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arXiv:2210.08315 · Applied Physics Letters

Latched readout for the quantum dot hybrid qubit

J. Corrigan, J. P. Dodson, Brandur Thorgrimsson, Samuel F. Neyens, T. J. Knapp, Thomas McJunkin, S. N. Coppersmith, M. A. Eriksson

Published Feb 13, 2023• 4 citations
arXivHTMLPDF

Abstract

A primary method of reading out a quantum dot hybrid qubit involves projection of the logical basis onto distinct charge states that are readily detected by an integrated charge sensing dot. However, in the simplest configuration, the excited charge state decays rapidly, making a single-shot readout challenging. Here, we demonstrate a readout procedure where the qubit excited state is latched to a metastable charge configuration whose lifetime is tunnel-rate limited, persisting here as long as 2.5 ms. Additionally, we show that working in the (4,1)–(3,2) charge configuration enables a latched readout window that is larger and more tunable than typical charge configurations, because the size of the readout window is determined by an orbital splitting rather than a valley splitting.

DOI
10.1063/5.0130865

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A primary method of reading out a quantum dot hybrid qubit involves projection of the logical basis onto distinct charge states that are readily detected by an integrated charge sensing dot. However, in the simplest configuration, the excited charge state decays rapidly, making a single-shot readout challenging. Here, we demonstrate a readout procedure where the qubit excited state is latched to a metastable charge configuration whose lifetime is tunnel-rate limited, persisting here as long as 2.5 ms. Additionally, we show that working in the (4,1)–(3,2) charge configuration enables a latched readout window that is larger and more tunable than typical charge configurations, because the size of the readout window is determined by an orbital splitting rather than a valley splitting.
Qubit latching techniques require precise tuning of multiple tunnel rates, which can be challenging when a qubit is coupled to a single reservoir. Here, we present such a method for single-shot measurement, in which the barrier gate is pulsed to dynamically control qubit-to-reservoir tunnel rates. We use it to reduce the qubit reset time in measurements of coherent Larmor oscillations of a Si/SiGe quantum dot hybrid qubit. The method is also readily applicable to other types of spin-based qubits.
We report a single-shot-based projective readout of a semiconductor hybrid qubit formed by three electrons in a GaAs double quantum dot. Voltage-controlled adiabatic transitions between the qubit operations and readout conditions allow high-fidelity mapping of quantum states. We show that a large ratio both in relaxation time vs tunneling time (≈50) and singlet-triplet splitting vs thermal energy (≈20) allows energy-selective tunneling-based spin-to-charge conversion with a readout visibility of ≈92.6%. Combined with ac driving, we demonstrate high visibility coherent Rabi and Ramsey oscillations of a hybrid qubit in GaAs. Further, we discuss the generality of the method for use in other materials, including silicon.
Spin qubits in gate-defined semiconductor quantum dots are a promising platform for realising a scalable quantum computer. This platform encodes quantum information into the intrinsic angular momentum (spin) of individual electrons or holes. In particular, hole spins in quantum dots enable the possibility of all-electrical spin control due to their intrinsic spin-orbit coupling. One important criterion of a quantum computer is the ability to reliably read out the state of the qubit. This is done by mapping spin information to the charge degree of freedom, combined with a method for charge detection. Charge detection can be facilitated by coupling a resonator directly to the quantum dot gate or to a charge sensor and measuring the resonator's phase or amplitude response. In this thesis, we describe a set of experiments relating to the implementation of hole spin qubit readout with superconducting resonators. First, we study gate-based charge sensing in a Si fin field-effect transistor (finFET) device using a superconducting NbN nanowire resonator. We find that the resonator amplifies capacitive crosstalk when electrical control pulses are applied to gate electrodes in this device. This occurs whenever a spectral component of qubit control pulses overlaps with the resonance frequency, which we show may occur for both baseband and radio-frequency (RF) drive signals. We quantify this crosstalk in a transport-based method, and attribute its source to the capacitance between the bondpads of neighbouring gate electrodes. In a second experiment, we use an RF charge sensor with the same nanowire technology to read out hole spin qubits formed in the quantum well of a Ge/SiGe heterostructure. A screening layer approach is used to facilitate RF matching due to large RF dissipation found in the heterostructure. We implement a double-latched readout scheme in this system to reduce errors arising from both spin and charge decay during readout. The spin-to-charge conversion process and its dependence on magnetic field orientation is studied. Spin and charge decay processes arising after spin-to-charge conversion are mitigated by choosing an optimal working point for the field orientation, field amplitude and readout pulse times. By doing so, we achieve an average single-shot state-preparation-and-measurement fidelity of 97%, primarily bounded by imperfect state preparation and limited signal-to-noise ratio. Finally, we study coplanar waveguide resonators fabricated from NbN. We investigate the feasibility of integrating this type of resonator onto the same chip as the Si finFET quantum dots. We then develop a novel resonator readout technique by the interference of two microwave sidebands. We demonstrate the insensitivity of this technique to common-mode phase and amplitude noise both theoretically and experimentally. This technique is then used to study resonator noise arising from two-level system defects. These results highlight both the practical constraints and the potential for achieving high readout fidelities using resonator-based hole spin qubit readout. This work establishes a blueprint for mitigating resonator-induced crosstalk and enabling high-fidelity readout in lossy materials by incorporating a screening layer.
We investigate the coherence decay of a single-exciton-based qubit with electrical readout. Pairs of laser pulses initialize the qubit state and then map the target basis onto the measurement basis, probing the decay of either the population inversion or electronic polarization components of the state vector. A comparison of coherence and population decay confirms that the exciton coherence is lifetime limited by fast voltage-tunable electron tunneling. Optimum coherence times are limited by a heavy-hole tunneling rate slow compared with the repetition rate of the laser.
Qubit readout schemes often deviate from ideal projective measurements, introducing critical issues that limit quantum computing performance. In this Letter, we model charge-sensing-based readout for semiconductor spin qubits in double quantum dots, and identify key error mechanisms caused by the backaction of the charge sensor. We quantify how the charge noise of the sensor, residual tunneling, and g-tensor modulation degrade readout fidelity, induce a mixed postmeasurement state, and cause leakage from the computational subspace. For state-of-the-art systems with strong spin-orbit interaction and electrically tunable g tensors, we identify a readout sweet spot, that is, a special device configuration where readout is closest to projective. Our framework provides a foundation for developing effective readout error mitigation strategies, with broad applications for optimizing readout performance for a variety of charge-sensing techniques, advancing quantum protocols, and improving adaptive circuits for error correction.
In quantum computing based on semiconductor quantum dots, adding dots (or just electrons) allows a wider search for an optimal qubit-encoding scheme that is both controllable and coherent. This work reports experiments to realize a hybrid charge-spin qubit in a linear triple quantum dot with asymmetric tunnel couplings, in a multielectron charge configuration. This qubit's energy splitting can be tuned conveniently over a wide range, and the authors attain qualitative understanding of the observations in terms of a three-electron system. This should stimulate further exploration of quantum coherent dynamics in the few-electron regime for semiconductor quantum processors.