Publications by Anasua Chatterjee
- 2025
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Automated in situ optimization and disorder mitigation in a quantum
device -
Abstract
- We investigate automated in situ optimization of the potential landscape in a quantum point contact device, using a $3 \times 3$ gate array patterned atop the constriction. Optimization is performed using the covariance matrix adaptation evolutionary strategy, for which we introduce a metric for how "step-like" the conductance is as the channel becomes constricted. We first perform the optimization of the gate voltages in a tight-binding simulation and show how such in situ tuning can be used to mitigate a random disorder potential. The optimization is then performed in a physical device in experiment, where we also observe a marked improvement in the quantization of the conductance resulting from the optimization procedure.
- 2412.04997v2 [pdf]
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Autonomous bootstrapping of quantum dot devices -
Abstract
- Semiconductor quantum dots (QDs) are a promising platform for multiple different qubit implementations, all of which are voltage controlled by programmable gate electrodes. However, as the QD arrays grow in size and complexity, tuning procedures that can fully autonomously handle the increasing number of control parameters are becoming essential for enabling scalability. We propose a bootstrapping algorithm for initializing a depletion-mode QD device in preparation for subsequent phases of tuning. During bootstrapping, the QD device functionality is validated, all gates are characterized, and the QD charge sensor is made operational. We demonstrate the bootstrapping protocol in conjunction with a coarse-tuning module, showing that the combined algorithm can efficiently and reliably take a cooled-down QD device to a desired global-state configuration in under 8 min with a success rate of 96 %. Finally, by following heuristic approaches to QD device initialization and combining the efficient ray-based measurement with the rapid radio-frequency reflectometry measurements, the proposed algorithm establishes a reference in terms of performance, reliability, and efficiency against which alternative algorithms can be benchmarked.
Anton Zubchenko, Danielle Middlebrooks, Torbjørn Rasmussen, Lara Lausen, Ferdinand Kuemmeth, Anasua Chatterjee, Justyna P. Zwolak Journal reference: Phys. Rev. Applied 23, 014072 (2025) [pdf] DOI: 10.1103/PhysRevApplied.23.014072
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Efficient Qubit Calibration by Binary-Search Hamiltonian Tracking -
Abstract
- We present a real-time method for calibrating the frequency of a resonantly driven qubit. The real-time processing capabilities of a controller dynamically compute adaptive probing sequences for qubit-frequency estimation. Each probing time and drive frequency are calculated to divide the prior probability distribution into two branches, following a locally optimal strategy that mimics a conventional binary search. We show the algorithm's efficacy by stabilizing a flux-tunable transmon qubit, leading to improved coherence and gate fidelity. By feeding forward the updated qubit frequency, the FPGA-powered control electronics also mitigates non-Markovian noise in the system, which is detrimental to quantum error correction. Our protocol highlights the importance of feedback in improving the calibration and stability of qubits subject to drift and can be readily applied to other qubit platforms.
- 2501.05386v1 [pdf]
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A quantum dot in germanium proximitized by a superconductor -
Abstract
- Planar germanium quantum wells have recently been shown to host hard-gapped superconductivity. Additionally, quantum dot spin qubits in germanium are well-suited for quantum information processing, with isotopic purification to a nuclear spin-free material expected to yield long coherence times. Therefore, as one of the few group IV materials with the potential to host superconductor-semiconductor hybrid devices, proximitized quantum dots in germanium is a compelling platform to achieve and combine topological superconductivity with existing and novel qubit modalities. Here we demonstrate a quantum dot (QD) in a Ge/SiGe heterostructure proximitized by a platinum germanosilicide (PtGeSi) superconducting lead (SC), forming a SC-QD-SC junction. We show tunability of the QD-SC coupling strength, as well as gate control of the ratio of charging energy and the induced gap. We further exploit this tunability by exhibiting control of the ground state of the system between even and odd parity. Furthermore, we characterize the critical magnetic field strengths, finding a critical out-of-plane field of 0.90(4). Finally we explore sub-gap spin splitting in the device, observing rich physics in the resulting spectra, that we model using a zero-bandwidth model in the Yu-Shiba-Rusinov limit. The demonstration of controllable proximitization at the nanoscale of a germanium quantum dot opens up the physics of novel spin and superconducting qubits, and Josephson junction arrays in a group IV material.
