Publications by Charles M. Marcus

  • 2025
    • From two dimensions to wire networks in a dice-lattice Josephson array - Abstract
      • We investigate Josephson arrays consisting of a dice-lattice network of superconducting weak links surrounding rhombic plaquettes of proximitized semiconductor. Josephson coupling of the weak links and electron density in the plaquettes are independently controlled by separate electrostatic gates. Applied magnetic flux results in an intricate pattern of switching currents associated with frustration, $f$. For depleted plaquettes, the switching current is nearly periodic in $f$, expected for a phase-only description, while occupied plaquettes yield a decreasing envelope of switching currents with increasing $f$. A model of flux dependence based on ballistic small-area junctions and diffusive large-area plaquettes yields excellent agreement with experiment.
    • 2510.07412v1 [pdf]
      J. D. Bondar, L. Banszerus, W. Marshall, T. Lindemann, T. Zhang, M. J. Manfra, C. M. Marcus, S. Vaitiekėnas
      [pdf]
    • Voltage-Tuned Anomalous-Metal to Metal Transition in Hybrid Josephson Junction Arrays - Abstract
      • We report voltage-tuned phase transitions in arrays of hybrid semiconductor-superconductor islands arranged in a square lattice. A double-layer electrostatic gate geometry enables independent tuning of inter-island coupling and proximity-induced superconductivity. This design enables access to the superconductor-insulator, superconductor-metal, and metal-insulator transitions in a single device, revealing critical points and emergent intermediate phases. We find that the superconductor-insulator transition is interrupted by an anomalous metallic phase with saturating low-temperature resistivity. Across gate voltages, this regime extends over three orders of magnitude in resistivity and can be continuously tuned into the conventional metallic phase. The signature of the anomalous metallic phase is suppressed by magnetic frustration.
    • S. Sasmal, M. Efthymiou-Tsironi, G. Nagda, E. Fugl, L. L. Olsen, F. Krizek, C. M. Marcus, S. Vaitiekėnas
      Journal reference: Phys. Rev. Lett. 135, 156301 (2025) [pdf]
      DOI: 10.1103/xbm4-37cf
    • Caroli–de Gennes–Matricon Analogs in Full-Shell Hybrid Nanowires - Abstract
      • We report tunneling spectroscopy of Andreev subgap states in hybrid nanowires with a thin superconducting full-shell surrounding a semiconducting core. The combination of the quantized fluxoid of the shell and the Andreev reflection at the superconductor-semiconductor interface gives rise to analogs of Caroli-de Gennes-Matricon (CdGM) states found in Abrikosov vortices in type-II superconductors. Unlike in metallic superconductors, CdGM analogs in full-shell hybrid nanowires manifest as one-dimensional van Hove singularities with energy spacings comparable to the superconducting gap and independent of the Fermi energy, making them readily observable. Evolution of these analogs with axial magnetic field, skewed within the Little-Parks lobe structure, is consistent with theory and yields information about the radial distribution and angular momenta of the corresponding subbands.
    • M. T. Deng, Carlos Payá, Pablo San-Jose, Elsa Prada, C. M. Marcus, S. Vaitiekėnas
      Journal reference: Phys. Rev. Lett. 134, 206302 (2025) [pdf]
      DOI: 10.1103/PhysRevLett.134.206302
    • Hybrid Josephson Rhombus: A Superconducting Element with Tailored Current-Phase Relation - Abstract
      • Controlling the current-phase relation (CPR) of Josephson elements is essential for tailoring the eigenstates of superconducting qubits, tuning the properties of parametric amplifiers, and designing nonreciprocal superconducting devices. Here, we introduce the hybrid Josephson rhombus, a highly tunable superconducting circuit containing four semiconductor-superconductor hybrid Josephson junctions embedded in a loop. Combining magnetic frustration with gate-voltage-controlled tuning of individual Josephson couplings provides deterministic control of the harmonic content of the rhombus CPR. We show that for balanced Josephson couplings at full frustration, the hybrid rhombus displays a $π$-periodic $\cos(2\varphi)$ potential, indicating coherent charge-$4e$ transport. Tuning away from the balanced configuration, we observe a superconducting diode effect with efficiency exceeding 25%. These results showcase the potential of hybrid Josephson rhombi as fundamental building blocks for noise-resilient qubits and quantum devices with custom transport properties.
