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arXiv:2112.09801 · PRX Quantum

Fast and High-Fidelity State Preparation and Measurement in Triple-Quantum-Dot Spin Qubits

Jacob Blumoff, Andrew Pan, Tyler Keating, Reed W. Andrews, David W. Barnes, T. Brecht, E. T. Croke, Larken E. Euliss, Jacob Fast, Clayton A. C. Jackson, Aaron M. Jones, Joseph Kerckhoff, R. K. Lanza, Kate Raach, Bryan J. Thomas, Roland Velunta, Aaron Weinstein, Thaddeus D. Ladd, Kevin Eng, Matthew Borselli, A. T. Hunter, Matthew T. Rakher

Published Mar 29, 2022• 59 citations

Abstract

We demonstrate rapid high-fidelity state preparation and measurement in exchange-only Si/SiGe triplequantum-dot qubits.Fast measurement integration (980-ns) and initialization (approximately 300-ns) operations are performed with all-electrical baseband control.We emphasize a leakage-sensitive joint initialization and measurement metric, developed in the context of exchange-only qubits but applicable more broadly, and report an infidelity of 2.5 ± 0.5×10 -3 .This result is enabled by a high-valley-splitting heterostructure, initialization at the two-to three-electron charge boundary, and careful assessment and mitigation of T 1 during spin-to-charge conversion.The ultimate fidelity is limited by a number of comparably important factors and we identify clear paths toward further improved fidelity and speed.Along with an observed single-qubit randomized benchmarking error rate of 1.7 × 10 -3 , this work demonstrates initialization, control, and measurement of Si/SiGe triple-dot qubits at fidelities and durations that are promising for scalable quantum information processing.

DOI: 10.1103/prxquantum.3.010352

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We demonstrate rapid high-fidelity state preparation and measurement in exchange-only Si/SiGe triplequantum-dot qubits.Fast measurement integration (980-ns) and initialization (approximately 300-ns) operations are performed with all-electrical baseband control.We emphasize a leakage-sensitive joint initialization and measurement metric, developed in the context of exchange-only qubits but applicable more broadly, and report an infidelity of 2.5 ± 0.5×10 -3 .This result is enabled by a high-valley-splitting heterostructure, initialization at the two-to three-electron charge boundary, and careful assessment and mitigation of T 1 during spin-to-charge conversion.The ultimate fidelity is limited by a number of comparably important factors and we identify clear paths toward further improved fidelity and speed.Along with an observed single-qubit randomized benchmarking error rate of 1.7 × 10 -3 , this work demonstrates initialization, control, and measurement of Si/SiGe triple-dot qubits at fidelities and durations that are promising for scalable quantum information processing.

We use Si/SiGe triple quantum dots to operate two and three spin qubits with high fidelities. For the two qubit we speed up the single qubit and exchange control to achieve the fault tolerance threshold fidelity of

> 99

%. For the three qubits we control two exchange couplings to generate a three-spin entangled state.

Scalable quantum processors require repeated qubit initialization throughout large arrays. In semiconductor spin qubits, fast initialization commonly relies on local reservoir access or measurement-based feedback, requiring dedicated infrastructure that becomes increasingly difficult to distribute as processors scale. Here, we demonstrate reservoir- and measurement-free initialization of a silicon spin-qubit pair in an industrially fabricated Si/SiGe quantum-dot device using a fixed sequence of microwave and baseband pulses. Odd spin-parity states relax to the singlet charge state, whereas blocked even spin-parity states are microwave-driven through the triplet manifold and subsequently converted to the singlet by singlet-triplet mixing and charge hybridization. Repeated cycles produce the singlet-associated charge outcome with a median probability of 99.4% across the sampled preparation states, while exchange spectroscopy independently verifies mapping to the target

|\uparrow\downarrow\rangle

operational state. Microwave spectroscopy and time-domain measurements identify the dark-state-limited single-cycle transfer and the subsequent blockade-lifting dynamics that set the initialization time scale. The demonstrated pumping sequence uses approximately

12\,μ\mathrm{s}

of microwave bursts and mixing dwells, while we project sub-microsecond initialization under improved device conditions. These results establish fixed-sequence microwave initialization as a scalable control primitive for semiconductor spin-qubit processors, based on singlet-triplet physics that can be adapted to platforms with suitable Pauli-blockade transitions.

Abstract Fast electrical spin rotations in quantum dots require strong spin-charge hybridization, but the same hybridization can increase sensitivity to detuning noise. In a double-quantum-dot (DQD) flopping-mode qubit, the sweet-spot width and the Rabi enhancement are both tied to the same orbital splitting, limiting post-fabrication optimization of the speed-coherence trade-off. Here, we demonstrate a GaAs triple-quantum-dot (TQD) flopping-mode qubit in which the detuning of the third dot, ϵ 34 , provides an additional electrical tuning parameter for orbital-band flattening. Tuning ϵ 34 flattens the qubit-frequency dependence on the detuning of the target DQD while maintaining strong spin-charge hybridization, enabling electrically driven Rabi frequencies above 100 MHz. We then use feedforward-neural-network (FNN) feedback to track slow qubit-frequency drift and connect this engineered operating point to high-fidelity gates. This feedback suppresses slow qubit-frequency fluctuations, reducing their standard deviation to 388 kHz, and yields a π /2 gate fidelity of 99.56% with a 4.05 ns gate time. These results identify TQD orbital-band engineering as an electrical route for tuning the speed-coherence trade-off, complementary to material-intrinsic spin-orbit coupling and fixed micromagnet geometry.

Initialization, two-spin coherent manipulation, and readout of a three-spin qubit are demonstrated using a few-electron triple quantum dot. The three-spin qubit is designed to allow all operations for full qubit control to be tuned via nearest-neighbor exchange interaction. Fast readout of charge states takes advantage of multiplexed reflectometry. Decoherence measured in a two-spin subspace is found to be consistent with predictions based on gate voltage noise with a uniform power spectrum. The theory of the exchange-only qubit is developed and it is shown that initialization of only two spins suffices for operation. Requirements for full multiqubit control using only exchange and electrostatic interactions are outlined.

Abstract Electron spins in semiconductors are promising qubits because their long coherence times enable nearly 10 9 coherent quantum gate operations. However, developing a scalable high-fidelity two-qubit gate remains challenging. Here, we demonstrate an entangling gate between two double-quantum-dot spin qubits in GaAs by using a magnetic field gradient between the two dots in each qubit to suppress decoherence due to charge noise. When the magnetic gradient dominates the voltage-controlled exchange interaction between electrons, qubit coherence times increase by an order of magnitude. Using randomized benchmarking, we measure single-qubit gate fidelities of ~ 99%, and through self-consistent quantum measurement, state, and process tomography, we measure an entangling gate fidelity of 90%. In the future, operating double quantum dot spin qubits with large gradients in nuclear-spin-free materials, such as Si, should enable a two-qubit gate fidelity surpassing the threshold for fault-tolerant quantum information processing.

We characterize single- and two-qubit operations in a SiGe quantum dot array, from the perspective of its quantum information processing capabilities. The analysis includes rigorous randomized benchmarking of single- and two-qubit gates, state preparation and measurement characterization, and Bell's state tomography, which are all basic functionalities required for universal quantum computation. To assess compatibility with integrated cryogenic electronics, we evaluate qubit performance at 350, 500, and 750 mK, with high-fidelity single and two qubit operations. The highest temperature, 750 mK, falls within the realistic thermal budget for practical integrated cryogenic electronics and represents the highest operating temperature reported for this qubit platform.