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arXiv:2207.10523

Parity and singlet-triplet high fidelity readout in a silicon double quantum dot at 0.5 K

David J. Niegemann, Victor El-Homsy, Baptiste Jadot, Martin Nurizzo, Bruna Cardoso Paz, Emmanuel Chanrion, Matthieu Dartiailh, Bernhard Klemt, Vivien Thiney, Christopher Bäuerle, Pierre-André Mortemousque, Benoît Bertrand, Heimanu Niebojewski, M. Vinet, Franck Balestro, Tristan Meunier, Matias Urdampilleta

Published Jul 21, 2022• 0 citations
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Abstract

We demonstrate singlet-triplet readout and parity readout allowing to distinguish T0 and the polarized triplet states. We achieve high fidelity spin readout with an average fidelity above $99.9\%$ for a readout time of $20~μ$s and $99\%$ for $4~μ$s at a temperature of $0.5~K$. We initialize a singlet state in a single dot with a fidelity higher than $99\%$ and separate the two electrons while keeping the same spin state with $a \approx 95.6\%$ fidelity.

DOI
10.48550/arxiv.2207.10523

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We demonstrate singlet-triplet readout and parity readout allowing to distinguish T0 and the polarized triplet states. We achieve high fidelity spin readout with an average fidelity above 99.9%99.9\%99.9% for a readout time of 20 μ20~μ20 μs and 99%99\%99% for 4 μ4~μ4 μs at a temperature of 0.5 K0.5~K0.5 K. We initialize a singlet state in a single dot with a fidelity higher than 99%99\%99% and separate the two electrons while keeping the same spin state with a≈95.6%a \approx 95.6\%a≈95.6% fidelity.
Pauli-spin-blockade (PSB) measurements have so far achieved the highest fidelity of spin readout in semiconductor quantum dots, overcoming the 99% threshold. Moreover, in contrast to energy-selective readout, PSB is less error prone to thermal energy, an important feature for large-scale architectures that could be operated at temperatures above a few hundreds of millikelvins. In this work, we use rf reflectometry on a single-lead quantum dot to perform charge sensing and to probe the spin state of a double quantum dot at 0.5 K. At this relatively elevated temperature, we characterize both singlet-triplet and parity readout, which are complementary measurements to perform a complete readout of a two-spin system. We demonstrate high-fidelity spin readout with an average fidelity above 99.9 % for a readout time of 20 μs and 99 % for 4 μs. Finally, we succeed in initializing a singlet state in a single dot with a fidelity higher than 99 % and separate the two electrons while retaining the same spin state with a 95.6 % fidelity
High-fidelity singlet-triplet state readout is essential for large-scale quantum computing. However, the widely used threshold method of comparing a mean value with the fixed threshold will limit the judgment accuracy, especially for the relaxed triplet state, under the restriction of relaxation time and signal-to-noise ratio. Here, we achieve an enhanced latching readout based on Pauli spin blockade in a Si-MOS double quantum dot device and demonstrate an average singlet-triplet-state readout fidelity of 97.59% by the threshold method. We reveal the inherent deficiency of the threshold method for the relaxed triplet-state classification and introduce machine learning as a noise-resilient and relaxation-independent readout method to reduce the misjudgment. The readout fidelity for classifying the simulated single-shot traces can be improved to 99.67% by the machine-learning method, better than the threshold method of 97.54%, which is consistent with the experimental result. This work indicates that the machine-learning method can be a strong potential candidate for alleviating the restrictions of stably achieving high-fidelity and high-accuracy singlet-triplet-state readout in large-scale quantum computing.
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Spins in silicon quantum devices are promising candidates for large-scale quantum computing. Gate-based sensing of spin qubits offers a compact and scalable readout with high fidelity, however, further improvements in sensitivity are required to meet the fidelity thresholds and measurement timescales needed for the implementation of fast feedback in error correction protocols. Here, we combine radio-frequency gate-based sensing at 622 MHz with a Josephson parametric amplifier, that operates in the 500-800 MHz band, to reduce the integration time required to read the state of a silicon double quantum dot formed in a nanowire transistor. Based on our achieved signal-to-noise ratio, we estimate that singlet-triplet single-shot readout with an average fidelity of 99.7% could be performed in 1 μs, well below the requirements for fault-tolerant readout and 30 times faster than without the Josephson parametric amplifier. Additionally, the Josephson parametric amplifier allows operation at a lower radio-frequency power while maintaining identical signal-to-noise ratio. We determine a noise temperature of 200 mK with a contribution from the Josephson parametric amplifier (25%), cryogenic amplifier (25%) and the resonator (50%), showing routes to further increase the readout speed.
We report repeated single-shot measurements of the two-electron spin state in a GaAs double quantum dot. The readout allows measurement with a fidelity above 90% with a approximately 7 micros cycle time. Hyperfine-induced precession between singlet and triplet states of the two-electron system are directly observed, as nuclear Overhauser fields are quasistatic on the time scale of the measurement cycle. Repeated measurements on millisecond to second time scales reveal the evolution of the nuclear environment.