    • L. Banszerus, C. W. Andersson, W. Marshall, T. Lindemann, M. J. Manfra, C. M. Marcus, S. Vaitiekėnas
      Journal reference: Phys. Rev. X 15, 011021 (2025) [pdf]
      DOI: 10.1103/PhysRevX.15.011021
  • 2024
    • Voltage-Controlled Synthesis of Higher Harmonics in Hybrid Josephson Junction Circuits - Abstract
      • We report measurements of the current-phase relation of two voltage-controlled semiconductor-superconductor hybrid Josephson junctions (JJs) in series. The two hybrid junctions behave similar to a single-mode JJ with effective transparency determined by the ratio of Josephson coupling strengths of the two junctions. Gate-voltage control of Josephson coupling (measured from switching currents) allows tuning of the harmonic content from sinusoidal, for asymmetric tuning, to highly nonsinusoidal, for symmetric tuning. The experimentally observed tunable harmonic content agrees with a model based on two conventional (sinusoidal) JJs in series.
    • L. Banszerus, W. Marshall, C. W. Andersson, T. Lindemann, M. J. Manfra, C. M. Marcus, S. Vaitiekėnas
      Journal reference: Phys. Rev. Lett. 133, 186303 (2024) [pdf]
      DOI: 10.1103/PhysRevLett.133.186303
    • Supercurrent transport through - Abstract
      • We experimentally investigate supercurrent through Coulomb islands, where island and leads are fabricated from semiconducting nanowires with fully surrounding superconducting shells. Applying flux along the wire yields a series of destructive Little-Parks lobes with reentrant supercurrent. We find Coulomb blockade with 2$e$ peak spacing in the zeroth lobe and 1$e$ average spacing, with regions of significant even-odd modulation, in the first lobe. Evolution of Coulomb-peak amplitude through the first lobe is consistent with a theoretical model of supercurrent carried predominantly by zero-energy states in the leads and the island.
    • D. Razmadze, R. Seoane Souto, E. C. T. O'Farrell, P. Krogstrup, M. Leijnse, C. M. Marcus, S. Vaitiekėnas
      Journal reference: Phys. Rev. B 109, L041302 (2024) [pdf]
      DOI: 10.1103/PhysRevB.109.L041302
  • 2023
    • Supercurrent reversal in ferromagnetic hybrid nanowire Josephson junctions - Abstract
      • We report supercurrent transport measurements in hybrid Josephson junctions comprised of semiconducting InAs nanowires with epitaxial ferromagnetic insulator EuS and superconducting Al coatings. The wires display a hysteretic superconducting window close to the coercivity, away from zero external magnetic field. Using a multi-interferometer setup, we measure the current-phase relation of multiple magnetic junctions and find an abrupt switch between $π$ and 0 phases within the superconducting window. We attribute the 0-$π$ transition to the discrete flipping of the EuS domains and provide a qualitative theory showing that a sizable exchange field can polarize the junction and lead to the supercurrent reversal. Both $0$ and $π$ phases can be realized at zero external field by demagnetizing the wire.
    • D. Razmadze, R. Seoane Souto, L. Galletti, A. Maiani, Y. Liu, P. Krogstrup, C. Schrade, A. Gyenis, C. M. Marcus, S. Vaitiekėnas
      Journal reference: Phys. Rev. B 107, L081301 (2023) [pdf]
      DOI: 10.1103/PhysRevB.107.L081301
    • Local and Nonlocal Transport Spectroscopy in Planar Josephson Junctions - Abstract
      • We report simultaneously acquired local and nonlocal transport spectroscopy in a phase-biased planar Josephson junction based on an epitaxial InAs/Al hybrid two-dimensional heterostructure. Quantum point contacts at the junction ends allow measurement of the 2 x 2 matrix of local and nonlocal tunneling conductances as a function of magnetic field along the junction, phase difference across the junction, and carrier density. A closing and reopening of a gap was observed in both the local and nonlocal tunneling spectra as a function of magnetic field. For particular tunings of junction density, gap reopenings were accompanied by zero-bias conductance peaks (ZBCPs) in local conductances. End-to-end correlation of gap reopening was strong, while correlation of local ZBCPs was weak. A simple, disorder-free model of the device shows comparable conductance matrix behavior associated with a topological phase transition. Phase dependence helps distinguish possible origins of the ZBCPs.
