesting a continuum of subgap states---a situation that nullifies topological protection. Here, we report a hard superconducting gap induced by proximity effect in a semiconductor, using epitaxial Al-InAs superconductor-semiconductor nanowires. The hard gap, along with favorable material properties and gate-tunability, makes this new hybrid system attractive for a number of applications, as well as fundamental studies of mesoscopic superconductivity.
    • W. Chang, S. M. Albrecht, T. S. Jespersen, F. Kuemmeth, P. Krogstrup, J. Nygård, C. M. Marcus
      Journal reference: Nature Nanotechnology 10, 232 (2015) [pdf]
      DOI: 10.1038/nnano.2014.306

    • Parity lifetime of bound states in a proximitized semiconductor nanowire - Abstract
      • Quasiparticle excitations can compromise the performance of superconducting devices, causing high frequency dissipation, decoherence in Josephson qubits, and braiding errors in proposed Majorana-based topological quantum computers. Quasiparticle dynamics have been studied in detail in metallic superconductors but remain relatively unexplored in semiconductor-superconductor structures, which are now being intensely pursued in the context of topological superconductivity. To this end, we introduce a new physical system comprised of a gate-confined semiconductor nanowire with an epitaxially grown superconductor layer, yielding an isolated, proximitized nanowire segment. We identify Andreev-like bound states in the semiconductor via bias spectroscopy, determine the characteristic temperatures and magnetic fields for quasiparticle excitations, and extract a parity lifetime (poisoning time) of the bound state in the semiconductor exceeding 10 ms.
    • A. P. Higginbotham, S. M. Albrecht, G. Kirsanskas, W. Chang, F. Kuemmeth, P. Krogstrup, T. S. Jespersen, J. Nygard, K. Flensberg, C. M. Marcus
      Journal reference: Nature Physics 11, 1017 (2015) [pdf]
      DOI: 10.1038/nphys3461

  • 2014
    • Epitaxy of semiconductor–superconductor nanowires - Abstract
      • Controlling the properties of semiconductor/metal interfaces is a powerful method for designing functionality and improving the performance of electrical devices. Recently semiconductor/superconductor hybrids have appeared as an important example where the atomic scale uniformity of the interface plays a key role for the quality of the induced superconducting gap. Here we present epitaxial growth of semiconductor-metal core-shell nanowires by molecular beam epitaxy, a method that provides a conceptually new route to controlled electrical contacting of nanostructures and for designing devices for specialized applications such as topological and gate-controlled superconducting electronics. Our materials of choice, InAs/Al, are grown with epitaxially matched single plane interfaces, and alternative semiconductor/metal combinations allowing epitaxial interface matching in nanowires are discussed. We formulate the grain growth kinetics of the metal phase in general terms of continuum parameters and bicrystal symmetries. The method realizes the ultimate limit of uniform interfaces and appears to solve the soft-gap problem in superconducting hybrid structures.
    • P. Krogstrup, N. L. B. Ziino, W. Chang, S. M. Albrecht, M. H. Madsen, E. Johnson, J. Nygård, C. M. Marcus, T. S. Jespersen
      Journal reference: Nature Materials 14, 400 (2015) [pdf]
      DOI: 10.1038/nmat4176

    • Hole Spin Coherence in a Ge/Si Heterostructure Nanowire - Abstract
      • Relaxation and dephasing of hole spins are measured in a gate-defined Ge/Si nanowire double quantum dot using a fast pulsed-gate method and dispersive readout. An inhomogeneous dephasing time $T_2^* \sim 0.18~\mathrm{\mu s}$ exceeds corresponding measurements in III-V semiconductors by more than an order of magnitude, as expected for predominately nuclear-spin-free materials. Dephasing is observed to be exponential in time, indicating the presence of a broadband noise source, rather than Gaussian, previously seen in systems with nuclear-spin-dominated dephasing.
    • A. P. Higginbotham, T. W. Larsen, J. Yao, H. Yan, C. M. Lieber, C. M. Marcus, F. Kuemmeth
      Journal reference: Nano Letters 14, 3582 (2014) [pdf]
      DOI: 10.1021/nl501242b

    • Antilocalization of Coulomb Blockade in a Ge/Si Nanowire - Abstract
      • The distribution of Coulomb blockade peak heights as a function of magnetic field is investigated experimentally in a Ge-Si nanowire quantum dot. Strong spin-orbit coupling in this hole-gas system leads to antilocalization of Coulomb blockade peaks, consistent with theory. In particular, the peak height distribution has its maximum away from zero at zero magnetic field, with an average that decreases with increasing field. Magnetoconductance in the open-wire regime places a bound on the spin-orbit length ($l_{so}$ < 20 nm), consistent with values extracted in the Coulomb blockade regime ($l_{so}$ < 25 nm).
    • A. P. Higginbotham, F. Kuemmeth, T. W. Larsen, M. Fitzpatrick, J. Yao, H. Yan, C. M. Lieber, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 112, 216806 (2014) [pdf]
      DOI: 10.1103/PhysRevLett.112.216806

  • 2013
    • Epitaxial aluminum contacts to InAs nanowires - Abstract
      • We report a method for making epitaxial superconducting contacts to semiconducting nanowires. The temperature and gate characteristics demonstrate barrier-free electrical contact, and the properties in the superconducting state are investigated at low temperature. Half-covering aluminum contacts are realized without the need of lithography and we demonstrate how to controllably insert high-band gap layers in the interface region. These developments are relevant to hybrid superconductor-nanowire devices that support Majorana zero energy states.
    • N. L. B. Ziino, P. Krogstrup, M. H. Madsen, E. Johnson, J. B. Wagner, C. M. Marcus, J. Nygård, T. S. Jespersen
      1309.4569v1 [pdf]

    • Single-layer graphene on silicon nitride micromembrane resonators - Abstract
      • Due to their exceptional mechanical and optical properties, dielectric silicon nitride (SiN) micromembrane resonators have become the centerpiece of many optomechanical experiments. Efficient capacitive coupling of the membrane to an electrical system would facilitate exciting hybrid optoelectromechanical devices. However, capacitive coupling of such dielectric membranes is rather weak. Here we add a single layer of graphene on SiN micromembranes and compare electromechanical coupling and mechanical properties to bare dielectric membranes and to membranes metallized with an aluminium layer. The electrostatic coupling of graphene coated membranes is found to be equal to a perfectly conductive membrane. Our results show that a single layer of graphene substantially enhances the electromechanical capacitive coupling without significantly adding mass, decreasing the superior mechanical quality factor or affecting the optical properties of SiN micromembrane resonators.
    • Silvan Schmid, Tolga Bagci, Emil Zeuthen, Jacob M. Taylor, Patrick K. Herring, Maja C. Cassidy, Charles M. Marcus, Luis Guillermo Villanueva, Bartolo Amato, Anja Boisen, Yong Cheol Shin, Jing Kong, Anders S. Sørensen, Koji Usami, Eugene S. Polzik
      Journal reference: J. Appl. Phys. 115, 054513 (2014) [pdf]
      DOI: 10.1063/1.4862296

    • Coherent Operations and Screening in Multielectron Spin Qubits - Abstract
      • The performance of multi-electron spin qubits is examined by comparing exchange oscillations in coupled single-electron and multi-electron quantum dots in the same device. Fast (> 1 GHz) exchange oscillations with a quality factor Q > 15 are found for the multi-electron case, compared to Q ~ 2 for the single-electron case, the latter consistent with previous experiments. A model of dephasing that includes voltage and hyperfine noise is developed that is in good agreement with both single- and multi-electron data, though in both cases additional exchange-independent dephasing is needed to obtain quantitative agreement across a broad parameter range.
    • A. P. Higginbotham, F. Kuemmeth, M. P. Hanson, A. C. Gossard, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 112, 026801 (2014) [pdf]
      DOI: 10.1103/PhysRevLett.112.026801

    • In vivo magnetic resonance imaging of hyperpolarized silicon particles - Abstract
      • Silicon-based micro and nanoparticles have gained popularity in a wide range of biomedical applications due to their biocompatibility and biodegradability in-vivo, as well as a flexible surface chemistry, which allows drug loading, functionalization and targeting. Here we report direct in-vivo imaging of hyperpolarized 29Si nuclei in silicon microparticles by MRI. Natural physical properties of silicon provide surface electronic states for dynamic nuclear polarization (DNP), extremely long depolarization times, insensitivity to the in-vivo environment or particle tumbling, and surfaces favorable for functionalization. Potential applications to gastrointestinal, intravascular, and tumor perfusion imaging at sub-picomolar concentrations are presented. These results demonstrate a new background-free imaging modality applicable to a range of inexpensive, readily available, and biocompatible Si particles.
    • M. C. Cassidy, H. R. Chan, B. D. Ross, P. K. Bhattacharya, C. M. Marcus
      Journal reference: Nature Nanotechnology 8, 363 (2013) [pdf]
      DOI: 10.1038/nnano.2013.65

    • Synthesis of Long-T1 Silicon Nanoparticles for Hyperpolarized 29Si Magnetic Resonance Imaging - Abstract
      • We describe the synthesis, materials characterization and dynamic nuclear polarization (DNP) of amorphous and crystalline silicon nanoparticles for use as hyperpolarized magnetic resonance imaging (MRI) agents. The particles were synthesized by means of a metathesis reaction between sodium silicide (Na4Si4) and silicon tetrachloride (SiCl4) and were surface functionalized with a variety of passivating ligands. The synthesis scheme results in particles of diameter ~10 nm with long size-adjusted 29Si spin lattice relaxation (T1) times (> 600 s), which are retained after hyperpolarization by low temperature DNP.
    • Tonya M. Atkins, Maja C. Cassidy, Menyoung Lee, Shreyashi Ganguly, Charles M. Marcus, Susan M. Kauzlarich
      Journal reference: ACS Nano 7, 1609 (2013) [pdf]
      DOI: 10.1021/nn305462y

    • Quantum-Dot-Based Resonant Exchange Qubit - Abstract
      • We introduce a solid-state qubit in which exchange interactions among confined electrons provide both the static longitudinal field and the oscillatory transverse field, allowing rapid and full qubit control via rf gate-voltage pulses. We demonstrate two-axis control at a detuning sweet-spot, where leakage due to hyperfine coupling is suppressed by the large exchange gap. A {\pi}/2-gate time of 2.5 ns and a coherence time of 19 {\mu}s, using multi-pulse echo, are also demonstrated. Model calculations that include effects of hyperfine noise are in excellent quantitative agreement with experiment.
    • J. Medford, J. Beil, J. M. Taylor, E. I. Rashba, H. Lu, A. C. Gossard, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 111, 050501 (2013) [pdf]
      DOI: 10.1103/PhysRevLett.111.050501

    • Superconductor-nanowire devices from tunneling to the multichannel regime: Zero-bias oscillations and magnetoconductance crossover - Abstract
      • We present transport measurements in superconductor-nanowire devices with a gated constriction forming a quantum point contact. Zero-bias features in tunneling spectroscopy appear at finite magnetic fields, and oscillate in amplitude and split away from zero bias as a function of magnetic field and gate voltage. A crossover in magnetoconductance is observed: Magnetic fields above ~ 0.5 T enhance conductance in the low-conductance (tunneling) regime but suppress conductance in the high-conductance (multichannel) regime. We consider these results in the context of Majorana zero modes as well as alternatives, including Kondo effect and analogs of 0.7 structure in a disordered nanowire.
    • H. O. H. Churchill, V. Fatemi, K. Grove-Rasmussen, M. T. Deng, P. Caroff, H. Q. Xu, C. M. Marcus
      Journal reference: Phys. Rev. B 87, 241401(R) (2013) [pdf]
      DOI: 10.1103/PhysRevB.87.241401

    • Self-consistent measurement and state tomography of an exchange-only spin qubit - Abstract
      • We report initialization, complete electrical control, and single-shot readout of an exchange-only spin qubit. Full control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in under 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements, and non-orthogonal control axes.
    • J. Medford, J. Beil, J. M. Taylor, S. D. Bartlett, A. C. Doherty, E. I. Rashba, D. P. DiVincenzo, H. Lu, A. C. Gossard, C. M. Marcus
      Journal reference: Nature Nanotechnology 8, 654 (2013) [pdf]
      DOI: 10.1038/nnano.2013.168

  • 2012
    • Tunneling Spectroscopy of Quasiparticle Bound States in a Spinful Josephson Junction - Abstract
      • The spectrum of a segment of InAs nanowire, confined between two superconducting leads, was measured as function of gate voltage and superconducting phase difference using a third normal-metal tunnel probe. Sub-gap resonances for odd electron occupancy---interpreted as bound states involving a confined electron and a quasiparticle from the superconducting leads, reminiscent of Yu-Shiba-Rusinov states---evolve into Kondo-related resonances at higher magnetic fields. An additional zero bias peak of unknown origin is observed to coexist with the quasiparticle bound states.
    • W. Chang, V. E. Manucharyan, T. S. Jespersen, J. Nygard, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 110, 217005 (2013) [pdf]
      DOI: 10.1103/PhysRevLett.110.217005

