Independent State and Measurement Characterization for Quantum Computers

TL;DR

This paper introduces a protocol for separately estimating state preparation and measurement errors in quantum computers, even with imperfect gates, improving the reliability of quantum device characterization.

Contribution

It presents a novel method to independently characterize SPAM errors and resolve gauge ambiguities assuming a qubit uncorrelated with others.

Findings

Protocol successfully estimates SPAM errors on a five-qubit QPU

Bounds on SPAM error rates derived from gate error measures

Validation through comparison with simulations

Abstract

Correctly characterizing state preparation and measurement (SPAM) processes is a necessary step towards building reliable quantum processing units (QPUs). In this work, we discuss the subtleties behind separately measuring SPAM errors. We propose a protocol that can separately estimate SPAM errors, in the case where quantum gates are ideal. In the case where the quantum gates are imperfect, we derive bounds on the estimated SPAM error rates, based on gate error measures which can be estimated independently of SPAM processes. Our method shows that the gauge ambiguity in characterizing SPAM operations can be resolved, by assuming that there exists one qubit whose initial state is uncorrelated with other qubits in a QPU. We test the protocol on a publicly available five-qubit QPU and demonstrate its validity by comparing our results with simulations.