    • A. Banerjee, O. Lesser, M. A. Rahman, C. Thomas, T. Wang, M. J. Manfra, E. Berg, Y. Oreg, Ady Stern, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 130, 096202 (2023) [pdf]
      DOI: 10.1103/PhysRevLett.130.096202
    • Parity switching in a full-shell superconductor-semiconductor nanowire qubit - Abstract
      • The rate of charge-parity switching in a full-shell superconductor-semiconductor nanowire qubit is measured by directly monitoring the dispersive shift of a readout resonator. At zero magnetic field, the measured switching time scale $T_P$ is on the order of 100 ms. Two-tone spectroscopy data post-selected on charge-parity is demonstrated. With increasing temperature or magnetic field, TP is at first constant, then exponentially suppressed, consistent with a model that includes both non-equilibrium and thermally activated quasiparticles. As TP is suppressed, qubit lifetime T1 also decreases. The long $T_P\sim 0.1$ s at zero field is promising for future development of qubits based on hybrid nanowires.
    • O. Erlandsson, D. Sabonis, A. Kringhøj, T. W. Larsen, P. Krogstrup, K. D. Petersson, C. M. Marcus
      Journal reference: Phys. Rev. B 108, L121406 (2023) [pdf]
      DOI: 10.1103/PhysRevB.108.L121406
    • Electrostatic control of quasiparticle poisoning in a hybrid semiconductor-superconductor island - Abstract
      • The performance of superconducting devices is often degraded by the uncontrolled appearance and disappearance of quasiparticles, a process known as poisoning. We demonstrate electrostatic control of quasiparticle poisoning in the form of single-charge tunneling across a fixed barrier onto a Coulomb island in an InAs/Al hybrid nanowire. High-bandwidth charge sensing was used to monitor charge occupancy of the island across Coulomb blockade peaks, where tunneling rates were maximal, and Coulomb valleys, where tunneling was absent. Electrostatic gates changed on-peak tunneling rates by two orders of magnitude for a barrier with fixed normal-state resistance, which we attribute to gate dependence of the size and softness of the induced superconducting gap on the island, corroborated by separate density-of-states measurements. Temperature and magnetic field dependence of tunneling rates are also investigated.
    • H. Q. Nguyen, D. Sabonis, D. Razmadze, E. T. Mannila, V. F. Maisi, D. M. T. van Zanten, E. C. T. O'Farrell, P. Krogstrup, F. Kuemmeth, J. P. Pekola, C. M. Marcus
      Journal reference: Phys. Rev. B 108, L041302 (2023) [pdf]
      DOI: 10.1103/PhysRevB.108.L041302
  • 2022
    • Gate-Tunable Transmon Using Selective-Area-Grown Superconductor-Semiconductor Hybrid Structures on Silicon - Abstract
      • We present a gate-voltage tunable transmon qubit (gatemon) based on planar InAs nanowires that are selectively grown on a high resistivity silicon substrate using III-V buffer layers. We show that low loss superconducting resonators with an internal quality of $2\times 10^5$ can readily be realized using these substrates after the removal of buffer layers. We demonstrate coherent control and readout of a gatemon device with a relaxation time, $T_{1}\approx 700\,\mathrm{ns}$, and dephasing times, $T_2^{\ast}\approx 20\,\mathrm{ns}$ and $T_{\mathrm{2,echo}} \approx 1.3\,\mathrm{μs}$. Further, we infer a high junction transparency of $0.4 - 0.9$ from an analysis of the qubit anharmonicity.
    • A. Hertel, M. Eichinger, L. O. Andersen, D. M. T. van Zanten, S. Kallatt, P. Scarlino, A. Kringhøj, J. M. Chavez-Garcia, G. C. Gardner, S. Gronin, M. J. Manfra, A. Gyenis, M. Kjaergaard, C. M. Marcus, K. D. Petersson
      Journal reference: Phys. Rev. Applied 18, 034042 (2022) [pdf]
      DOI: 10.1103/PhysRevApplied.18.034042
    • Evidence for spin-polarized bound states in semiconductor–superconductor–ferromagnetic-insulator islands - Abstract
      • We report Coulomb blockade transport studies of semiconducting InAs nanowires grown with epitaxial superconducting Al and ferromagnetic insulator EuS on overlapping facets. Comparing experiment to a theoretical model, we associate cotunneling features in even-odd bias spectra with spin-polarized Andreev levels. Results are consistent with zero-field spin splitting exceeding the induced superconducting gap. Energies of subgap states are tunable on either side of zero via electrostatic gates.