    • g-Tensor Control in Bent Carbon Nanotube Quantum Dots - Abstract
      • We demonstrate gate control of the electronic g-tensor in single and double quantum dots formed along a bend in a carbon nanotube. From the dependence of the single-dot excitation spectrum on magnetic field magnitude and direction, we extract spin-orbit coupling, valley coupling, spin and orbital magnetic moments. Gate control of the g-tensor is measured using the splitting of the Kondo peak in conductance as a sensitive probe of Zeeman energy. In the double quantum dot regime, the magnetic field dependence of the position of cotunneling lines in the two dimensional charge stability diagram is used to infer the positions of the two dots along the nanotube.
    • R. A. Lai, H. O. H. Churchill, C. M. Marcus
      Journal reference: Physical Review B 89, 121303(R) 2014 [pdf]
      DOI: 10.1103/PhysRevB.89.121303

    • Radical-free dynamic nuclear polarization using electronic defects in silicon - Abstract
      • Direct dynamic nuclear polarization of 1H nuclei in frozen water and water-ethanol mixtures is demonstrated using silicon nanoparticles as the polarizing agent. Electron spins at dangling-bond sites near the silicon surface are identified as the source of the nuclear hyperpolarization. This novel polarization method open new avenues for the fabrication of surface engineered nanostructures to create high nuclear-spin polarized solutions without introducing contaminating radicals, and for the study of molecules adsorbed onto surfaces.
    • M. C. Cassidy, C. Ramanathan, D. G. Cory, J. W. Ager, C. M. Marcus
      Journal reference: Phys. Rev. B 87, 161306(R) (2013) [pdf]
      DOI: 10.1103/PhysRevB.87.161306

    • Scaling of Dynamical Decoupling for Spin Qubits - Abstract
      • We investigate scaling of coherence time, T2, with the number of {\pi}-pulses, n_{\pi}, in a singlet- triplet spin qubit using Carr-Purcell-Meiboom-Gill (CPMG) and concatenated dynamical decoupling (CDD) pulse sequences. For an even numbers of CPMG pulses, we find a power law, T2 = (n_{\pi})^({\gamma}_e), with {\gamma}_e = 0.72\pm0.01, essentially independent of the envelope function used to extract T2. From this surprisingly robust value, a power-law model of the noise spectrum of the environment, S({\omega}) ~ {\omega}^(-{\beta}), yields {\beta} = {\gamma}_e/(1 - {\gamma}_e) = 2.6 \pm 0.1. Model values for T2(n_{\pi}) using {\beta} = 2.6 for CPMG with both even and odd n_{\pi} up to 32 and CDD orders 3 through 6 compare very well with experiment.
    • J. Medford, L. Cywinski, C. Barthel, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 108, 086802 (2012) [pdf]
      DOI: 10.1103/PhysRevLett.108.086802

    • Coulomb Oscillations in Antidots in the Integer and Fractional Quantum Hall Regimes - Abstract
      • We report measurements of resistance oscillations in micron-scale antidots in both the integer and fractional quantum Hall regimes. In the integer regime, we conclude that oscillations are of the Coulomb type from the scaling of magnetic field period with the number of edges bound to the antidot. Based on both gate-voltage and field periods, we find at filling factor {\nu} = 2 a tunneling charge of e and two charged edges. Generalizing this picture to the fractional regime, we find (again, based on field and gate-voltage periods) at {\nu} = 2/3 a tunneling charge of (2/3)e and a single charged edge.
    • A. Kou, C. M. Marcus, L. N. Pfeiffer, K. W. West
      Journal reference: PRL 108, 256803 (2012) [pdf]
      DOI: 10.1103/PhysRevLett.108.256803

  • 2011
    • Fabry-Perot Interferometry with Fractional Charges - Abstract
      • Resistance oscillations in electronic Fabry-Perot interferometers near fractional quantum Hall (FQH) filling factors 1/3, 2/3, 4/3 and 5/3 in the constrictions are compared to corresponding oscillations near integer quantum Hall (IQH) filling factors in the constrictions, appearing in the same devices and at the same gate voltages. Two-dimensional plots of resistance versus gate voltage and magnetic field indicate that all oscillations are Coulomb dominated. Applying a Coulomb charging model yields an effective tunneling charge e* \approx e/3 for all FQH constrictions and e* \approx e for IQH constrictions. Surprisingly, we find a common characteristic temperature for FQH oscillations and a different common characteristic temperature for IQH oscillations.
    • D. T. McClure, W. Chang, C. M. Marcus, L. N. Pfeiffer, K. W. West
      Journal reference: PRL 108, 256804 (2012) [pdf]
      DOI: 10.1103/PhysRevLett.108.256804

    • Magnetic field dependence of Pauli spin blockade: A window into the sources of spin relaxation in silicon quantum dots - Abstract
      • We investigate spin relaxation in a silicon double quantum dot via leakage current through Pauli blockade as a function of interdot detuning and magnetic field. A dip in leakage current as a function of magnetic field on a \sim 40 mT field scale is attributed to spin-orbit mediated spin relaxation. On a larger (\sim 400 mT) field scale, a peak in leakage current is seen in some, but not all, Pauli-blocked transitions, and is attributed to spin-flip cotunneling. Both dip and peak structure show good agreement between theory and experiment.
    • G. Yamahata, T. Kodera, H. O. H. Churchill, K. Uchida, C. M. Marcus, S. Oda
      Journal reference: Phys Rev. B 86, 115322 (2012) [pdf]
      DOI: 10.1103/PhysRevB.86.115322

    • Hole spin relaxation in Ge–Si core–shell nanowire qubits - Abstract
      • Controlling decoherence is the most challenging task in realizing quantum information hardware. Single electron spins in gallium arsenide are a leading candidate among solid- state implementations, however strong coupling to nuclear spins in the substrate hinders this approach. To realize spin qubits in a nuclear-spin-free system, intensive studies based on group-IV semiconductor are being pursued. In this case, the challenge is primarily control of materials and interfaces, and device nanofabrication. We report important steps toward implementing spin qubits in a predominantly nuclear-spin-free system by demonstrating state preparation, pulsed gate control, and charge-sensing spin readout of confined hole spins in a one-dimensional Ge/Si nanowire. With fast gating, we measure T1 spin relaxation times in coupled quantum dots approaching 1 ms, increasing with lower magnetic field, consistent with a spin-orbit mechanism that is usually masked by hyperfine contributions.
    • Yongjie Hu, Ferdinand Kuemmeth, Charles M. Lieber, Charles M. Marcus
      Journal reference: Nature Nanotechnology 7, 47--50 (2012) [pdf]
      DOI: 10.1038/nnano.2011.234

    • Hot Carrier Transport and Photocurrent Response in Graphene - Abstract
      • Strong electron-electron interactions in graphene are expected to result in multiple-excitation generation by the absorption of a single photon. We show that the impact of carrier multiplication on photocurrent response is enhanced by very inefficient electron cooling, resulting in an abundance of hot carriers. The hot-carrier-mediated energy transport dominates the photoresponse and manifests itself in quantum efficiencies that can exceed unity, as well as in a characteristic dependence of the photocurrent on gate voltages. The pattern of multiple photocurrent sign changes as a function of gate voltage provides a fingerprint of hot-carrier-dominated transport and carrier multiplication.
    • Justin C. W. Song, Mark S. Rudner, Charles M. Marcus, Leonid S. Levitov
      Journal reference: Nano Lett., 11 (11), pp 4688-4692 (2011) [pdf]
      DOI: 10.1021/nl202318u

    • Gate-Activated Photoresponse in a Graphene p–n Junction - Abstract
      • We study photodetection in graphene near a local electrostatic gate, which enables active control of the potential landscape and carrier polarity. We find that a strong photoresponse only appears when and where a p-n junction is formed, allowing on-off control of photodetection. Photocurrents generated near p-n junctions do not require biasing and can be realized using submicron gates. Locally modulated photoresponse enables a new range of applications for graphene-based photodetectors including, for example, pixilated infrared imaging with control of response on subwavelength dimensions.
    • M. C. Lemme, F. H. L. Koppens, A. L. Falk, M. S. Rudner, H. Park, L. S. Levitov, C. M. Marcus
      Journal reference: Nano Letters 11, 4134 (2011) [pdf]
      DOI: 10.1021/nl2019068

    • Relaxation and readout visibility of a singlet-triplet qubit in an Overhauser field gradient - Abstract
      • Using single-shot charge detection in a GaAs double quantum dot, we investigate spin relaxation time T_1 and readout visibility of a two-electron singlet-triplet qubit following single-electron dynamic nuclear polarization (DNP). For magnetic fields up to 2 T, the DNP cycle is in all cases found to increase Overhauser field gradients, which in turn decrease T_1 and consequently reduce readout visibility. This effect was previously attributed to a suppression of singlet-triplet dephasing under a similar DNP cycle. A model describing relaxation after singlet-triplet mixing agrees well with experiment. Effects of pulse bandwidth on visibility are also investigated.
    • C. Barthel, J. Medford, H. Bluhm, A. Yacoby, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. B 85, 035306 (2012) [pdf]
      DOI: 10.1103/PhysRevB.85.035306

    • Laser cooling and optical detection of excitations in a LC electrical circuit - Abstract
      • We explore a method for laser cooling and optical detection of excitations in a LC electrical circuit. Our approach uses a nanomechanical oscillator as a transducer between optical and electronic excitations. An experimentally feasible system with the oscillator capacitively coupled to the LC and at the same time interacting with light via an optomechanical force is shown to provide strong electro-mechanical coupling. Conditions for improved sensitivity and quantum limited readout of electrical signals with such an "optical loud speaker" are outlined.
    • J. M. Taylor, A. S. Sørensen, C. M. Marcus, E. S. Polzik
      Journal reference: Phys. Rev. Lett. 107, 273601 (2011) [pdf]
      DOI: 10.1103/PhysRevLett.107.273601

    • Direct graphene growth on insulator - Abstract
      • Fabrication of graphene devices is often hindered by incompatibility between the silicon technology and the methods of graphene growth. Exfoliation from graphite yields excellent films but is good mainly for research. Graphene grown on metal has a technological potential but requires mechanical transfer. Growth by SiC decomposition requires a temperature budget exceeding the technological limits. These issues could be circumvented by growing graphene directly on insulator, implying Van der Waals growth. During growth, the insulator acts as a support defining the growth plane. In the device, it insulates graphene from the Si substrate. We demonstrate planar growth of graphene on mica surface. This was achieved by molecular beam deposition above 600{\deg}C. High resolution Raman scans illustrate the effect of growth parameters and substrate topography on the film perfection. Ab initio calculations suggest a growth model. Data analysis highlights the competition between nucleation at surface steps and flat surface. As a proof of concept, we show the evidence of electric field effect in a transistor with a directly grown channel.
    • Gunther Lippert, Jarek Dabrowski, Max C. Lemme, Charles M. Marcus, Olaf Seifarth, Grzegorz Lupina
      Journal reference: Phys. Status Solidi B, 248, No. 11, 2619--2622 (2011) [pdf]
      DOI: 10.1002/pssb.201100052

    • Decay of nuclear hyperpolarization in silicon microparticles - Abstract
      • We investigate the low-field relaxation of nuclear hyperpolarization in undoped and highly doped silicon microparticles at room temperature following removal from high field. For nominally undoped particles, two relaxation time scales are identified for ambient fields above 0.2 mT. The slower, T_1s, is roughly independent of ambient field; the faster, T_1f, decreases with increasing ambient field. A model in which nuclear spin relaxation occurs at the particle surface via a two-electron mechanism is shown to be in good agreement with the experimental data, particularly the field-independence of T_1s. For boron-doped particles, a single relaxation time scale is observed. This suggests that for doped particles, mobile carriers and bulk ionized acceptor sites, rather than paramagnetic surface states, are the dominant relaxation mechanisms. Relaxation times for the undoped particles are not affected by tumbling in a liquid solution.
    • M. Lee, M. C. Cassidy, C. Ramanathan, C. M. Marcus
      Journal reference: Phys. Rev. B 84 035304 (2011) [pdf]
      DOI: 10.1103/PhysRevB.84.035304

    • Charge-State Conditional Operation of a Spin Qubit - Abstract
      • We report coherent operation of a singlet-triplet qubit controlled by the arrangement of two electrons in an adjacent double quantum dot. The system we investigate consists of two pairs of capacitively coupled double quantum dots fabricated by electrostatic gates on the surface of a GaAs heterostructure. We extract the strength of the capacitive coupling between qubit and double quantum dot and show that the present geometry allows fast conditional gate operation, opening pathways to multi-qubit control and implementation of quantum algorithms with spin qubits.
    • I. van Weperen, B. D. Armstrong, E. A. Laird, J. Medford, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 107 030506 (2011) [pdf]
      DOI: 10.1103/PhysRevLett.107.030506

