Estimating the Coherence of Noise

TL;DR

This paper introduces a method to quantify the coherence of noise in quantum systems using unitarity, which can be efficiently estimated and provides bounds on gate fidelity.

Contribution

It defines unitarity as a measure of noise coherence, relates it to purity changes, and presents a robust randomized benchmarking protocol for its estimation.

Findings

Unitarity effectively quantifies noise coherence.

The protocol for estimating unitarity is efficient and robust.

Unitarity bounds the minimal gate infidelity achievable.

Abstract

Noise mechanisms in quantum systems can be broadly characterized as either coherent (i.e., unitary) or incoherent. For a given fixed average error rate, coherent noise mechanisms will generally lead to a larger worst-case error than incoherent noise. We show that the coherence of a noise source can be quantified by the unitarity, which we relate to the average change in purity averaged over input pure states. We then show that the unitarity can be efficiently estimated using a protocol based on randomized benchmarking that is efficient and robust to state-preparation and measurement errors. We also show that the unitarity provides a lower bound on the optimal achievable gate infidelity under a given noisy process.