    • S. Vaitiekėnas, R. Seoane Souto, Y. Liu, P. Krogstrup, K. Flensberg, M. Leijnse, C. M. Marcus
      Journal reference: Phys. Rev. B 105, L041304 (2022) [pdf]
      DOI: 10.1103/PhysRevB.105.L041304
    • Signatures of a topological phase transition in a planar Josephson junction - Abstract
      • A growing body of work suggests that planar Josephson junctions fabricated using superconducting hybrid materials provide a highly controllable route toward one-dimensional topological superconductivity. Among the experimental controls are in-plane magnetic field, phase difference across the junction, and carrier density set by electrostatic gate voltages. Here, we investigate planar Josephson junctions with an improved design based on an epitaxial InAs/Al heterostructure, embedded in a superconducting loop, probed with integrated quantum point contacts (QPCs) at both ends of the junction. For particular ranges of in-plane field and gate voltages, a closing and reopening of the superconducting gap is observed, along with a zero-bias conductance peak (ZBCP) that appears upon reopening of the gap. Consistency with a simple theoretical model supports the interpretation of a topological phase transition. While gap closings and reopenings generally occurred together at the two ends of the junction, the height, shape, and even presence of ZBCPs typically differed between the ends, presumably due to disorder and variation of couplings to local probes.
    • A. Banerjee, O. Lesser, M. A. Rahman, H. -R. Wang, M. -R. Li, A. Kringhøj, A. M. Whiticar, A. C. C. Drachmann, C. Thomas, T. Wang, M. J. Manfra, E. Berg, Y. Oreg, Ady Stern, C. M. Marcus
      [pdf]
      DOI: 10.1103/PhysRevB.107.245304
      2201.03453v1 [pdf]
  • 2021
    • Electrical Properties of Selective-Area-Grown Superconductor-Semiconductor Hybrid Structures on Silicon - Abstract
      • We present a superconductor-semiconductor material system that is both scalable and monolithically integrated on a silicon substrate. It uses selective area growth of Al-InAs hybrid structures on a planar III-V buffer layer, grown directly on a high resistivity silicon substrate. We characterized the electrical properties of this material system at millikelvin temperatures and observed a high average field-effect mobility of $μ\approx 3200\,\mathrm{cm^2/Vs}$ for the InAs channel, and a hard induced superconducting gap. Josephson junctions exhibited a high interface transmission, $\mathcal{T} \approx 0.75 $, gate voltage tunable switching current with a product of critical current and normal state resistance, $I_{\mathrm{C}}R_{\mathrm{N}} \approx 83\,\mathrm{μV}$, and signatures of multiple Andreev reflections. These results pave the way for scalable and high coherent gate voltage tunable transmon devices and other superconductor-semiconductor hybrids fabricated directly on silicon.
    • A. Hertel, L. O. Andersen, D. M. T. van Zanten, M. Eichinger, P. Scarlino, S. Yadav, J. Karthik, S. Gronin, G. C. Gardner, M. J. Manfra, C. M. Marcus, K. D. Petersson
      Journal reference: Phys. Rev. Applied 16, 044015 (2021) [pdf]
      DOI: 10.1103/PhysRevApplied.16.044015
    • Zeeman-driven parity transitions in an Andreev quantum dot - Abstract
      • The Andreev spectrum of a quantum dot embedded in a hybrid semiconductor-superconductor interferometer can be modulated by electrostatic gating, magnetic flux through the interferometer, and Zeeman splitting from in-plane magnetic field. We demonstrate parity transitions in the embedded quantum dot system, and show that the Zeeman-driven transition is accompanied by a 0-π transition in the superconducting phase across the dot. We further demonstrate that flux through the interferometer modulates both dot parity and 0-π transitions.