  • 2010
    • Gate-controlled guiding of electrons in graphene - Abstract
      • Ballistic semiconductor structures have allowed the realization of optics-like phenomena in electronics, including magnetic focusing and lensing. An extension that appears unique to graphene is to use both n and p carrier types to create electronic analogs of optical devices having both positive and negative indices of refraction. Here, we use gate-controlled density with both p and n carrier types to demonstrate the analog of the fiber-optic guiding in graphene. Two basic effects are investigated: (1) bipolar p-n junction guiding, based on the principle of angle-selective transmission though the graphene p-n interface, and (2) unipolar fiber-optic guiding, using total internal reflection controlled by carrier density. Modulation of guiding efficiency through gating is demonstrated and compared to numerical simulations, which indicates that interface roughness limits guiding performance, with few-nanometer effective roughness extracted. The development of p-n and fiber-optic guiding in graphene may lead to electrically reconfigurable wiring in high-mobility devices.
    • J. R. Williams, Tony Low, M. S. Lundstrom, C. M. Marcus
      Journal reference: Nature Nanotechnology 6, 222-225 (2011) [pdf]
      DOI: 10.1038/nnano.2011.3

    • Interlaced Dynamical Decoupling and Coherent Operation of a Singlet-Triplet Qubit - Abstract
      • We experimentally demonstrate coherence recovery of singlet-triplet superpositions by interlacing qubit rotations between Carr-Purcell (CP) echo sequences. We then compare performance of Hahn, CP, concatenated dynamical decoupling (CDD) and Uhrig dynamical decoupling (UDD) for singlet recovery. In the present case, where gate noise and drift combined with spatially varying hyperfine coupling contribute significantly to dephasing, and pulses have limited bandwidth, CP and CDD yield comparable results, with T2 ~ 80 microseconds.
    • C. Barthel, J. Medford, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 105 266808 (2010) [pdf]
      DOI: 10.1103/PhysRevLett.105.266808

    • Snake States in Graphene p-n Junctions - Abstract
      • We investigate transport in locally-gated graphene devices, where carriers are injected and collected along, rather than across, the gate edge. Tuning densities into the p-n regime significantly reduces resistance along the p-n interface, while resistance across the interface increases. This provides an experimental signature of snake states, which zig-zag along the p-n interface and remain stable as applied perpendicular magnetic field approaches zero. Snake states appear as a peak in transverse resistance measured along the p-n interface. The generic role of snake states disordered graphene is also discussed.
    • J. R. Williams, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 107, 046602 (2011) [pdf]
      DOI: 10.1103/PhysRevLett.107.046602

    • Gate-Defined Graphene Quantum Point Contact in the Quantum Hall Regime - Abstract
      • We investigate transport in a gate-defined graphene quantum point contact in the quantum Hall regime. Edge states confined to the interface of p and n regions in the graphene sheet are controllably brought together from opposite sides of the sample and allowed to mix in this split-gate geometry. Among the expected quantum Hall features, an unexpected additional plateau at 0.5 h/e^2 is observed. We propose that chaotic mixing of edge channels gives rise to the extra plateau.
    • S. Nakaharai, J. R. Williams, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 107 036602 (2011) [pdf]
      DOI: 10.1103/PhysRevLett.107.036602

    • Dynamic Nuclear Polarization in Double Quantum Dots - Abstract
      • We theoretically investigate the controlled dynamic polarization of lattice nuclear spins in GaAs double quantum dots containing two electrons. Three regimes of long-term dynamics are identified, including the build up of a large difference in the Overhauser fields across the dots, the saturation of the nuclear polarization process associated with formation of so-called "dark states," and the elimination of the difference field. We show that in the case of unequal dots, build up of difference fields generally accompanies the nuclear polarization process, whereas for nearly identical dots, build up of difference fields competes with polarization saturation in dark states. The elimination of the difference field does not, in general, correspond to a stable steady state of the polarization process.
    • M. Gullans, J. J. Krich, J. M. Taylor, H. Bluhm, B. I. Halperin, C. M. Marcus, M. Stopa, A. Yacoby, M. D. Lukin
      Journal reference: Phys. Rev. Lett. 104, 226807 (2010) [pdf]
      DOI: 10.1103/PhysRevLett.104.226807

    • Bends in nanotubes allow electric spin control and coupling - Abstract
      • We investigate combined effects of spin-orbit coupling and magnetic field in carbon nanotubes containing one or more bends along their length. We show how bends can be used to provide electrical control of confined spins, while spins confined in straight segments remain insensitive to electric fields. Device geometries that allow general rotation of single spins are presented and analyzed. In addition, capacitive coupling along bends provides coherent spin-spin interaction, including between otherwise disconnected nanotubes, completing a universal set of one- and two-qubit gates.
    • K. Flensberg, C. M. Marcus
      Journal reference: Phys. Rev. B 81, 195418 (2010) [pdf]
      DOI: 10.1103/PhysRevB.81.195418

    • Coherent spin manipulation in an exchange-only qubit - Abstract
      • Initialization, manipulation, and measurement of a three-spin qubit are demonstrated using a few-electron triple quantum dot, where all operations can be driven by tuning the nearest-neighbor exchange interaction. Multiplexed reflectometry, applied to two nearby charge sensors, allows for qubit readout. Decoherence is found to be consistent with predictions based on gate voltage noise with a uniform power spectrum. The theory of the exchange-only qubit is developed and it is shown that initialization of only two spins suffices for operation. Requirements for full multi-qubit control using only exchange and electrostatic interactions are outlined.
    • E. A. Laird, J. M. Taylor, D. P. DiVincenzo, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. B 82 075403 (2010) [pdf]
      DOI: 10.1103/PhysRevB.82.075403

    • Dynamic Nuclear Polarization in the Fractional Quantum Hall Regime - Abstract
      • We investigate dynamic nuclear polarization in quantum point contacts (QPCs) in the integer and fractional quantum Hall regimes. Following the application of a dc bias, fractional plateaus in the QPC shift symmetrically about half filling of the lowest Landau level, \nu=1/2, suggesting an interpretation in terms of composite fermions. Polarizing and detecting at different filling factors indicates that Zeeman energy is reduced by the induced nuclear polarization. Mapping effects from integer to fractional regimes extends the composite fermion picture to include hyperfine coupling.
    • A. Kou, D. T. McClure, C. M. Marcus, L. N. Pfeiffer, K. W. West
      Journal reference: Phys.Rev.Lett.105:056804,2010 [pdf]
      DOI: 10.1103/PhysRevLett.105.056804

    • Fast sensing of double-dot charge arrangement and spin state with a radio-frequency sensor quantum dot - Abstract
      • Single-shot measurement of the charge arrangement and spin state of a double quantum dot are reported, with measurement times down to ~ 100 ns. Sensing uses radio-frequency reflectometry of a proximal quantum dot in the Coulomb blockade regime. The sensor quantum dot is up to 30 times more sensitive than a comparable quantum point contact sensor, and yields three times greater signal to noise in rf single-shot measurements. Numerical modeling is qualitatively consistent with experiment and shows that the improved sensitivity of the sensor quantum dot results from reduced screening and lifetime broadening.
    • C. Barthel, M. Kjaergaard, J. Medford, M. Stopa, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Physical Review B 81 161308(R), 2010 [pdf]
      DOI: 10.1103/PhysRevB.81.161308

  • 2009
    • Carbon nanotubes for coherent spintronics - Abstract
      • Carbon nanotubes bridge the molecular and crystalline quantum worlds, and their extraordinary electronic, mechanical and optical properties have attracted enormous attention from a broad scientific community. We review the basic principles of fabricating spin-electronic devices based on individual, electrically-gated carbon nanotubes, and present experimental efforts to understand their electronic and nuclear spin degrees of freedom, which in the future may enable quantum applications.
    • F. Kuemmeth, H. O. H. Churchill, P. K. Herring, C. M. Marcus
      Journal reference: Materials Today 13, 18 (2010) [pdf]
      DOI: 10.1016/S1369-7021(10)70030-4

    • Rapid Single-Shot Measurement of a Singlet-Triplet Qubit - Abstract
      • We report repeated single-shot measurements of the two-electron spin state in a GaAs double quantum dot. The readout scheme allows measurement with fidelity above 90% with a 7 microsecond cycle time. Hyperfine-induced precession between singlet and triplet states of the two-electron system are directly observed, as nuclear Overhauser fields are quasi-static on the time scale of the measurement cycle. Repeated measurements on millisecond to second time scales reveal evolution of the nuclear environment.
    • C. Barthel, D. J. Reilly, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 103 160503 (2009) [pdf]
      DOI: 10.1103/PhysRevLett.103.160503

    • Precision cutting and patterning of graphene with helium ions - Abstract
      • We report nanoscale patterning of graphene using a helium ion microscope configured for lithography. Helium ion lithography is a direct-write lithography process, comparable to conventional focused ion beam patterning, with no resist or other material contacting the sample surface. In the present application, graphene samples on Si/SiO2 substrates are cut using helium ions, with computer controlled alignment, patterning, and exposure. Once suitable beam doses are determined, sharp edge profiles and clean etching are obtained, with little evident damage or doping to the sample. This technique provides fast lithography compatible with graphene, with ~15 nm feature sizes.
    • D. C. Bell, M. C. Lemme, L. A. Stern, J. R. Williams, C. M. Marcus
      Journal reference: Nanotechnology 20, 455301 (2009) [pdf]
      DOI: 10.1088/0957-4484/20/45/455301

    • Etching of Graphene Devices with a Helium Ion Beam - Abstract
      • We report on the etching of graphene devices with a helium ion beam, including in situ electrical measurement during lithography. The etching process can be used to nanostructure and electrically isolate different regions in a graphene device, as demonstrated by etching a channel in a suspended graphene device with etched gaps down to about 10 nm. Graphene devices on silicon dioxide (SiO2) substrates etch with lower He ion doses and are found to have a residual conductivity after etching, which we attribute to contamination by hydrocarbons.
    • M. C. Lemme, D. C. Bell, J. R. Williams, L. A. Stern, B. W. H. Baugher, P. Jarillo-Herrero, C. M. Marcus
      Journal reference: ACSNano 3, 2674 (2009) [pdf]
      DOI: 10.1021/nn900744z

    • Edge-State Velocity and Coherence in a Quantum Hall Fabry-Pérot Interferometer - Abstract
      • We investigate nonlinear transport in electronic Fabry-Perot interferometers in the integer quantum Hall regime. For interferometers sufficiently large that Coulomb blockade effects are absent, a checkerboard-like pattern of conductance oscillations as a function of dc bias and perpendicular magnetic field is observed. Edge-state velocities extracted from the checkerboard data are compared to model calculations and found to be consistent with a crossover from skipping orbits at low fields to E x B drift at high fields. Suppression of visibility as a function of bias and magnetic field is accounted for by including energy- and field-dependent dephasing of edge electrons.
    • D. T. McClure, Yiming Zhang, B. Rosenow, E. M. Levenson-Falk, C. M. Marcus, L. N. Pfeiffer, K. W. West
      Journal reference: Phys. Rev. Lett. 103 206806 (2009). [pdf]
      DOI: 10.1103/PhysRevLett.103.206806

    • Silicon Nanoparticles as Hyperpolarized Magnetic Resonance Imaging Agents - Abstract
      • Silicon nanoparticles are experimentally investigated as a potential hyperpolarized, targetable MRI imaging agent. Nuclear T_1 times at room temperature for a variety of Si nanoparticles are found to be remarkably long (10^2 to 10^4 s) - roughly consistent with predictions of a core-shell diffusion model - allowing them to be transported, administered and imaged on practical time scales without significant loss of polarization. We also report surface functionalization of Si nanoparticles, comparable to approaches used in other biologically targeted nanoparticle systems.
    • J. W. Aptekar, M. C. Cassidy, A. C. Johnson, R. A. Barton, M. Y. Lee, A. C. Ogier, C. Vo, M. N. Anahtar, Y. Ren, S. N. Bhatia, C. Ramanathan, D. G. Cory, A. L. Hill, R. W. Mair, M. S. Rosen, R. L. Walsworth, C. M. Marcus
      Journal reference: ACS Nano 3, 4003 (2009) [pdf]
      DOI: 10.1021/nn900996p

  • 2008
    • Distinct signatures for Coulomb blockade and Aharonov-Bohm interference in electronic Fabry-Perot interferometers - Abstract
      • Two distinct types of magnetoresistance oscillations are observed in two electronic Fabry-Perot interferometers of different sizes in the integer quantum Hall regime. Measuring these oscillations as a function of magnetic field and gate voltages, we observe three signatures that distinguish the two types. The oscillations observed in a 2.0 square micron device are understood to arise from the Coulomb blockade mechanism, and those observed in an 18 square micron device from the Aharonov-Bohm mechanism. This work clarifies, provides ways to distinguish, and demonstrates control over, these distinct physical origins of resistance oscillations seen in electronic Fabry-Perot interferometers.
    • Yiming Zhang, D. T. McClure, E. M. Levenson-Falk, C. M. Marcus, L. N. Pfeiffer, K. W. West
      Journal reference: Phys. Rev. B (RC) 79, 241304 (2009) [pdf]
      DOI: 10.1103/PhysRevB.79.241304

