Session R38: DQI Invited Session: Quantum State Preparation and Measurement (SPAM) in Semiconductor Qubits

Quantifying high-fidelity state preparation and measurement in triple-quantum-dot qubits

Exchange-only triple-quantum-dot spin qubits are a promising approach to scalable quantum information processing. In such systems, state preparation is generally performed by biasing the device to exchange electrons with a cold electron reservoir, while measurement is realized by spin-to-charge conversion and detection with a nearby electrometer. In this talk I will discuss implementation of these techniques in isotopically-enriched Si/SiGe triple dots using a direct dot-sensor electrometer with a cryogenic HEMT amplifier. I will examine the non-idealities in these operations, and explain how those factors can be hidden or revealed by choice of metric, showing data which exhibits SPAM infidelity in the low parts-per-thousand level. Further, I’ll detail a budget for what performance is expected, given properties of the device and instrumentation.   

Quantum non-demolition measurement of an electron spin qubit

While single-shot spin detection of electron spin qubits in semiconductor quantum dots is now a laboratory routine, the need for quantum error correction in a large-scale system demands a quantum non-demolition (QND) implementation. The QND spin readout imposes minimal disturbance to the probed spin polarization and can therefore be repeated to extinguish measurement errors. Furthermore, it also works as a high-fidelity state preparation device. However, its realization is challenging as it involves exquisite exposure of the system to the external readout circuitry while maintaining the qubit coherence and integrity. In this talk, we show that an electron spin qubit in quantum dots can be measured in a highly non-demolition manner by probing a neighboring electron spin qubit Ising-coupled to the qubit spin. The high QND fidelity enables enhancement of the measurement fidelity of a single electron spin state by repeating the readout process several dozens of times. We describe the analysis method based on statistical inference, which is crucial to achieve the optimal performance in the presence of the qubit relaxation. We will further discuss how such QND measurements can improve the fidelity of measurement-based state preparation in addition to the measurement fidelity.   

Time-domain Multiplexing and Optimization of Gate-based Sensing for Silicon-based Quantum Devices

In the quest for scaling up silicon-based quantum computing, readout by already existingvgate electrodes has gained prominence due to its reduced impact in the qubit layout and comparable sensitivities to conventional charge sensors. Recently, single-shot spin readout has been achieved with this technique [1-3] but further improvements are necessary, first, to meet the fidelity thresholds and timescales needed for the implementation of fast feedback in error correction protocols and, secondly, to be able to scale the method.
In this talk, I will first present the steps we have taken to reduce the measurement timescales of gate-based sensors beyond the state-of-the-art using silicon CMOS systems and off-chip superconducting technology. By combining the large gate lever arm of CMOS devices with high Q electrical resonators [4] and Josephson parametric amplification in the 600 MHz range, we reduce the integration time required to read the state of a silicon double quantum dot below the thresholds for fault-tolerant readout [5].
Secondly, I will present a strategy based on time-domain multiplexing to scale gate-based sensing. Here, we interfaced a single electrical resonator to two silicon quantum devices via digital transistors at milikelvin temperatures to demonstrate the building blocks of a dynamic random access architecture for efficient readout of complex quantum circuits. Our results demonstrate a path to reducing the number of resonators required to read large arrays of devices and facilitate scaling.

References
[1] A. West, et al. Nat Nano 14 437 (2019)
[2] P. Pakkiam, et al. PRX 8 041032 (2018)
[3] G. Zheng et al., Nat Nano 14 742 (2019)
[4] I. Ahmed, et al. Phys. Rev. App. 10, 014018 (2018).
[5] Schaal et al. Phys Rev Lett 124 067701 (2020)
[6] Schaal et al. Nat Elect 2 236 (2019)