Estimating Detector Error Models on Google's Willow

TL;DR

This paper develops algorithms to estimate Detector Error Models directly from syndromes, applies them to Google's quantum chips, and reveals insights into error correlations and artifacts affecting quantum error correction.

Contribution

It formalizes algorithms for DEM estimation from syndromes without decoders and applies them to real quantum hardware, uncovering error correlations and artifacts.

Findings

DEM estimates align closely with unseen syndromes

Long-range detector correlations are identified on 105-qubit chip

Artifacts such as detector pair flips and radiation signatures are observed

Abstract

We consolidate recent theoretical advances in Detector Error Model (DEM) estimation and formalize several algorithms to learn DEM parameters and structure from syndromes without using a decoder, demonstrating recovery of known DEMs from simulated syndromes with precision limited only by finite-sample effects. We then apply these algorithms to estimate DEMs from Google's 72- and 105-qubit chips. Using a likelihood function that is tractable for small DEMs, we show that DEMs estimated directly from syndromes agree more closely with unseen syndromes than DEMs trained to optimize logical performance, whereas the latter outperform the former as priors for decoders in logical memory experiments. We used a time-series of estimated DEMs to track both global error and specific local errors over the course of a QEC experiment, suggesting applications in online characterization. We employ a sequence of DEM estimation techniques to discover and quantify long-range detector correlations spanning the width of the 105-qubit chip, for which DEM analysis suggests correlated measurement errors rather than high-weight Pauli errors as the most likely explanation. Finally, we present two artifacts in repetition code syndromes that are \emph{not} well-modeled by a DEM: correlated flipping of pairs of adjacent detectors in many consecutive rounds of QEC, and signatures consistent with radiation events occurring more frequently than previously reported.