Estimating the bias of CX gates via character randomized benchmarking

TL;DR

This paper introduces bias randomized benchmarking (BRB), a highly accurate, SPAM-error-immune method for measuring the bias of CX gates in biased-noise qubit systems, facilitating better error correction strategies.

Contribution

The paper proposes two novel BRB protocols, CX-dihedral BRB and interleaved bias RB, specifically designed to measure bias in CX gates for biased-noise qubits.

Findings

BRB accurately measures gate bias despite SPAM errors

Protocols are suitable for biased-noise qubit platforms like Kerr cats

Provides a middle ground between fidelity estimation and full noise tomography

Abstract

Recent work has demonstrated that high-threshold quantum error correction is possible for biased-noise qubits, provided one can implement a controlled-not (CX) gate that preserves the bias. Bias-preserving CX gates have been proposed for several biased-noise qubit platforms, most notably Kerr cats. However, experimentally measuring the noise bias is challenging as it requires accurately estimating certain low-probability Pauli errors in the presence of much larger state preparation and measurement (SPAM) errors. In this paper, we introduce bias randomized benchmarking (BRB) as a technique for measuring bias in quantum gates. BRB, like all RB protocols, is highly accurate and immune to SPAM errors. Our first protocol, CX-dihedral BRB, is a straightforward method to measure the bias of the entire CX-dihedral group. Our second protocol, interleaved bias randomized benchmarking (IBRB), is a generalization of interleaved RB tailored to the experimental constraints biased-noise qubits; this is a more involved procedure that directly targets the bias of the CX gate alone. Our BRB procedures occupy a middle ground between classic RB protocols that only estimate the average fidelity, and tomographic RB protocols that provide more detailed characterization of noise but require more measurements as well as experimental capabilities that are not necessarily available in biased-noise qubits.