    • A new mechanism of electric dipole spin resonance: hyperfine coupling in quantum dots - Abstract
      • A recently discovered mechanism of electric dipole spin resonance, mediated by the hyperfine interaction, is investigated experimentally and theoretically. The effect is studied using a spin-selective transition in a GaAs double quantum dot. The resonant frequency is sensitive to the instantaneous hyperfine effective field, revealing a nuclear polarization created by driving the resonance. A device incorporating a micromagnet exhibits a magnetic field difference between dots, allowing electrons in either dot to be addressed selectively. An unexplained additional signal at half the resonant frequency is presented.
    • E. A. Laird, C. Barthel, E. I. Rashba, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Semicond. Sci. Technol. 24 (2009) 064004. [pdf]
      DOI: 10.1088/0268-1242/24/6/064004

    • Electron–nuclear interaction in 13C nanotube double quantum dots - Abstract
      • For coherent electron spins, hyperfine coupling to nuclei in the host material can either be a dominant source of unwanted spin decoherence or, if controlled effectively, a resource allowing storage and retrieval of quantum information. To investigate the effect of a controllable nuclear environment on the evolution of confined electron spins, we have fabricated and measured gate-defined double quantum dots with integrated charge sensors made from single-walled carbon nanotubes with a variable concentration of 13C (nuclear spin I=1/2) among the majority zero-nuclear-spin 12C atoms. Spin-sensitive transport in double-dot devices grown using methane with the natural abundance (~ 1%) of 13C is compared with similar devices grown using an enhanced (~99%) concentration of 13C. We observe strong isotope effects in spin-blockaded transport, and from the dependence on external magnetic field, estimate the hyperfine coupling in 13C nanotubes to be on the order of 100 micro-eV, two orders of magnitude larger than anticipated theoretically. 13C-enhanced nanotubes are an interesting new system for spin-based quantum information processing and memory, with nuclei that are strongly coupled to gate-controlled electrons, differ from nuclei in the substrate, are naturally confined to one dimension, lack quadrupolar coupling, and have a readily controllable concentration from less than one to 10^5 per electron.
    • H. O. H. Churchill, A. J. Bestwick, J. W. Harlow, F. Kuemmeth, D. Marcos, C. H. Stwertka, S. K. Watson, C. M. Marcus
      Journal reference: Nature Physics 5, 321 (2009) [pdf]
      DOI: 10.1038/nphys1247

    • Relaxation and Dephasing in a Two-electron 13C Nanotube Double Quantum Dot - Abstract
      • We use charge sensing of Pauli blockade (including spin and isospin) in a two-electron 13C nanotube double quantum dot to measure relaxation and dephasing times. The relaxation time, T1, first decreases with parallel magnetic field then goes through a minimum in a field of 1.4 T. We attribute both results to the spin-orbit-modified electronic spectrum of carbon nanotubes, which suppresses hyperfine mediated relaxation and enhances relaxation due to soft phonons. The inhomogeneous dephasing time, T2*, is consistent with previous data on hyperfine coupling strength in 13C nanotubes.
    • H. O. H. Churchill, F. Kuemmeth, J. W. Harlow, A. J. Bestwick, E. I. Rashba, K. Flensberg, C. H. Stwertka, T. Taychatanapat, S. K. Watson, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 102, 166802 (2009). [pdf]
      DOI: 10.1103/PhysRevLett.102.166802

    • Quantum Hall conductance of two-terminal graphene devices - Abstract
      • Measurement and theory of the two-terminal conductance of monolayer and bilayer graphene in the quantum Hall regime are compared. We examine features of conductance as a function of gate voltage that allow monolayer, bilayer, and gapped samples to be distinguished, including N-shaped distortions of quantum Hall plateaus and conductance peaks and dips at the charge neutrality point. Generally good agreement is found between measurement and theory. Possible origins of discrepancies are discussed.
    • J. R. Williams, D. A. Abanin, L. DiCarlo, L. S. Levitov, C. M. Marcus
      Journal reference: Phys. Rev. B 80, 045408 (2009) [pdf]
      DOI: 10.1103/PhysRevB.80.045408

    • Measurement of Temporal Correlations of the Overhauser Field in a Double Quantum Dot - Abstract
      • In quantum dots made from materials with nonzero nuclear spins, hyperfine coupling creates a fluctuating effective Zeeman field (Overhauser field) felt by electrons, which can be a dominant source of spin qubit decoherence. We characterize the spectral properties of the fluctuating Overhauser field in a GaAs double quantum dot by measuring correlation functions and power spectra of the rate of singlet-triplet mixing of two separated electrons. Away from zero field, spectral weight is concentrated below 10 Hz, with 1/f^2 dependence on frequency, f. This is consistent with a model of nuclear spin diffusion, and indicates that decoherence can be largely suppressed by echo techniques.
    • D. J. Reilly, J. M. Taylor, E. A. Laird, J. R. Petta, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 101, 236803 (2008). [pdf]
      DOI: 10.1103/PhysRevLett.101.236803

    • Suppressing Spin Qubit Dephasing by Nuclear State Preparation - Abstract
      • Coherent spin states in semiconductor quantum dots offer promise as electrically controllable quantum bits (qubits) with scalable fabrication. For few-electron quantum dots made from gallium arsenide (GaAs), fluctuating nuclear spins in the host lattice are the dominant source of spin decoherence. We report a method of preparing the nuclear spin environment that suppresses the relevant component of nuclear spin fluctuations below its equilibrium value by a factor of ~ 70, extending the inhomogeneous dephasing time for the two-electron spin state beyond 1 microsecond. The nuclear state can be readily prepared by electrical gate manipulation and persists for > 10 seconds.
    • D. J. Reilly, J. M. Taylor, J. R. Petta, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Science 321, 817 (2008) [pdf]
      DOI: 10.1126/science.1159221

    • Quasiparticle Tunneling in the Fractional Quantum Hall State at ν= 5/2 - Abstract
      • Theory predicts that quasiparticle tunneling between the counter-propagating edges in a fractional quantum Hall state can be used to measure the effective quasiparticle charge e* and dimensionless interaction parameter g, and thereby characterize the many-body wavefunction describing the state. We report measurements of quasiparticle tunneling in a high mobility GaAs two dimensional electron system in the fractional quantum Hall state at nu=5/2 using a gate-defined constriction to bring the edges close together. We find the dc-bias peaks in the tunneling conductance at different temperatures collapse onto a single curve when scaled, in agreement with weak tunneling theory. Various models for the \nu=5/2 state predict different values for g. Among these models, the non-abelian states with e*=1/4 and g=1/2 are most consistent with the data.
    • Iuliana P. Radu, J. B. Miller, C. M. Marcus, M. A. Kastner, L. N. Pfeiffer, K. W. West
      Journal reference: Science 320, 899 (2008). [pdf]

    • Exchange Control of Nuclear Spin Diffusion in a Double Quantum Dot - Abstract
      • Coherent two-level systems, or qubits, based on electron spins in GaAs quantum dots are strongly coupled to the nuclear spins of the host lattice via the hyperfine interaction. Realizing nuclear spin control would likely improve electron spin coherence and potentially enable the nuclear environment to be harnessed for the long-term storage of quantum information. Toward this goal, we report experimental control of the relaxation of nuclear spin polarization in a gate-defined two-electron GaAs double quantum dot. A cyclic gate-pulse sequence transfers the spin of an electron pair to the host nuclear system, establishing a local nuclear polarization that relaxes on a time scale of seconds. We find nuclear relaxation depends on magnetic field and gate-controlled two-electron exchange, consistent with a model of electron mediated nuclear spin diffusion.
    • D. J. Reilly, J. M. Taylor, J. R. Petta, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Physical Review Letters 104, 236802 (2010) [pdf]
      DOI: 10.1103/PhysRevLett.104.236802

    • Shot Noise in Graphene - Abstract
      • We report measurements of current noise in single- and multi-layer graphene devices. In four single-layer devices, including a p-n junction, the Fano factor remains constant to within +/-10% upon varying carrier type and density, and averages between 0.35 and 0.38. The Fano factor in a multi-layer device is found to decrease from a maximal value of 0.33 at the charge-neutrality point to 0.25 at high carrier density. These results are compared to theoretical predictions for shot noise in ballistic and disordered graphene.
    • L. DiCarlo, J. R. Williams, Yiming Zhang, D. T. McClure, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 100, 156801 (2008). [pdf]
      DOI: 10.1103/PhysRevLett.100.156801

  • 2007
    • Hyperfine-Mediated Gate-Driven Electron Spin Resonance - Abstract
      • An all-electrical spin resonance effect in a GaAs few-electron double quantum dot is investigated experimentally and theoretically. The magnetic field dependence and absence of associated Rabi oscillations are consistent with a novel hyperfine mechanism. The resonant frequency is sensitive to the instantaneous hyperfine effective field, and the effect can be used to detect and create sizable nuclear polarizations. A device incorporating a micromagnet exhibits a magnetic field difference between dots, allowing electrons in either dot to be addressed selectively.
    • E. A. Laird, C. Barthel, E. I. Rashba, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 99 246601 (2007) [pdf]
      DOI: 10.1103/PhysRevLett.99.246601

    • Dynamic Nuclear Polarization with Single Electron Spins - Abstract
      • We polarize nuclear spins in a GaAs double quantum dot by controlling two-electron spin states near the anti-crossing of the singlet (S) and m_S=+1 triplet (T+) using pulsed gates. An initialized S state is cyclically brought into resonance with the T+ state, where hyperfine fields drive rapid rotations between S and T+, 'flipping' an electron spin and 'flopping' a nuclear spin. The resulting Overhauser field approaches 80 mT, in agreement with a simple rate-equation model. A self-limiting pulse sequence is developed that allows the steady-state nuclear polarization to be set using a gate voltage.
    • J. R. Petta, J. M. Taylor, A. C. Johnson, A. Yacoby, M. D. Lukin, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 100, 067601 (2008). [pdf]
      DOI: 10.1103/PhysRevLett.100.067601

    • Quantum Hall Effect in a Graphene p-n Junction - Abstract
      • We report on the fabrication and transport studies of a single-layer graphene p-n junction. Carrier type and density in two adjacent regions are individually controlled by electrostatic gating using a local top gate and a global back gate. A functionalized Al203 oxide that adheres to graphene and does not significantly affect its electronic properties is described. Measurements in the quantum Hall regime reveal new plateaus of two-terminal conductance across the junction at 1 and 3/2 times the quantum of conductance, e2/h, consistent with theory.
    • J. R. Williams, L. DiCarlo, C. M. Marcus
      Journal reference: Science 317, 638 (2007) [pdf]

    • Fast single-charge sensing with a rf quantum point contact - Abstract
      • We report high-bandwidth charge sensing measurements using a GaAs quantum point contact embedded in a radio frequency impedance matching circuit (rf-QPC). With the rf-QPC biased near pinch-off where it is most sensitive to charge, we demonstrate a conductance sensitivity of 5x10^(-6) e^(2)/h Hz^(-1/2) with a bandwidth of 8 MHz. Single-shot readout of a proximal few-electron double quantum dot is investigated in a mode where the rf-QPC back-action is rapidly switched.
    • D. J. Reilly, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Appl. Phys. Lett. 91, 162101 (2007) [pdf]
      DOI: 10.1063/1.2794995

    • A Ge/Si heterostructure nanowire-based double quantum dot with integrated charge sensor - Abstract
      • Coupled electron spins in semiconductor double quantum dots hold promise as the basis for solid-state qubits. To date, most experiments have used III-V materials, in which coherence is limited by hyperfine interactions. Ge/Si heterostructure nanowires seem ideally suited to overcome this limitation: the predominance of spin-zero nuclei suppresses the hyperfine interaction and chemical synthesis creates a clean and defect-free system with highly controllable properties. Here we present a top gate-defined double quantum dot based on Ge/Si heterostructure nanowires with fully tunable coupling between the dots and to the leads. We also demonstrate a novel approach to charge sensing in a one-dimensional nanostructure by capacitively coupling the double dot to a single dot on an adjacent nanowire. The double quantum dot and integrated charge sensor serve as an essential building block required to form a solid-state spin qubit free of nuclear spin.
    • Yongjie Hu, Hugh H. O. Churchill, David J. Reilly, Jie Xiang, Charles M. Lieber, Charles M. Marcus
      Journal reference: Nature Nanotechnology 2, 622 (2007). [pdf]
      DOI: 10.1038/nnano.2007.302

