Silicon qubit fidelities approaching incoherent noise limits via pulse engineering (1807.09500v3)

Published 25 Jul 2018 in cond-mat.mes-hall and quant-ph

Abstract: The performance requirements for fault-tolerant quantum computing are very stringent. Qubits must be manipulated, coupled, and measured with error rates well below 1%. For semiconductor implementations, silicon quantum dot spin qubits have demonstrated average single-qubit Clifford gate error rates that approach this threshold, notably with error rates of 0.14% in isotopically enriched $^{28}$Si/SiGe devices. This gate performance, together with high-fidelity two-qubit gates and measurements, is only known to meet the threshold for fault-tolerant quantum computing in some architectures when assuming that the noise is incoherent, and still lower error rates are needed to reduce overhead. Here we experimentally show that pulse engineering techniques, widely used in magnetic resonance, improve average Clifford gate error rates for silicon quantum dot spin qubits to 0.043%,a factor of 3 improvement on previous best results for silicon quantum dot devices. By including tomographically complete measurements in randomised benchmarking, we infer a higher-order feature of the noise called the unitarity, which measures the coherence of noise. This in turn allows us to theoretically predict that average gate error rates as low as 0.026% may be achievable with further pulse improvements. These fidelities are ultimately limited by Markovian noise, which we attribute to charge noise emanating from the silicon device structure itself, or the environment.

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Summary

Silicon Qubit Fidelities Approaching Incoherent Noise Limits via Pulse Engineering

The paper under review presents an experimental investigation into the enhancement of silicon qubit gate fidelities using pulse engineering techniques. This paper is particularly relevant for quantum computing systems where achieving error rates below the threshold for fault-tolerant computing is imperative. The research focuses on silicon quantum dot spin qubits, which have previously demonstrated promising low error rates, but require further improvement to meet the stringent requirements of large-scale, fault-tolerant quantum computing.

Key Contributions and Methods

The authors employ pulse engineering, specifically Gradient Ascent Pulse Engineering (GRAPE), to optimize qubit control pulses against quasi-static noise. This method systematically adjusts the pulse sequences to increase the resilience of the manipulated qubits to prevalent low-frequency noise. The paper reports achieving an average single-qubit Clifford gate error rate of 0.043%, representing a threefold improvement over the previously recorded best for silicon quantum dot devices. The research further suggests that these improvements could reduce gate error rates to as low as 0.026% with additional refinement of the pulse sequences.

An innovative aspect of the paper is the utilization of tomographically complete measurements in the randomised benchmarking protocol to extract higher-order noise characteristics, specifically the unitarity. Unitarity provides a quantification of the coherence of noise sources, in contrast to standard average gate fidelity measurements, which primarily indicate overall infidelity. This higher-order analysis corroborates that pulse engineering effectively mitigates coherent noise components, affirming the potential for control optimizations in minimizing gate errors.

Numerical Results and Implications

The research highlights a critical numerical achievement: reducing the randomised benchmarking decay factor to 99.914%, correlating with an average per-Clifford gate fidelity of 99.957%. These findings establish a benchmark for silicon-based qubit systems, offering a path toward attaining the criteria necessary for fault-tolerant architectures. The paper underscores the significance of characterizing noise through unitarity, identifying that the enhanced gates have primarily impacted the reduction of unitary noise, which remains a major component of fidelity losses even post-optimization.

Future Directions and Speculations

The implications of this research are profound for both practical and theoretical advancements in quantum computing. Practically, the techniques and results advocate for further exploration of pulse sequence optimization and noise characterization across various qubit platforms. Future investigations could focus on the integration of such enhanced gates in larger, multi-qubit systems to assess scalability and impact on overall computational fidelity.

Theoretically, the approach of leveraging unitarity as a measure of noise opens new possibilities in the systematic diagnosis and reduction of coherent error sources across quantum systems. This could lead to refined models for noise in quantum gates and inform the development of more accurate predictive tools for gate performance under various environmental conditions.