    • A. M. Whiticar, A. Fornieri, A. Banerjee, A. C. C. Drachmann, S. Gronin, G. C. Gardner, T. Lindemann, M. J. Manfra, C. M. Marcus
      Journal reference: Phys. Rev. B 103, 245308 (2021) [pdf]
      DOI: 10.1103/PhysRevB.103.245308
    • Magnetic-Field-Compatible Superconducting Transmon Qubit - Abstract
      • We present a hybrid semiconductor-based superconducting qubit device which remains coherent at magnetic fields up to 1 T. The qubit transition frequency exhibits periodic oscillations with magnetic field, consistent with interference effects due to the magnetic flux threading the cross section of the proximitized semiconductor nanowire junction. As induced superconductivity revives, additional coherent modes emerge at high magnetic fields, which we attribute to the interaction of the qubit and low-energy Andreev states.
    • A. Kringhøj, T. W. Larsen, O. Erlandsson, W. Uilhoorn, J. G. Kroll, M. Hesselberg, R. P. G. McNeil, P. Krogstrup, L. Casparis, C. M. Marcus, K. D. Petersson
      Journal reference: Phys. Rev. Applied 15, 054001 (2021) [pdf]
      DOI: 10.1103/PhysRevApplied.15.054001
    • Andreev Modes from Phase Winding in a Full-Shell Nanowire-Based Transmon - Abstract
      • We investigate transmon qubits made from semiconductor nanowires with a fully surrounding superconducting shell. In the regime of reentrant superconductivity associated with the destructive Little-Parks effect, numerous coherent transitions are observed in the first reentrant lobe, where the shell carries 2π winding of superconducting phase, and are absent in the zeroth lobe. As junction density was increased by gate voltage, qubit coherence was suppressed then lost in the first lobe. These observations and numerical simulations highlight the role of winding-induced Andreev states in the junction.
    • A. Kringhøj, G. W. Winkler, T. W. Larsen, D. Sabonis, O. Erlandsson, P. Krogstrup, B. van Heck, K. D. Petersson, C. M. Marcus
      Journal reference: Phys. Rev. Lett. 126, 047701 (2021) [pdf]
      DOI: 10.1103/PhysRevLett.126.047701
    • Closing of the induced gap in a hybrid superconductor-semiconductor nanowire - Abstract
      • Hybrid superconductor-semiconductor nanowires are predicted to undergo a field-induced phase transition from a trivial to a topological superconductor, marked by the closure and re-opening of the excitation gap, followed by the emergence of Majorana bound states at the nanowire ends. Many local density-of-states measurements have reported signatures of the topological phase, however this interpretation has been challenged by alternative explanations. Here, by measuring nonlocal conductance, we identify the closure of the excitation gap in the bulk of the semiconductor before the emergence of zero-bias peaks. This observation is inconsistent with scenarios where zero-bias peaks occur due to end-states with a trivially gapped bulk, which have been extensively considered in the theoretical and experimental literature. We observe that after the gap closes, nonlocal signals fluctuate strongly and persist irrespective of the presence of local-conductance zero-bias peaks. Thus, our observations are also incompatible with a simple picture of clean topological superconductivity. This work presents a new experimental approach for probing the spatial extent of states in Majorana wires, and reveals the presence of a regime with a continuum of spatially extended states and uncorrelated zero-bias peaks.