    • Fractional quantum Hall effect in a quantum point contact at filling fraction 5/2 - Abstract
      • Recent theories suggest that the excitations of certain quantum Hall states may have exotic braiding statistics which could be used to build topological quantum gates. This has prompted an experimental push to study such states using confined geometries where the statistics can be tested. We study the transport properties of quantum point contacts (QPCs) fabricated on a GaAs/AlGaAs two dimensional electron gas that exhibits well-developed fractional quantum Hall effect, including at bulk filling fraction 5/2. We find that a plateau at effective QPC filling factor 5/2 is identifiable in point contacts with lithographic widths of 1.2 microns and 0.8 microns, but not 0.5 microns. We study the temperature and dc-current-bias dependence of the 5/2 plateau in the QPC, as well as neighboring fractional and integer plateaus in the QPC while keeping the bulk at filling factor 3. Transport near QPC filling factor 5/2 is consistent with a picture of chiral Luttinger liquid edge-states with inter-edge tunneling, suggesting that an incompressible state at 5/2 forms in this confined geometry.
    • J. B. Miller, I. P. Radu, D. M. Zumbuhl, E. M. Levenson-Falk, M. A. Kastner, C. M. Marcus, L. N. Pfeiffer, K. W. West
      Journal reference: Nature Physics 3, 561 (2007). [ cond-mat/0703161v2 ]
      DOI: 10.1038/nphys658

    • Current Noise in Quantum Point Contacts - Abstract
      • We present measurements of current noise in quantum point contacts as a function of source-drain bias, gate voltage, and in-plane magnetic field. At zero bias, Johnson noise provides a measure of the electron temperature. At finite bias, shot noise at zero field exhibits an asymmetry related to the 0.7 structure in conductance. The asymmetry in noise evolves smoothly into the symmetric signature of spin-resolved electron transmission at high field. Comparison to a phenomenological model with density-dependent level splitting yields quantitative agreement. Additionally, a device-specific contribution to the finite-bias noise, particularly visible on conductance plateaus (where shot noise vanishes), agrees quantitatively with a model of bias-dependent electron heating.
    • L. DiCarlo, Yiming Zhang, D. T. McClure, D. J. Reilly, C. M. Marcus, L. N. Pfeiffer, K. W. West, M. P. Hanson, A. C. Gossard
      0704.3892v1 [pdf]

    • Noise Correlations in a Coulomb-Blockaded Quantum Dot - Abstract
      • We report measurements of current noise auto- and cross-correlation in a tunable quantum dot with two or three leads. As the Coulomb blockade is lifted at finite source-drain bias, the auto-correlation evolves from super-Poissonian to sub-Poissonian in the two-lead case, and the cross-correlation evolves from positive to negative in the three-lead case, consistent with transport through multiple levels. Cross-correlations in the three-lead dot are found to be proportional to the noise in excess of the Poissonian value in the limit of weak output tunneling.
    • Y. Zhang, L. DiCarlo, D. T. McClure, M. Yamamoto, S. Tarucha, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 99, 036603 (2007). [ cond-mat/0703419v1 ]
      DOI: 10.1103/PhysRevLett.99.036603

    • Relaxation, dephasing, and quantum control of electron spins in double quantum dots - Abstract
      • Recent experiments have demonstrated quantum manipulation of two-electron spin states in double quantum dots using electrically controlled exchange interactions. Here, we present a detailed theory for electron spin dynamics in two-electron double dot systems that was used to guide these experiments and analyze experimental results. The theory treats both charge and spin degrees of freedom on an equal basis. Specifically, we analyze the relaxation and dephasing mechanisms that are relevant to experiments and discuss practical approaches for quantum control of two-electron system. We show that both charge and spin dephasing play important roles in the dynamics of the two-spin system, but neither represents a fundamental limit for electrical control of spin degrees of freedom in semiconductor quantum bits.
    • J. M. Taylor, J. R. Petta, A. C. Johnson, A. Yacoby, C. M. Marcus, M. D. Lukin
      Journal reference: Phys. Rev. B 76, 035315 (2007) [ cond-mat/0602470v2 ]
      DOI: 10.1103/PhysRevB.76.035315

  • 2006
    • Tunable Noise Cross Correlations in a Double Quantum Dot - Abstract
      • We report measurements of the cross-correlation between current noise fluctuations in two capacitively coupled quantum dots in the Coulomb blockade regime. The sign of the cross-spectral density is found to be tunable by gate voltage and source-drain bias. Good agreement is found with a model of sequential tunneling through the dots in the presence of inter-dot capacitive coupling.
    • D. T. McClure, L. DiCarlo, Y. Zhang, H. -A. Engel, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 98, 056801 (2007) [ cond-mat/0607280v1 ]
      DOI: 10.1103/PhysRevLett.98.056801

    • Effect of Exchange Interaction on Spin Dephasing in a Double Quantum Dot - Abstract
      • We measure singlet-triplet dephasing in a two-electron double quantum dot in the presence of an exchange interaction which can be electrically tuned from much smaller to much larger than the hyperfine energy. Saturation of dephasing and damped oscillations of the spin correlator as a function of time are observed when the two interaction strengths are comparable. Both features of the data are compared with predictions from a quasistatic model of the hyperfine field.
    • E. A. Laird, J. R. Petta, A. C. Johnson, C. M. Marcus, A. Yacoby, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 97, 056801 (2006). [ cond-mat/0512077v3 ]
      DOI: 10.1103/PhysRevLett.97.056801

    • System for measuring auto- and cross correlation of current noise at low temperatures - Abstract
      • We describe the construction and operation of a two-channel noise detection system for measuring power and cross spectral densities of current fluctuations near 2 MHz in electronic devices at low temperatures. The system employs cryogenic amplification and fast-Fourier-transform based spectral measurement. The gain and electron temperature are calibrated using Johnson noise thermometry. Full shot noise of 100 pA can be resolved with an integration time of 10 s. We report a demonstration measurement of bias-dependent current noise in a gate defined GaAs/AlGaAs quantum point contact.
    • L. DiCarlo, Y. Zhang, D. T. McClure, C. M. Marcus, L. N. Pfeiffer, K. W. West
      Journal reference: Rev. Sci. Instru. 77, 073906 (2006). [ cond-mat/0604018v2 ]
      DOI: 10.1063/1.2221541

    • Shot-Noise Signatures of 0.7 Structure and Spin in a Quantum Point Contact - Abstract
      • We report simultaneous measurement of shot noise and dc transport in a quantum point contact as a function of source-drain bias, gate voltage, and in-plane magnetic field. Shot noise at zero field exhibits an asymmetry related to the 0.7 structure in conductance. The asymmetry in noise evolves smoothly into the symmetric signature of spin-resolved electron transmission at high field. Comparison to a phenomenological model with density-dependent level splitting yields good quantitative agreement.
    • L. DiCarlo, Y. Zhang, D. T. McClure, D. J. Reilly, C. M. Marcus, L. N. Pfeiffer, K. W. West
      Journal reference: Phys. Rev. Lett. 97, 036810 (2006) [ cond-mat/0604019v2 ]
      DOI: 10.1103/PhysRevLett.97.036810

    • Magnetic Field Control of Exchange and Noise Immunity in Double Quantum Dots - Abstract
      • We employ density functional calculated eigenstates as a basis for exact diagonalization studies of semiconductor double quantum dots, with two electrons, through the transition from the symmetric bias regime to the regime where both electrons occupy the same dot. We calculate the singlet-triplet splitting $J(\epsilon)$ as a function of bias detuning $\epsilon$ and explain its functional shape with a simple, double anti-crossing model. A voltage noise suppression "sweet spot," where $dJ(\epsilon)/d\epsilon=0$ with nonzero $J(\epsilon)$, is predicted and shown to be tunable with a magnetic field $B$.
    • M. Stopa, C. M. Marcus
      cond-mat/0604008v1 [pdf]

    • Non-equilibrium singlet–triplet Kondo effect in carbon nanotubes - Abstract
      • The Kondo-effect is a many-body phenomenon arising due to conduction electrons scattering off a localized spin. Coherent spin-flip scattering off such a quantum impurity correlates the conduction electrons and at low temperature this leads to a zero-bias conductance anomaly. This has become a common signature in bias-spectroscopy of single-electron transistors, observed in GaAs quantum dots as well as in various single-molecule transistors. While the zero-bias Kondo effect is well established it remains uncertain to what extent Kondo correlations persist in non-equilibrium situations where inelastic processes induce decoherence. Here we report on a pronounced conductance peak observed at finite bias-voltage in a carbon nanotube quantum dot in the spin singlet ground state. We explain this finite-bias conductance anomaly by a nonequilibrium Kondo-effect involving excitations into a spin triplet state. Excellent agreement between calculated and measured nonlinear conductance is obtained, thus strongly supporting the correlated nature of this nonequilibrium resonance.
    • J. Paaske, A. Rosch, P. Woelfle, N. Mason, C. M. Marcus, J. Nygard
      Journal reference: Nature Physics, vol. 2, p.460 - 464 (2006) [ cond-mat/0602581v1 ]
      DOI: 10.1038/nphys340

  • 2005
    • Charge sensing in carbon nanotube quantum dots on microsecond timescales - Abstract
      • We report fast, simultaneous charge sensing and transport measurements of gate-defined carbon nanotube quantum dots. Aluminum radio frequency single electron transistors (rf-SETs) capacitively coupled to the nanotube dot provide single-electron charge sensing on microsecond timescales. Simultaneously, rf reflectometry allows fast measurement of transport through the nanotube dot. Charge stability diagrams for the nanotube dot in the Coulomb blockade regime show extended Coulomb diamonds into the high-bias regime, as well as even-odd filling effects, revealed in charge sensing data.
    • M. J. Biercuk, D. J. Reilly, T. M. Buehler, V. C. Chan, J. M. Chow, R. G. Clark, C. M. Marcus
      DOI: 10.1103/PhysRevB.73.201402
      cond-mat/0510550v1 [pdf]

    • Charge and spin manipulation in a few-electron double dot - Abstract
      • We demonstrate high speed manipulation of a few-electron double quantum dot. In the one-electron regime, the double dot forms a charge qubit. Microwaves are used drive transitions between the (1,0) and (0,1) charge states of the double dot. A local quantum point contact charge detector measures the photon-induced change in occupancy of the charge states. Charge detection is used to measure T1~16 ns and also provides a lower bound estimate for T2* of 400 ps for the charge qubit. In the two-electron regime we use pulsed-gate techniques to measure the singlet-triplet relaxation time for nearly-degenerate spin states. These experiments demonstrate that the hyperfine interaction leads to fast spin relaxation at low magnetic fields. Finally, we discuss how two-electron spin states can be used to form a logical spin qubit.
    • J. R. Petta, A. C. Johnson, J. M. Taylor, A. Yacoby, M. D. Lukin, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Physica E 34, 42 (2006) [ cond-mat/0510151v1 ]
      DOI: 10.1016/j.physe.2006.02.033

    • Asymmetry of Nonlinear Transport and Electron Interactions in Quantum Dots - Abstract
      • The symmetry properties of transport beyond the linear regime in chaotic quantum dots are investigated experimentally. A component of differential conductance that is antisymmetric in both applied source-drain bias V and magnetic field B, absent in linear transport, is found to exhibit mesoscopic fluctuations around a zero average. Typical values of this component allow a measurement of the electron interaction strength.
    • D. M. Zumbuhl, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 96, 206802 (2006) [ cond-mat/0508766v1 ]
      DOI: 10.1103/PhysRevLett.96.206802

    • Triplet–singlet spin relaxation via nuclei in a double quantum dot - Abstract
      • The spin of a confined electron, when oriented originally in some direction, will lose memory of that orientation after some time. Physical mechanisms leading to this relaxation of spin memory typically involve either coupling of the electron spin to its orbital motion or to nuclear spins. Relaxation of confined electron spin has been previously measured only for Zeeman or exchange split spin states, where spin-orbit effects dominate relaxation, while spin flips due to nuclei have been observed in optical spectroscopy studies. Using an isolated GaAs double quantum dot defined by electrostatic gates and direct time domain measurements, we investigate in detail spin relaxation for arbitrary splitting of spin states. Results demonstrate that electron spin flips are dominated by nuclear interactions and are slowed by several orders of magnitude when a magnetic field of a few millitesla is applied. These results have significant implications for spin-based information processing.
    • A. C. Johnson, J. R. Petta, J. M. Taylor, A. Yacoby, M. D. Lukin, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Nature 435, 925 (2005) [ cond-mat/0503687v3 ]
      DOI: 10.1038/nature03815

    • Gate-Defined Quantum Dots on Carbon Nanotubes - Abstract
      • We report the realization of nanotube-based quantum dot structures that use local electrostatic gating to produce individually controllable dots in series along a nanotube. Electrostatic top-gates produce depletion regions in the underlying tube; a pair of such depletion regions defines a quantum dot. Transparencies of tunnel barriers as well as the electrostatic energies, within single and multiple dots, can be tuned by gate voltages. The approach allows accurate control over multiple devices on a single tube, and serves as a design paradigm for nanotube-based electronics and quantum systems.
    • M. J. Biercuk, S. Garaj, N. Mason, J. M. Chow, C. M. Marcus
      Journal reference: Nano Letters 5, 1267 (2005) [ cond-mat/0502634v2 ]
      DOI: 10.1021/nl050364v