Lazar Lakic, William I. L. Lawrie, David van Driel, Lucas E. A. Stehouwer, Yao Su, Menno Veldhorst, Giordano Scappucci, Ferdinand Kuemmeth, Anasua Chatterjee Journal reference: Nature Materials (2025) [pdf] DOI: 10.1038/s41563-024-02095-5
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Automated in situ optimization and disorder mitigation in a quantum
device -
Abstract
- 2024
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Physics-informed tracking of qubit fluctuations -
Abstract
- Environmental fluctuations degrade the performance of solid-state qubits but can in principle be mitigated by real-time Hamiltonian estimation down to time scales set by the estimation efficiency. We implement a physics-informed and an adaptive Bayesian estimation strategy and apply them in real time to a semiconductor spin qubit. The physics-informed strategy propagates a probability distribution inside the quantum controller according to the Fokker-Planck equation, appropriate for describing the effects of nuclear spin diffusion in gallium-arsenide. Evaluating and narrowing the anticipated distribution by a predetermined qubit probe sequence enables improved dynamical tracking of the uncontrolled magnetic field gradient within the singlet-triplet qubit. The adaptive strategy replaces the probe sequence by a small number of qubit probe cycles, with each probe time conditioned on the previous measurement outcomes, thereby further increasing the estimation efficiency. The combined real-time estimation strategy efficiently tracks low-frequency nuclear spin fluctuations in solid-state qubits, and can be applied to other qubit platforms by tailoring the appropriate update equation to capture their distinct noise sources.
Fabrizio Berritta, Jan A. Krzywda, Jacob Benestad, Joost van der Heijden, Federico Fedele, Saeed Fallahi, Geoffrey C. Gardner, Michael J. Manfra, Evert van Nieuwenburg, Jeroen Danon, Anasua Chatterjee, Ferdinand Kuemmeth Journal reference: Phys. Rev. Applied 22, 014033 (2024) [pdf] DOI: 10.1103/PhysRevApplied.22.014033
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Real-time two-axis control of a spin qubit -
Abstract
- Optimal control of qubits requires the ability to adapt continuously to their ever-changing environment. We demonstrate a real-time control protocol for a two-electron singlet-triplet qubit with two fluctuating Hamiltonian parameters. Our approach leverages single-shot readout classification and dynamic waveform generation, allowing full Hamiltonian estimation to dynamically stabilize and optimize the qubit performance. Powered by a field-programmable gate array (FPGA), the quantum control electronics estimates the Overhauser field gradient between the two electrons in real time, enabling controlled Overhauser-driven spin rotations and thus bypassing the need for micromagnets or nuclear polarization protocols. It also estimates the exchange interaction between the two electrons and adjusts their detuning, resulting in extended coherence of Hadamard rotations when correcting for fluctuations of both qubit axes. Our study emphasizes the critical role of feedback in enhancing the performance and stability of quantum devices affected by quasistatic noise. Feedback will play an essential role in improving performance in various qubit implementations that go beyond spin qubits, helping realize the full potential of quantum devices for quantum technology applications.
Fabrizio Berritta, Torbjørn Rasmussen, Jan A. Krzywda, Joost van der Heijden, Federico Fedele, Saeed Fallahi, Geoffrey C. Gardner, Michael J. Manfra, Evert van Nieuwenburg, Jeroen Danon, Anasua Chatterjee, Ferdinand Kuemmeth Journal reference: Nature Communications 15, 1676 (2024) [pdf] DOI: 10.1038/s41467-024-45857-0
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Elongated quantum dot as a distributed charge sensor -
Abstract
- Increasing the separation between semiconductor quantum dots offers scaling advantages by fa- cilitating gate routing and the integration of sensors and charge reservoirs. Elongated quantum dots have been utilized for this purpose in GaAs heterostructures to extend the range of spin-spin interactions. Here, we study a metal-oxide-semiconductor (MOS) device where two quantum dot arrays are separated by an elongated quantum dot (340 nm long, 50 nm wide). We monitor charge transitions of the elongated quantum dot by measuring radiofrequency single-electron currents to a reservoir to which we connect a lumped-element resonator. We operate the dot as a single electron box to achieve charge sensing of remote quantum dots in each array, separated by a distance of 510 nm. Simultaneous charge detection on both ends of the elongated dot demonstrates that the charge is well distributed across its nominal length, supported by the simulated quantum-mechanical electron density. Our results illustrate how single-electron boxes can be realised with versatile foot- prints that may enable novel and compact quantum processor layouts, offering distributed charge sensing in addition to the possibility of mediated coupling.