    • D. Puglia, E. A. Martinez, G. C. Ménard, A. Pöschl, S. Gronin, G. C. Gardner, R. Kallaher, M. J. Manfra, C. M. Marcus, A. P. Higginbotham, L. Casparis
      Journal reference: Phys. Rev. B 103, 235201 (2021) [pdf]
      DOI: 10.1103/PhysRevB.103.235201
    • Zero-bias peaks at zero magnetic field 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
      Journal reference: Nat. Phys. 17, 43 (2021) [pdf]
      DOI: 10.1038/s41567-020-1017-3
  • 2020
    • 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(2φ) Josephson element, reflecting coherent transport of pairs of Cooper pairs. We demonstrate that relaxation of the qubit can be suppressed tenfold 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
      Journal reference: Phys. Rev. Lett. 125, 056801 (2020) [pdf]
      DOI: 10.1103/PhysRevLett.125.056801
    • Coherent transport through a Majorana island in an Aharonov–Bohm interferometer - Abstract
      • Majorana zero modes are leading candidates for topological quantum computation due to non-local qubit encoding and non-abelian exchange statistics. Spatially separated Majorana modes are expected to allow phase-coherent single-electron transport through a topological superconducting island via a mechanism referred to as teleportation. Here we experimentally investigate such a system by patterning an elongated epitaxial InAs-Al island embedded in an Aharonov-Bohm interferometer. With increasing parallel magnetic field, a discrete sub-gap state in the island is lowered to zero energy yielding persistent 1e-periodic Coulomb blockade conductance peaks (e is the elementary charge). In this condition, conductance through the interferometer is observed to oscillate in a perpendicular magnetic field with a flux period of h/e (h is Planck's constant), indicating coherent transport of single electrons through the islands, a signature of electron teleportation via Majorana modes, 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
      Journal reference: Nat. Comm. 11, 3212 (2020) [pdf]
      DOI: 10.1038/s41467-020-16988-x
    • 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 π-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
      Journal reference: Phys. Rev. Lett. 125, 116803 (2020) [pdf]
      DOI: 10.1103/PhysRevLett.125.116803
    • 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) [pdf]
      DOI: 10.1103/PhysRevB.101.060507
    • 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
      Journal reference: Phys. Rev. Lett. 124, 226801 (2020) [pdf]
      DOI: 10.1103/PhysRevLett.124.226801
    • 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
      Journal reference: Phys. Rev. Lett. 124, 246803 (2020) [pdf]
      DOI: 10.1103/PhysRevLett.124.246803
    • 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) [pdf]
      DOI: 10.1103/PhysRevLett.124.056801
    • 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) [pdf]
      DOI: 10.1103/PhysRevLett.124.036802
    • 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 Φ_0 = h/2e, corresponding to 2π 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) [pdf]
      DOI: 10.1126/science.aav3392
  • 2019
    • 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) [pdf]
      DOI: 10.1103/PhysRevB.100.205412
    • 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.
    • 1902.07570v1 [pdf]
      J. T. Mlack, K. S. Wickramasinghe, T. D. Mishima, M. B. Santos, C. M. Marcus
      [pdf]
    • 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) [pdf]
      DOI: 10.1103/PhysRevB.99.085434
  • 2018
    • 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) [pdf]
      DOI: 10.1103/PhysRevB.98.085125
    • Effective - 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
    • 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) [pdf]
      DOI: 10.1103/PhysRevLett.121.256803
    • 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) [pdf]
      DOI: 10.1103/PhysRevB.97.060508
  • 2017
    • 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) [pdf]
      DOI: 10.1038/nnano.2016.170
    • 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 μs) and sets a bound for a weakly coupled island (> 10 μs). 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) [pdf]
      DOI: 10.1103/PhysRevLett.118.137701
    • Proximity 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 $Δ^*=0.50~\mathrm{meV}$ and a critical temperature of $7.8~\mathrm{K}$. Tunneling spectroscopy yields a hard induced gap in the InAs adjacent to the superconductor of $Δ^*=0.43~\mathrm{meV}$ 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) [pdf]
      DOI: 10.1021/acs.nanolett.6b04964
    • 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) [pdf]
      DOI: 10.1103/PhysRevB.95.035307
    • 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 μeV, 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) [pdf]
      DOI: 10.1103/PhysRevApplied.7.034029
    • 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) [pdf]
      DOI: 10.1038/nnano.2016.227
  • 2016
    • 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) [pdf]
      DOI: 10.1126/science.aaf3961
    • 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) [pdf]
      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) [pdf]
      DOI: 10.1103/PhysRevApplied.6.054017
    • 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
    • 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) [pdf]
      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
  • 2015
    • 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 μs) and dephasing times (1 μs), 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
    • 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
    • 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
  • 2014
    • 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 $T_2^* \sim 0.18~\mathrm{μs}$ 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
    • 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
  • 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.
    • 1309.4569v1 [pdf]
      N. L. B. Ziino, P. Krogstrup, M. H. Madsen, E. Johnson, J. B. Wagner, C. M. Marcus, J. Nygård, T. S. Jespersen
      [pdf]
    • 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
    • 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 π/2-gate time of 2.5 ns and a coherence time of 19 μs, 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
    • 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
    • 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
  • 2012
    • - 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