Detailed, interpretable characterization of mid-circuit measurement on a transmon qubit

TL;DR

This paper introduces a method to interpret mid-circuit measurement errors on transmon qubits using an adapted error generator formalism, revealing physical error mechanisms and their dependence on measurement parameters.

Contribution

It adapts the error generator formalism to characterize and interpret mid-circuit measurement errors in quantum devices, providing physically meaningful insights.

Findings

Identified dominant error mechanisms including amplitude damping and readout errors.

Demonstrated how error magnitudes vary with readout pulse amplitude.

Validated a simplified model with few parameters to describe measurement errors.

Abstract

Mid-circuit measurements (MCMs) are critical components of the quantum error correction protocols expected to enable utility-scale quantum computing. MCMs can be modeled by quantum instruments (a type of quantum operation or process), which can be characterized self-consistently using gate set tomography. However, experimentally estimated quantum instruments are often hard to interpret or relate to device physics. We address this challenge by adapting the error generator formalism -- previously used to interpret noisy quantum gates by decomposing their error processes into physically meaningful sums of "elementary errors" -- to MCMs. We deploy our new analysis on a transmon qubit device to tease out and quantify error mechanisms including amplitude damping, readout error, and imperfect collapse. We examine in detail how the magnitudes of these errors vary with the readout pulse amplitude, recover the key features of dispersive readout predicted by theory, and show that these features can be modeled parsimoniously using a reduced model with just a few parameters.