S. M. Patomäki, J. Williams, F. Berritta, C. Laine, M. A. Fogarty, R. C. C. Leon, J. Jussot, S. Kubicek, A. Chatterjee, B. Govoreanu, F. Kuemmeth, J. J. L. Morton, M. F. Gonzalez-Zalba Journal reference: Phys. Rev. Applied 21, 054042 (2024) [pdf] DOI: 10.1103/PhysRevApplied.21.054042
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Physics-informed tracking of qubit fluctuations -
Abstract
- 2023
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Gate reflectometry in dense quantum dot arrays -
Abstract
- Silicon quantum devices are maturing from academic single- and two-qubit devices to industrially-fabricated dense quantum-dot (QD) arrays, increasing operational complexity and the need for better pulsed-gate and readout techniques. We perform gate-voltage pulsing and gate-based reflectometry measurements on a dense 2$\times$2 array of silicon quantum dots fabricated in a 300-mm-wafer foundry. Utilizing the strong capacitive couplings within the array, it is sufficient to monitor only one gate electrode via high-frequency reflectometry to establish single-electron occupation in each of the four dots and to detect single-electron movements with high bandwidth. A global top-gate electrode adjusts the overall tunneling times, while linear combinations of side-gate voltages yield detailed charge stability diagrams. To test for spin physics and Pauli spin blockade at finite magnetic fields, we implement symmetric gate-voltage pulses that directly reveal bidirectional interdot charge relaxation as a function of the detuning between two dots. Charge sensing within the array can be established without the involvement of adjacent electron reservoirs, important for scaling such split-gate devices towards longer 2$\times$N arrays. Our techniques may find use in the scaling of few-dot spin-qubit devices to large-scale quantum processors.
Fabio Ansaloni, Heorhii Bohuslavskyi, Federico Fedele, Torbjørn Rasmussen, Bertram Brovang, Fabrizio Berritta, Amber Heskes, Jing Li, Louis Hutin, Benjamin Venitucci, Benoit Bertrand, Maud Vinet, Yann-Michel Niquet, Anasua Chatterjee, Ferdinand Kuemmeth Journal reference: New J. Phys. 25, 033023 (2023) [pdf] DOI: 10.1088/1367-2630/acc126
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Probing quantum devices with radio-frequency reflectometry -
Abstract
- Many important phenomena in quantum devices are dynamic, meaning that they cannot be studied using time-averaged measurements alone. Experiments that measure such transient effects are collectively known as fast readout. One of the most useful techniques in fast electrical readout is radio-frequency reflectometry, which can measure changes in impedance (both resistive and reactive) even when their duration is extremely short, down to a microsecond or less. Examples of reflectometry experiments, some of which have been realised and others so far only proposed, include projective measurements of qubits and Majorana devices for quantum computing, real-time measurements of mechanical motion and detection of non-equilibrium temperature fluctuations. However, all of these experiments must overcome the central challenge of fast readout: the large mismatch between the typical impedance of quantum devices (set by the resistance quantum) and of transmission lines (set by the impedance of free space). Here, we review the physical principles of radio-frequency reflectometry and its close cousins, measurements of radio-frequency transmission and emission. We explain how to optimise the speed and sensitivity of a radio-frequency measurement, and how to incorporate new tools such as superconducting circuit elements and quantum-limited amplifiers into advanced radio-frequency experiments. Our aim is three-fold: to introduce the readers to the technique, to review the advances to date and to motivate new experiments in fast quantum device dynamics. Our intended audience includes experimentalists in the field of quantum electronics who want to implement radio-frequency experiments or improve them, together with physicists in related fields who want to understand how the most important radio-frequency measurements work.