    • Conductance fluctuations and partially broken spin symmetries in quantum dots - Abstract
      • Conductance fluctuations in GaAs quantum dots with spin-orbit and Zeeman coupling are investigated experimentally and compared to a random matrix theory formulation that defines a number of regimes of spin symmetry depending on experimental parameters. Accounting for orbital coupling of the in-plane magnetic field, which can break time-reversal symmetry, yields excellent overall agreement between experiment and theory.
    • D. M. Zumbuhl, J. B. Miller, D. Goldhaber-Gordon, C. M. Marcus, J. S. Harris, Jr., K. Campman, A. C. Gossard
      Journal reference: Phys. Rev. B 72, 081305 (2005) [ cond-mat/0501622v2 ]
      DOI: 10.1103/PhysRevB.72.081305

    • Solid-state circuit for spin entanglement generation and purification - Abstract
      • We show how realistic charge manipulation and measurement techniques, combined with the exchange interaction, allow for the robust generation and purification of four-particle spin entangled states in electrically controlled semiconductor quantum dots. The generated states are immunized to the dominant sources of noise via a dynamical decoherence-free subspace; all additional errors are corrected by a purification protocol. This approach may find application in quantum computation, communication, and metrology.
    • J. M. Taylor, W. Dür, P. Zoller, A. Yacoby, C. M. Marcus, M. D. Lukin
      DOI: 10.1103/PhysRevLett.94.236803
      cond-mat/0503255v2 [pdf]

  • 2004
    • Pulsed-gate measurements of the singlet-triplet relaxation time in a two-electron double quantum dot - Abstract
      • A pulsed-gate technique with charge sensing is used to measure the singlet-triplet relaxation time for nearly-degenerate spin states in a two-electron double quantum dot. Transitions from the (1,1) charge occupancy state to the (0,2) state, measured as a function of pulse cycle duration and magnetic field, allow the (1,1) singlet-triplet relaxation time (~70 microseconds) and the (0,2) singlet-triplet splitting to be measured. The use of charge sensing rather than current measurement allows long relaxation times to be readily probed.
    • J. R. Petta, A. C. Johnson, A. Yacoby, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. B (RC) 72, 161301 (2005) [ cond-mat/0412048v2 ]
      DOI: 10.1103/PhysRevB.72.161301

    • Singlet-triplet spin blockade and charge sensing in a few-electron double quantum dot - Abstract
      • Singlet-triplet spin blockade in a few-electron lateral double quantum dot is investigated using simultaneous transport and charge-sensing measurements. Transport from the (1,1) to the (0,2) electron occupancy states is strongly suppressed relative to the opposite bias [(0,2) to (1,1)]. At large bias, spin blockade ceases as the (0,2) triplet state enters the transport window, giving a direct measure of exchange splitting as a function of magnetic field. A simple model for current and steady-state charge distribution in spin-blockade conditions is developed and found to be in excellent agreement with experiment.
    • A. C. Johnson, J. R. Petta, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. B 72, 165308 (2005). [ cond-mat/0410679v1 ]
      DOI: 10.1103/PhysRevB.72.165308

    • Zero-field splitting of Kondo resonances in a carbon nanotube quantum dot - Abstract
      • We present low-temperature electron transport measurements on a single-wall carbon nanotube quantum dot exhibiting Kondo resonances at low temperature. Contrary to the usual behavior for the spin-1/2 Kondo effect we find that the temperature dependence of the zero bias conductance is nonmonotonic. In nonlinear transport measurements low-energy splittings of the Kondo resonances are observed at zero magnetic field. We suggest that these anomalies reflect interactions between the nanotube and a magnetic (catalyst) particle. The nanotube device may effectively act as a ferromagnetically contacted Kondo dot.
    • J. Nygard, W. F. Koehl, N. Mason, L. DiCarlo, C. M. Marcus
      cond-mat/0410467v2 [pdf]

    • Photovoltaic and rectification currents in quantum dots - Abstract
      • We investigate theoretically and experimentally the statistical properties of dc current through an open quantum dot subject to ac excitation of a shape-defining gate. The symmetries of rectification current and photovoltaic current with respect to applied magnetic field are examined. Theory and experiment are found to be in good agreement throughout a broad range of frequency and ac power, ranging from adiabatic to nonadiabatic regimes.
    • M. G. Vavilov, L. DiCarlo, C. M. Marcus
      Journal reference: Phys. Rev. B 71, 241309(R) (2005) [ cond-mat/0410042v1 ]
      DOI: 10.1103/PhysRevB.71.241309

    • Controlling Spin Qubits in Quantum Dots - Abstract
      • We review progress on the spintronics proposal for quantum computing where the quantum bits (qubits) are implemented with electron spins. We calculate the exchange interaction of coupled quantum dots and present experiments, where the exchange coupling is measured via transport. Then, experiments on single spins on dots are described, where long spin relaxation times, on the order of a millisecond, are observed. We consider spin-orbit interaction as sources of spin decoherence and find theoretically that also long decoherence times are expected. Further, we describe the concept of spin filtering using quantum dots and show data of successful experiments. We also show an implementation of a read out scheme for spin qubits and define how qubits can be measured with high precision. Then, we propose new experiments, where the spin decoherence time and the Rabi oscillations of single electrons can be measured via charge transport through quantum dots. Finally, all these achievements have promising applications both in conventional and quantum information processing.
    • Hans-Andreas Engel, L. P. Kouwenhoven, Daniel Loss, C. M. Marcus
      Journal reference: Quantum Information Processing 3, 115 (2004) [ cond-mat/0409294v1 ]
      DOI: 10.1007/s11128-004-3103-3

    • Anomalous Conductance Quantization in Carbon Nanotubes - Abstract
      • Conductance measurements of carbon nanotubes containing gated local depletion regions exhibit plateaus as a function of gate voltage, spaced by approximately e2/h, the quantum of conductance for a single (non-degenerate) mode. Plateau structure is investigated as a function of bias voltage, temperature, and magnetic field. We speculate on the origin of this surprising quantization, which appears to lack band and spin degeneracy.
    • M. J. Biercuk, N. Mason, J. Martin, A. Yacoby, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 94, 026801 (2005). [ cond-mat/0406652v2 ]
      DOI: 10.1103/PhysRevLett.94.026801

    • Cotunneling Spectroscopy in Few-Electron Quantum Dots - Abstract
      • Few-electron quantum dots are investigated in the regime of strong tunneling to the leads. Inelastic cotunneling is used to measure the two-electron singlet-triplet splitting above and below a magnetic field driven singlet-triplet transition. Evidence for a non-equilibrium two-electron singlet-triplet Kondo effect is presented. Cotunneling allows orbital correlations and parameters characterizing entanglement of the two-electron singlet ground state to be extracted from dc transport.
    • D. M. Zumbuhl, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 93, 256801 (2004) [ cond-mat/0408276v1 ]
      DOI: 10.1103/PhysRevLett.93.256801

    • Manipulation of a Single Charge in a Double Quantum Dot - Abstract
      • We manipulate a single electron in a fully tunable double quantum dot using microwave excitation. Under resonant conditions, microwaves drive transitions between the (1,0) and (0,1) charge states of the double dot. Local quantum point contact charge detectors enable a direct measurement of the photon-induced change in occupancy of the charge states. From charge sensing measurements, we find T1~16 ns and a lower bound estimate for T2* of 400 ps for the charge two-level system.
    • J. R. Petta, A. C. Johnson, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 93, 186802 (2004) [ cond-mat/0408139v1 ]
      DOI: 10.1103/PhysRevLett.93.186802

    • Voltage Dependence of Spin Polarized Tunneling - Abstract
      • A mesoscopic spin valve is used to determine the effective spin polarization of electrons tunneling from and into ferromagnetic transition metals at finite voltages. The tunneling spin polarization from the ferromagnet (FM) slowly decreases with bias, but drops faster and even inverts with voltage when electrons tunnel into it. A bias-dependent free electron model shows that in the former case electrons originate near the Fermi level of the FM with large polarization whereas in the latter, electrons tunnel into hot electron states for which the polarization is significantly reduced. The change in sign is ascribed to the detailed matching of the electron wave function through the tunnel barrier.
    • S. O. Valenzuela, D. J. Monsma, C. M. Marcus, V. Narayanamurti, M. Tinkham
      cond-mat/0407489v1 [pdf]

    • Tunable Nonlocal Spin Control in a Coupled-Quantum Dot System - Abstract
      • The effective interaction between magnetic impurities in metals that can lead to various magnetic ground states often competes with a tendency for electrons near impurities to screen the local moment (Kondo effect). The simplest system exhibiting the richness of this competition, the two-impurity Kondo system, is here realized experimentally in the form of two quantum dots coupled through an open conducting region. We demonstrate non-local spin control by suppressing and splitting Kondo resonances in one quantum dot by changing electron number and coupling of the other dot. Results suggest an approach to non-local spin control relevant to quantum information processing.
    • N. J. Craig, J. M. Taylor, E. A. Lester, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Science 304, 565 (2004) [ cond-mat/0404213v2 ]
      DOI: 10.1126/science.1095452

    • Differential Charge Sensing and Charge Delocalization in a Tunable Double Quantum Dot - Abstract
      • We report measurements of a tunable double quantum dot, operating in the quantum regime, with integrated local charge sensors. The spatial resolution of the sensors is sufficient to allow the charge distribution within the double dot system to be resolved at fixed total charge. We use this readout scheme to investigate charge delocalization as a function of temperature and strength of tunnel coupling, showing that local charge sensing allows an accurate determination of interdot tunnel coupling in the absence of transport.
    • L. DiCarlo, H. J. Lynch, A. C. Johnson, L. I. Childress, K. Crockett, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 92, 226801 (2004) [ cond-mat/0311308v4 ]
      DOI: 10.1103/PhysRevLett.92.226801

    • Coulomb-Modified Fano Resonance in a One-Lead Quantum Dot - Abstract
      • We investigate a tunable Fano interferometer consisting of a quantum dot coupled via tunneling to a one-dimensional channel. In addition to Fano resonance, the channel shows strong Coulomb response to the dot, with a single electron modulating channel conductance by factors of up to 100. Where these effects coexist, lineshapes with up to four extrema are found. A model of Coulomb-modified Fano resonance is developed and gives excellent agreement with experiment.
    • A. C. Johnson, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 93, 106803 (2004). [ cond-mat/0312571v2 ]
      DOI: 10.1103/PhysRevLett.93.106803

    • Charge sensing of excited states in an isolated double quantum dot - Abstract
      • Pulsed electrostatic gating combined with capacitive charge sensing is used to perform excited state spectroscopy of an electrically isolated double-quantum-dot system. The tunneling rate of a single charge moving between the two dots is affected by the alignment of quantized energy levels; measured tunneling probabilities thereby reveal spectral features. Two pulse sequences are investigated, one of which, termed latched detection, allows measurement of a single tunneling event without repetition. Both provide excited-state spectroscopy without electrical contact to the double-dot system.
    • A. C. Johnson, C. M. Marcus, M. P. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. B 71, 115333 (2005) [ cond-mat/0312717v2 ]
      DOI: 10.1103/PhysRevB.71.115333

  • 2003
    • Orbital effects of in-plane magnetic fields probed by mesoscopic conductance fluctuations - Abstract
      • We use the high sensitivity to magnetic flux of mesoscopic conductance fluctuations in large quantum dots to investigate changes in the two-dimensional electron dispersion caused by an in-plane magnetic field. In particular, changes in effective mass and the breaking of momentum reversal symmetry in the electron dispersion are extracted quantitatively from correlations of conductance fluctuations. New theory is presented, and good agreement between theory and experiment is found.
    • D. M. Zumbuhl, J. B. Miller, C. M. Marcus, V. I. Fal'ko, T. Jungwirth, J. S. Harris Jr
      Journal reference: Phys. Rev. B 69, 121305(R) (2004) [ cond-mat/0304404v2 ]
      DOI: 10.1103/PhysRevB.69.121305

    • Locally addressable tunnel barriers within a carbon nanotube - Abstract
      • We report the realization and characterization of independently controllable tunnel barriers within a carbon nanotube. The nanotubes are mechanically bent or kinked using an atomic force microscope, and top gates are subsequently placed near each kink. Transport measurements indicate that the kinks form gate-controlled tunnel barriers, and that gates placed away from the kinks have little or no effect on conductance. The overall conductance of the nanotube can be controlled by tuning the transmissions of either the kinks or the metal-nanotube contacts.
    • M. J. Biercuk, N. Mason, J. M. Chow, C. M. Marcus
      DOI: 10.1021/nl0486721
      cond-mat/0312276v1 [pdf]

