Efficient characterization of quantum nondemolition qubit readout

TL;DR

This paper introduces an efficient method to quantitatively evaluate the performance of quantum nondemolition (QND) qubit readout using classical trace distance, enabling practical assessment of measurement fidelity and projectivity.

Contribution

It proposes a new experimentally efficient scheme to evaluate QND fidelity, readout fidelity, and projectivity from two consecutive measurements, with theoretical validation for superconducting qubits.

Findings

The method accurately quantifies QND fidelity in superconducting qubits.

The approach simplifies measurement performance evaluation.

Relationships among QND fidelity, readout fidelity, and projectivity are established.

Abstract

We study the quantitative characterization of the performance of qubit measurements in this paper. In particular, the back-action evading nature of quantum nondemolition (QND) readout of qubits is fully quantified by quantum trace distance. Only computational basis states are necessary to be taken into consideration. Most importantly, we propose an experimentally efficient method to evaluate the QND fidelity based on the classical trace distance, which uses an experimental scheme with two consecutive measurements. The three key quantifiers of a measurement, i.e., QND fidelity, readout fidelity, and projectivity, can be derived directly from the same experimental scheme. Besides, we present the relationships among these three factors. Theoretical simulation results for the dispersive readout of superconducting qubits show the validity of the proposed QND fidelity. Efficient quantification of measurement performance is of practical significance for the diagnosis, performance improvement, and design of measurement apparatuses.