Florian Vigneau, Federico Fedele, Anasua Chatterjee, David Reilly, Ferdinand Kuemmeth, Fernando Gonzalez-Zalba, Edward Laird, Natalia Ares Journal reference: Applied Physics Reviews 10, 021305 (2023) [pdf] DOI: 10.1063/5.0088229
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Gate reflectometry in dense quantum dot arrays -
Abstract
- 2022
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Autonomous Estimation of High-Dimensional Coulomb Diamonds from Sparse Measurements -
Abstract
- Quantum dot arrays possess ground states governed by Coulomb energies, utilized prominently by singly occupied quantum dots, each implementing a spin qubit. For such quantum processors, the controlled transitions between ground states are of operational significance, as these allow the control of quantum information within the array such as qubit initialization and entangling gates. For few-dot arrays, ground states are traditionally mapped out by performing dense raster-scan measurements in control-voltage space. These become impractical for larger arrays due to the large number of measurements needed to sample the high-dimensional gate-voltage hypercube and the comparatively little information extracted. We develop a hardware-triggered detection method based on reflectometry, to acquire measurements directly corresponding to transitions between ground states. These measurements are distributed sparsely within the high-dimensional voltage space by executing line searches proposed by a learning algorithm. Our autonomous software-hardware algorithm accurately estimates the polytope of Coulomb blockade boundaries, experimentally demonstrated in a 2$\times$2 array of silicon quantum dots.
Anasua Chatterjee, Fabio Ansaloni, Torbjørn Rasmussen, Bertram Brovang, Federico Fedele, Heorhii Bohuslavskyi, Oswin Krause, Ferdinand Kuemmeth Journal reference: Physical Review Applied 18, 064040 (2022) [pdf] DOI: 10.1103/PhysRevApplied.18.064040
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Learning Coulomb Diamonds in Large Quantum Dot Arrays -
Abstract
- We introduce an algorithm that is able to find the facets of Coulomb diamonds in quantum dot arrays. We simulate these arrays using the constant-interaction model, and rely only on one-dimensional raster scans (rays) to learn a model of the device using regularized maximum likelihood estimation. This allows us to determine, for a given charge state of the device, which transitions exist and what the compensated gate voltages for these are. For smaller devices the simulator can also be used to compute the exact boundaries of the Coulomb diamonds, which we use to assess that our algorithm correctly finds the vast majority of transitions with high precision.
Oswin Krause, Anasua Chatterjee, Ferdinand Kuemmeth, Evert van Nieuwenburg Journal reference: SciPost Phys. 13, 084 (2022) [pdf] DOI: 10.21468/SciPostPhys.13.4.084
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Estimation of Convex Polytopes for Automatic Discovery of Charge State Transitions in Quantum Dot Arrays -
Abstract
- In spin based quantum dot arrays, material or fabrication imprecisions affect the behaviour of the device, which must be taken into account when controlling it. This requires measuring the shape of specific convex polytopes. In this work, we present an algorithm that automatically discovers count, shape and size of the facets of a convex polytope from measurements. Results on simulated devices as well as a real 2x2 spin qubit array show that we can reliably find the facets of the convex polytopes, including small facets with sizes on the order of the measurement precision.
Oswin Krause, Torbjørn Rasmussen, Bertram Brovang, Anasua Chatterjee, Ferdinand Kuemmeth Journal reference: Electronics 11, 2327 (2022) [pdf] DOI: 10.3390/electronics11152327
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Autonomous Estimation of High-Dimensional Coulomb Diamonds from Sparse Measurements -
Abstract
- 2021
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Protected solid-state qubits -
Abstract
- The implementation of large-scale fault-tolerant quantum computers calls for the integration of millions of physical qubits, with error rates of physical qubits significantly below 1%. This outstanding engineering challenge may benefit from emerging qubits that are protected from dominating noise sources in the qubits' environment. In addition to different noise reduction techniques, protective approaches typically encode qubits in global or local decoherence-free subspaces, or in dynamical sweet spots of driven systems. We exemplify such protective qubits by reviewing the state-of-art in protected solid-state qubits based on semiconductors, superconductors, and hybrid devices.
