Characterizing Coherent Errors using Matrix-Element Amplification

TL;DR

This paper introduces MEADD, a new protocol combining dynamical decoupling with matrix-element amplification, significantly improving the precision of quantum gate error characterization on superconducting qubits.

Contribution

The paper presents MEADD, a novel method that enhances the accuracy of systematic error estimation in quantum gates by mitigating noise and reducing measurement complexity.

Findings

MEADD achieves 5-10x better accuracy in estimating coherent errors.

It reaches a precision below one milliradian for single- and two-qubit gates.

Successfully detects previously unmeasurable coherent crosstalk.

Abstract

Repeating a gate sequence multiple times amplifies systematic errors coherently, making it a useful tool for characterizing quantum gates. However, the precision of such an approach is limited by low-frequency noises, while its efficiency hindered by time-consuming scans required to match up the phases of the off-diagonal matrix elements being amplified. Here, we overcome both challenges by interleaving the gate of interest with dynamical decoupling sequences in a protocol we call Matrix-Element Amplification using Dynamical Decoupling (MEADD). Using frequency-tunable superconducting qubits from a Google Sycamore quantum processor, we experimentally demonstrate that MEADD surpasses the accuracy and precision of existing characterization protocols for estimating systematic errors in single- and two-qubit gates. In particular, MEADD yields factors of 5 to 10 improvements in estimating coherent parameters of the $\mathrm{CZ}$ gates compared to existing methods, reaching a precision below one milliradian. We also use it to characterize coherent crosstalk in the processor which was previously too small to detect reliably.