    • NATO Science Series II: Mathematics, Physics and ChemistryFundamental Problems of Mesoscopic PhysicsEntanglement Production in a Chaotic Quantum Dot - Abstract
      • It has recently been shown theoretically that elastic scattering in the Fermi sea produces quantum mechanically entangled states. The mechanism is similar to entanglement by a beam splitter in optics, but a key distinction is that the electronic mechanism works even if the source is in local thermal equilibrium. An experimental realization was proposed using tunneling between two edge channels in a strong magnetic field. Here we investigate a low-magnetic field alternative, using multiple scattering in a quantum dot. Two pairs of single-channel point contacts define a pair of qubits. If the scattering is chaotic, a universal statistical description of the entanglement production (quantified by the concurrence) is possible. The mean concurrence turns out to be almost independent on whether time-reversal symmetry is broken or not. We show how the concurrence can be extracted from a Bell inequality using low-frequency noise measurements, without requiring the tunneling assumption of earlier work.
    • C. W. J. Beenakker, M. Kindermann, C. M. Marcus, A. Yacoby
      Journal reference: Fundamental Problems of Mesoscopic Physics, eds. I.V. Lerner, B.L. Altshuler, and Y. Gefen, NATO Science Series II. Vol. 154 (Kluwer, Dordrecht, 2004) [ cond-mat/0310199v1 ]
      DOI: 10.1007/1-4020-2193-3_10

    • Local Gating of Carbon Nanotubes - Abstract
      • Local effects of multiple electrostatic gates placed beneath carbon nanotubes grown by chemical vapor deposition (CVD) are reported. Single-walled carbon nanotubes were grown by CVD from Fe catalyst islands across thin Mo "finger gates" (150 x 10nm). Prior to tube growth, several finger gates were patterned lithogrpahically and subsequently coated with a patterned high-k dielectric using low-temperature atomic layer deposition. Transport measurements demonstrate that local finger gates have a distinct effect from a global backgate.
    • M. J. Biercuk, N. Mason, C. M. Marcus
      Journal reference: Nano Letters 4, 1 (2004). [ cond-mat/0308477v1 ]
      DOI: 10.1021/nl034696g

    • Low-temperature atomic-layer-deposition lift-off method for microelectronic and nanoelectronic applications - Abstract
      • We report a novel method for depositing patterned dielectric layers with sub-micron features using atomic layer deposition (ALD). The patterned films are superior to sputtered or evaporated films in continuity, smoothness, conformality, and minimum feature size. Films were deposited at 100-150C using several different precursors and patterned using either PMMA or photoresist. The low deposition temperature permits uniform film growth without significant outgassing or hardbaking of resist layers. A liftoff technique presented here gives sharp step edges with edge roughness as low as ~10 nm. We also measure dielectric constants (k) and breakdown fields for the high-k materials aluminum oxide (k ~ 8-9), hafnium oxide (k ~ 16-19) and zirconium oxide (k ~ 20-29), grown under similar low temperature conditions.
    • M. J. Biercuk, D. J. Monsma, C. M. Marcus, J. S. Becker, R. G. Gordon
      Journal reference: Appl. Phys. Lett. 83, 2405 (2003) [ cond-mat/0305711v2 ]
      DOI: 10.1063/1.1612904

    • Experimental Realization of a Quantum Spin Pump - Abstract
      • We demonstrate the operation of a quantum spin pump based on cyclic radio-frequency excitation of a GaAs quantum dot, including the ability to pump pure spin without pumping charge. The device takes advantage of bidirectional mesoscopic fluctuations of pumped current, made spin-dependent by the application of an in-plane Zeeman field. Spin currents are measured by placing the pump in a focusing geometry with a spin-selective collector.
    • Susan K. Watson, R. M. Potok, C. M. Marcus, V. Umansky
      Journal reference: Phys. Rev. Lett. 91, 258301 (2003) [ cond-mat/0302492v2 ]
      DOI: 10.1103/PhysRevLett.91.258301

    • Photocurrent, Rectification, and Magnetic Field Symmetry of Induced Current through Quantum Dots - Abstract
      • We report mesoscopic dc current generation in an open chaotic quantum dot with ac excitation applied to one of the shape-defining gates. For excitation frequencies large compared to the inverse dwell time of electrons in the dot (i.e., GHz), we find mesoscopic fluctuations of induced current that are fully asymmetric in the applied perpendicular magnetic field, as predicted by recent theory. Conductance, measured simultaneously, is found to be symmetric in field. In the adiabatic (i.e., MHz) regime, in contrast, the induced current is always symmetric in field, suggesting its origin is mesoscopic rectification.
    • L. DiCarlo, C. M. Marcus, J. S. Harris Jr
      Journal reference: Phys. Rev. Lett. 91, 246804 (2003) [ cond-mat/0304397v1 ]
      DOI: 10.1103/PhysRevLett.91.246804

    • Gate-Controlled Spin-Orbit Quantum Interference Effects in Lateral Transport - Abstract
      • In situ control of spin-orbit coupling in coherent transport using a clean GaAs/AlGaAs 2DEG is realized, leading to a gate-tunable crossover from weak localization to antilocalization. The necessary theory of 2D magnetotransport in the presence of spin-orbit coupling beyond the diffusive approximation is developed and used to analyze experimental data. With this theory the Rashba contribution and linear and cubic Dresselhaus contributions to spin-orbit coupling are separately estimated, allowing the angular dependence of spin-orbit precession to be extracted at various gate voltages.
    • J. B. Miller, D. M. Zumbuhl, C. M. Marcus, Y. B. Lyanda-Geller, D. Goldhaber-Gordon, K. Campman, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 90, 076807 (2003) [ cond-mat/0206375v4 ]
      DOI: 10.1103/PhysRevLett.90.076807

    • Spin and Polarized Current from Coulomb Blockaded Quantum Dots - Abstract
      • We report measurements of spin transitions for GaAs quantum dots in the Coulomb blockade regime, and compare ground and excited state transport spectroscopy to direct measurements of the spin polarization of emitted current. Transport spectroscopy reveals both spin-increasing and spin-decreasing transitions as well as higher-spin ground states, and allows g-factors to be measured down to a single electron. The spin of emitted current in the Coulomb blockade regime, measured using spin-sensitive electron focusing, is found to be polarized along the direction of the applied magnetic field regardless of the ground state spin transition.
    • R. M. Potok, J. A. Folk, C. M. Marcus, V. Umansky, M. Hanson, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 91, 016802 (2003) [ cond-mat/0303152v1 ]
      DOI: 10.1103/PhysRevLett.91.016802

    • Long-Lived Memory for Mesoscopic Quantum Bits - Abstract
      • We describe a technique to create long-lived quantum memory for quantum bits in mesoscopic systems. Specifically we show that electronic spin coherence can be reversibly mapped onto the collective state of the surrounding nuclei. The coherent transfer can be efficient and fast and it can be used, when combined with standard resonance techniques, to reversibly store coherent superpositions on the time scale of seconds. This method can also allow for ``engineering'' entangled states of nuclear ensembles and efficiently manipulating the stored states. We investigate the feasibility of this method through a detailed analysis of the coherence properties of the system.
    • J. M. Taylor, C. M. Marcus, M. D. Lukin
      Journal reference: Phys. Rev. Lett. 90, 206803 (2003) [ cond-mat/0301323v1 ]
      DOI: 10.1103/PhysRevLett.90.206803

  • 2002
    • Spin-Orbit Coupling, Antilocalization, and Parallel Magnetic Fields in Quantum Dots - Abstract
      • We investigate antilocalization due to spin-orbit coupling in ballistic GaAs quantum dots. Antilocalization that is prominent in large dots is suppressed in small dots, as anticipated theoretically. Parallel magnetic fields suppress both antilocalization and also, at larger fields, weak localization, consistent with random matrix theory results once orbital coupling of the parallel field is included. In situ control of spin-orbit coupling in dots is demonstrated as a gate-controlled crossover from weak localization to antilocalization.
    • D. M. Zumbuhl, J. B. Miller, C. M. Marcus, K. Campman, A. C. Gossard
      Journal reference: Phys. Rev. Lett. 89, 276803 (2002). [ cond-mat/0208436v2 ]
      DOI: 10.1103/PhysRevLett.89.276803

    • Detecting Spin-Polarized Currents in Ballistic Nanostructures - Abstract
      • We demonstrate a mesoscopic spin polarizer/analyzer system that allows the spin polarization of current from a quantum point contact in an in-plane magnetic field to be measured. A transverse focusing geometry is used to couple current from an emitter point contact into a collector point contact. At large in-plane fields, with the point contacts biased to transmit only a single spin (g < e^2/h), the voltage across the collector depends on the spin polarization of the current incident on it. Spin polarizations of greater than 80% are found for both emitter and collector at 300mK and 7T in-plane field.
    • R. M. Potok, J. A. Folk, C. M. Marcus, V. Umansky
      Journal reference: Phys. Rev. Lett. 89, 266602 (2002). [ cond-mat/0206379v1 ]
      DOI: 10.1103/PhysRevLett.89.266602

    • Low-Temperature Fate of the 0.7 Structure in a Point Contact: A Kondo-like Correlated State in an Open System - Abstract
      • Besides the usual conductance plateaus at multiples of 2e2/h, quantum point contacts typically show an extra plateau at ~ 0.7(2e2/h), believed to arise from electron-electron interactions that prohibit the two spin channels from being simultaneously occupied. We present evidence that the disappearance of the 0.7 structure at very low temperature signals the formation of a Kondo-like correlated spin state. Evidence includes a zero-bias conductance peak that splits in a parallel field, scaling of conductance to a modified Kondo form, and consistency between peak width and the Kondo temperature.
    • S. M. Cronenwett, H. J. Lynch, D. Goldhaber-Gordon, L. P. Kouwenhoven, C. M. Marcus, K. Hirose, N. S. Wingreen, V. Umansky
      Journal reference: Phys. Rev. Lett. 88, 226805 (2002). [ cond-mat/0201577v1 ]
      DOI: 10.1103/PhysRevLett.88.226805

  • 2001
    • Adiabatic Quantum Pump of Spin-Polarized Current - Abstract
      • We propose a mechanism by which an open quantum dot driven by two ac (radio frequency) gate voltages in the presence of a moderate in-plane magnetic field generates a spin polarized, phase coherent dc current. The idea combines adiabatic, non-quantized (but coherent) pumping through periodically modulated external parameters and the strong fluctuations of the electron wave function existent in chaotic cavities. We estimate that the spin polarization of the current can be observed for temperatures and Zeeman splitting energies of the order of the single-particle mean level spacing.
    • Eduardo R. Mucciolo, Claudio Chamon, Charles M. Marcus
      Journal reference: Phys. Rev. Lett. 89, 146802 (2002) [ cond-mat/0112157v1 ]
      DOI: 10.1103/PhysRevLett.89.146802

    • Decoherence in Nearly Isolated Quantum Dots - Abstract
      • Decoherence in nearly-isolated GaAs quantum dots is investigated using the change in average Coulomb blockade peak height upon breaking time-reversal symmetry. The normalized change in average peak height approaches the predicted universal value of 1/4 at temperatures well below the single-particle level spacing, but is greatly suppressed for temperature greater than the level spacing, suggesting that inelastic scattering or other dephasing mechanisms dominate in this regime.
    • J. A. Folk, C. M. Marcus, J. S. Harris Jr
      Journal reference: Phys. Rev. Lett. 87, 206802 (2001) [ cond-mat/0008052v2 ]
      DOI: 10.1103/PhysRevLett.87.206802

  • 2000
    • Spin Degeneracy and Conductance Fluctuations in Open Quantum Dots - Abstract
      • The dependence of mesoscopic conductance fluctuations on parallel magnetic field is used as a probe of spin degeneracy in open GaAs quantum dots. The variance of fluctuations at high parallel field is reduced from the low-field variance (with broken time-reversal symmetry) by factors ranging from roughly two in a 1 square-micron dot at low temperature, to four or greater in 8 square-micron dots. The factor of two is expected for simple Zeeman splitting of spin degenerate channels. A possible explanation for the unexpected larger factors in terms of field-dependent spin orbit scattering is proposed.
    • J. A. Folk, S. R. Patel, K. M. Birnbaum, C. M. Marcus, C. I. Duruoz, J. S. Harris Jr
      Journal reference: Phys. Rev. Lett. 86, 2102 (2001) [ cond-mat/0005066v3 ]
      DOI: 10.1103/PhysRevLett.86.2102

    • Ground state spin and Coulomb blockade peak motion in chaotic quantum dots - Abstract
      • We investigate experimentally and theoretically the behavior of Coulomb blockade (CB) peaks in a magnetic field that couples principally to the ground-state spin (rather than the orbital moment) of a chaotic quantum dot. In the first part, we discuss numerically observed features in the magnetic field dependence of CB peak and spacings that unambiguously identify changes in spin S of each ground state for successive numbers of electrons on the dot, N. We next evaluate the probability that the ground state of the dot has a particular spin S, as a function of the exchange strength, J, and external magnetic field, B. In the second part, we describe recent experiments on gate-defined GaAs quantum dots in which Coulomb peak motion and spacing are measured as a function of in-plane magnetic field, allowing changes in spin between N and N+1 electron ground states to be inferred.
    • J. A. Folk, C. M. Marcus, R. Berkovits, I. L. Kurland, I. L. Aleiner, B. L. Altshuler
      DOI: 10.1238/Physica.Topical.090a00026
      cond-mat/0010441v1 [pdf]