Jeroen Danon, Anasua Chatterjee, András Gyenis, Ferdinand Kuemmeth Journal reference: Appl. Phys. Lett. 119, 260502 (2021) [pdf] DOI: 10.1063/5.0073945
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Simultaneous Operations in a Two-Dimensional Array of Singlet-Triplet Qubits -
Abstract
- In many physical approaches to quantum computation, error-correction schemes assume the ability to form two-dimensional qubit arrays with nearest-neighbor couplings and parallel operations at multiple qubit sites. While semiconductor spin qubits exhibit long coherence times relative to their operation speed and single-qubit fidelities above error correction thresholds, multiqubit operations in two-dimensional arrays have been limited by fabrication, operation, and readout challenges. We present a two-by-two array of four singlet-triplet qubits in gallium arsenide and show simultaneous coherent operations and four-qubit measurements via exchange oscillations and frequency-multiplexed single-shot measurements. A larger multielectron quantum dot is fabricated in the center of the array as a tunable interqubit link, which we utilize to demonstrate coherent spin exchange with selected qubits. Our techniques are extensible to other materials, indicating a path towards quantum processors with gate-controlled spin qubits.
Federico Fedele, Anasua Chatterjee, Saeed Fallahi, Geoffrey C. Gardner, Michael J. Manfra, Ferdinand Kuemmeth Journal reference: PRX Quantum 2, 040306 (2021) [pdf] DOI: 10.1103/PRXQuantum.2.040306
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Semiconductor qubits in practice -
Abstract
- In recent years semiconducting qubits have undergone a remarkable evolution, making great strides in overcoming decoherence as well as in prospects for scalability, and have become one of the leading contenders for the development of large-scale quantum circuits. In this Review we describe the current state of the art in semiconductor charge and spin qubits based on gate-controlled semiconductor quantum dots, shallow dopants, and color centers in wide band gap materials. We frame the relative strengths of the different semiconductor qubit implementations in the context of quantum simulations, computing, sensing and networks. By highlighting the status and future perspectives of the basic types of semiconductor qubits, this Review aims to serve as a technical introduction for non-specialists as well as a forward-looking reference for scientists intending to work in this field.
Anasua Chatterjee, Paul Stevenson, Silvano De Franceschi, Andrea Morello, Nathalie de Leon, Ferdinand Kuemmeth Journal reference: Nature Reviews Physics (2021) [pdf] DOI: 10.1038/s42254-021-00283-9
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Protected solid-state qubits -
Abstract
- 2020
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Single-electron operations in a foundry-fabricated array of quantum dots -
Abstract
- Silicon spin qubits have achieved high-fidelity one- and two-qubit gates, above error correction thresholds, promising an industrial route to fault-tolerant quantum computation. A significant next step for the development of scalable multi-qubit processors is the operation of foundry-fabricated, extendable two-dimensional (2D) arrays. In gallium arsenide, 2D quantum-dot arrays recently allowed coherent spin operations and quantum simulations. In silicon, 2D arrays have been limited to transport measurements in the many-electron regime. Here, we operate a foundry-fabricated silicon 2x2 array in the few-electron regime, achieving single-electron occupation in each of the four gate-defined quantum dots, as well as reconfigurable single, double, and triple dots with tunable tunnel couplings. Pulsed-gate and gate-reflectometry techniques permit single-electron manipulation and single-shot charge readout, while the two-dimensionality allows the spatial exchange of electron pairs. The compact form factor of such arrays, whose foundry fabrication can be extended to larger 2xN arrays, along with the recent demonstration of coherent spin control and readout, paves the way for dense qubit arrays for quantum computation and simulation.
Fabio Ansaloni, Anasua Chatterjee, Heorhii Bohuslavskyi, Benoit Bertrand, Louis Hutin, Maud Vinet, Ferdinand Kuemmeth Journal reference: Nature Communications 11, 6399 (2020) [pdf] DOI: 10.1038/s41467-020-20280-3
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Single-electron operations in a foundry-fabricated array of quantum dots -
Abstract
- 2019
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Fast 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) [pdf] DOI: 10.1021/acs.nanolett.9b02149
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Fast Charge Sensing of Si/SiGe Quantum Dots via a High-Frequency Accumulation Gate -
Abstract