    • Spin orbit effects in a GaAs quantum dot in a parallel magnetic field - Abstract
      • We analyze the effects of spin-orbit coupling on fluctuations of the conductance of a quantum dot fabricated in a GaAs heterostructure. We argue that spin-orbit effects may become important in the presence of a large parallel magnetic field B_{||}, even if they are negligble for B_{||}=0. This should be manifest in the level repulsion of a closed dot, and in reduced conductance fluctuations in dots with a small number of open channels in each lead, for large B_{||}. Our picture is consistent with the experimental observations of Folk et al.
    • B. I. Halperin, Ady Stern, Y. Oreg, J. H. Cremers, J. Folk, C. M. Marcus
      DOI: 10.1103/PhysRevLett.86.2106
      cond-mat/0010064v1 [pdf]

  • 1999
    • Low-Temperature Saturation of the Dephasing Time and Effects of Microwave Radiation on Open Quantum Dots - Abstract
      • The dephasing time of electrons in open semiconductor quantum dots, measured using ballistic weak localization, is found to saturate below ~ 100 mK, roughly twice the electron base temperature, independent of dot size. Microwave radiation deliberately coupled to the dots affects quantum interference indistinguishably from elevated temperature, suggesting that direct dephasing due to radiation is not the cause of the observed saturation. Coulomb blockade measurements show that the applied microwaves create sufficient source drain voltages to account for dephasing due to Joule heating.
    • A. G. Huibers, J. A. Folk, S. R. Patel, C. M. Marcus, C. I. Duruoz, J. S. Harris, Jr
      Journal reference: Phys. Rev. Lett 83, 5090 (1999) [ cond-mat/9904274v1 ]
      DOI: 10.1103/PhysRevLett.83.5090

    • Coulomb Blockade Fluctuations in Strongly Coupled Quantum Dots - Abstract
      • Quantum fluctuations of Coulomb blockade are investigated as a function of the coupling to reservoirs in semiconductor quantum dots. We use fluctuations in the distance between peaks $\Delta N$ apart to characterize both the amplitude and correlation of peak motion. For strong coupling, peak motion is greatly enhanced at low temperature, but does not show an increase in peak-to-peak correlation. These effects can lead to anomalous temperature dependence in the Coulomb valleys, similar to behavior ascribed to Kondo physics.
    • S. M. Maurer, S. R. Patel, C. M. Marcus, C. I. Duruoz, J. S. Harris, Jr
      Journal reference: Phys. Rev. Lett. 83, 1403 (1999) [ cond-mat/9810283v2 ]
      DOI: 10.1103/PhysRevLett.83.1403

    • An Adiabatic Quantum Electron Pump - Abstract
      • A quantum pumping mechanism which produces dc current or voltage in response to a cyclic deformation of the confining potential in an open quantum dot is reported. The voltage produced at zero current bias is sinusoidal in the phase difference between the two ac voltages deforming the potential and shows random fluctuations in amplitude and direction with small changes in external parameters such as magnetic field. The amplitude of the pumping response increases linearly with the frequency of the deformation. Dependencies of pumping on the strength of the deformations, temperature, and breaking of time-reversal symmetry are also investigated.
    • M. Switkes, C. M. Marcus, K. Campman, A. C. Gossard
      Journal reference: Science 283, 1905 (1999) [ cond-mat/9904238v1 ]
      DOI: 10.1126/science.283.5409.1905

    • Controlled Fabrication of Metallic Electrodes with Atomic Separation - Abstract
      • We report a new technique for fabricating metallic electrodes on insulating substrates with separations on the 1 nm scale. The fabrication technique, which combines lithographic and electrochemical methods, provides atomic resolution without requiring sophisticated instrumentation. The process is simple, controllable, reversible, and robust, allowing rapid fabrication of electrode pairs with high yield. We expect the method to prove useful in interfacing molecular-scale structures to macroscopic probes and electronic devices .
    • A. F. Morpurgo, D. B. Robinson, C. M. Marcus
      DOI: 10.1063/1.123765
      cond-mat/9811265v2 [pdf]

  • 1998
    • Changing the Electronic Spectrum of a Quantum Dot by Adding Electrons - Abstract
      • The temperature dependence of Coulomb blockade peak height correlation is used to investigate how adding electrons to a quantum dot alters or "scrambles" its electronic spectrum. Deviations from finite-temperature random matrix theory with an unchanging spectrum indicate spectral scrambling after a small number of electrons are added. Enhanced peak-to-peak correlations at low temperature are observed. Peak height statistics show similar behavior in several dot configurations despite significant differences in correlations.
    • S. R. Patel, D. R. Stewart, C. M. Marcus, M. Gokcedag, Y. Alhassid, A. D. Stone, C. I. Duruoz, J. S. Harris Jr
      DOI: 10.1103/PhysRevLett.81.5900
      cond-mat/9808166v3 [pdf]

    • Feedback Control of a Quantum Dot - Abstract
      • Mesoscopic structures are generally operated in an open-loop configuration, whereas most practical electronics including quantum interference devices such as SQUIDs are operated closed-loop, taking advantage of feedback. This paper presents some basic considerations on the use of feedback in mesoscopic samples with universal statistical properties. The controllability of mesoscopic fluctuations is shown to be connected to problems in continuum percolation, leading to the requirement of two control parameters to achieve robust control.
    • C. M. Marcus
      cond-mat/9807380v1 [pdf]

    • Mesoscopic Coulomb Blockade in One-channel Quantum Dots - Abstract
      • Signatures of "mesoscopic Coulomb blockade" are reported for quantum dots with one fully transmitting point-contact lead, T1 = 1, T2 << 1. Unlike Coulomb blockade (CB) in weak-tunneling devices (T1, T2 << 1), one-channel CB is a mesoscopic effect requiring quantum coherence. Several distinctive features of mesoscopic CB are observed, including a reduction in CB upon breaking time-reversal symmetry with a magnetic field, relatively large fluctuations of peak position as a function of magnetic field, and strong temperature dependence on the scale of the quantum level spacing.
    • S. M. Cronenwett, S. M. Maurer, S. R. Patel, C. M. Marcus, C. I. Duruoz, J. S. Harris
      DOI: 10.1103/PhysRevLett.81.5904
      cond-mat/9807311v1 [pdf]

    • Statistics of Coulomb Blockade Peak Spacings - Abstract
      • Distributions of Coulomb blockade peak spacing are reported for large ensembles of both unbroken (magnetic field B = 0) and broken (B <> 0) time reversal symmetry in GaAs quantum dots. Both distributions are symmetric and roughly gaussian with a width ~ 2-6% of the average spacing, with broad, non-gaussian tails. The distribution is systematically wider at B = 0 by a factor of ~ 1.2 +- 0.1. No even-odd spacing correlations or bimodal structure in the spacing distribution is found, suggesting an absence of spin-degeneracy. There is no observed correlation between peak spacing and peak height.
    • S. R. Patel, S. M. Cronenwett, D. R. Stewart, A. G. Huibers, C. M. Marcus, C. I. Duruoz, J. S. Harris, K. Campman, A. C. Gossard
      DOI: 10.1103/PhysRevLett.80.4522
      cond-mat/9708090v2 [pdf]

    • Classical Advection of Guiding Centers in a Random Magnetic Field - Abstract
      • We investigate theoretically and experimentally classical advective transport in a 2D electron gas in a random magnetic field. For uniform external perpendicular magnetic fields large compared to the random field we observe a strong enhancement of conductance compared to the ordinary Drude value. This can be understood as resulting from advection of cyclotron guiding centers. For low disorder this enhancement shows non-trivial scaling as a function of scattering time, with consistency between theory and experiment.
    • L. Zielinski, K. Chaltikian, K. Birnbaum, C. M. Marcus, K. Campman, A. C. Gossard
      DOI: 10.1209/epl/i1998-00554-7
      cond-mat/9704058v2 [pdf]

    • Distributions of the Conductance and its Parametric Derivatives in Quantum Dots - Abstract
      • Full distributions of conductance through quantum dots with single-mode leads are reported for both broken and unbroken time-reversal symmetry. Distributions are nongaussian and agree well with random matrix theory calculations that account for a finite dephasing time, $\tau_\phi$, once broadening due to finite temperature $T$ is also included. Full distributions of the derivatives of conductance with respect to gate voltage $P(dg/dV_g)$ are also investigated.
    • A. G. Huibers, S. R. Patel, C. M. Marcus, P. W. Brouwer, C. I. Duruoz, J. S. Harris, Jr
      DOI: 10.1103/PhysRevLett.81.1917
      cond-mat/9801174v1 [pdf]

  • 1997
    • Correlations between Ground and Excited State Spectra of a Quantum Dot - Abstract
      • We have studied the ground and excited state spectra of a semiconductor quantum dot for successive numbers of electron occupancy using linear and nonlinear magnetoconductance measurements. We present the first observation of direct correlation between the mth excited state of the N electron system and the ground state of the N+m electron system for m up to 4. Results are consistent with a non-spin-degenerate single particle picture of the filling of levels. Electron-electron interaction effects are also observed as a perturbation to this model. Magnetoconductance fluctuations of ground states are shown as anticrossings where wavefunction characteristics are exchanged between adjacent levels.
    • D. R. Stewart, D. Sprinzak, C. M. Marcus, C. I. Duruoz, J. S. Harris Jr
      DOI: 10.1126/science.278.5344.1784
      cond-mat/9709126v2 [pdf]

    • High bias transport and magnetometer design in open quantum dots - Abstract
      • We report transport measurements as a function of bias in open semiconductor quantum dots. These measurements are well described by an effective electron temperature derived from Joule heating at the point contacts and cooling by Wiedemann-Franz out-diffusion of thermal electrons. Using this model, we propose and analyze a quantum dot based sensor which measures absolute magnetic field at micron scales with a noise floor of $\sim 50 \mu\phi_{0} / \sqrt{Hz}$ at 300 mK.
    • M. Switkes, A. G. Huibers, C. M. Marcus, K. Campman, A. C. Gossard
      Journal reference: Appl. Phys. Lett. 72, p. 471 (1998) [ cond-mat/9708224v1 ]
      DOI: 10.1063/1.120789

    • Dephasing in Open Quantum Dots - Abstract
      • Shape-averaged magnetoconductance (weak localization) is used for the first time to obtain the electron phase coherence time $\tau_{\phi}$ in open ballistic GaAs quantum dots. Values for $\tau_{\phi}$ in the range of temperature T from 0.335 to 4 K are found to be independent of dot area, and are not consistent with the $\tau_{\phi} \propto T^{-2}$ behavior expected for isolated dots. Surprisingly, $\tau_{\phi}(T)$ agrees quantitatively with the predicted dephasing time for disordered two-dimensional electron systems.
    • A. G. Huibers, M. Switkes, C. M. Marcus, K. Campman, A. C. Gossard
      DOI: 10.1016/S0921-4526(98)00129-X
      cond-mat/9708170v1 [pdf]

    • Mesoscopic Fluctuations of Elastic Cotunneling in Coulomb Blockaded Quantum Dots - Abstract
      • We report measurements of mesoscopic fluctuations of elastic cotunneling in Coulomb blockaded quantum dots. Unlike resonant tunneling on Coulomb peaks, cotunneling in the valleys is sensitive to charging effects. We observe a larger magnetic field scale for the cotunneling (valley) fluctuations compared to the peaks, as well as an absence of "weak localization" (reduced conductance at B = 0) in valleys. Cotunneling fluctuations remain correlated over several valleys while peak conductance correlations decreases quickly.
    • S. M. Cronenwett, S. R. Patel, C. M. Marcus, K. Campman, A. C. Gossard
      Journal reference: S.M. Cronenwett et al., PRL 79, 2312 (1997) [ cond-mat/9707274v1 ]
      DOI: 10.1103/PhysRevLett.79.2312

    • Quantum Chaos in Open versus Closed Quantum Dots: Signatures of Interacting Particles - Abstract
      • This paper reviews recent studies of mesoscopic fluctuations in transport through ballistic quantum dots, emphasizing differences between conduction through open dots and tunneling through nearly isolated dots. Both the open dots and the tunnel-contacted dots show random, repeatable conductance fluctuations with universal statistical proper-ties that are accurately characterized by a variety of theoretical models including random matrix theory, semiclassical methods and nonlinear sigma model calculations. We apply these results in open dots to extract the dephasing rate of electrons within the dot. In the tunneling regime, electron interaction dominates transport since the tunneling of a single electron onto a small dot may be sufficiently energetically costly (due to the small capacitance) that conduction is suppressed altogether. How interactions combine with quantum interference are best seen in this regime.
    • C. M. Marcus, S. R. Patel, A. G. Huibers, S. M. Cronenwett, M. Switkes, I. H. Chan, R. M. Clarke, J. A. Folk, S. F. Godijn, K. Campman, A. C. Gossard
      DOI: 10.1016/S0960-0779(97)00019-2
      cond-mat/9703038v2 [pdf]