Efficient evaluation of quantum observables using entangled measurements

TL;DR

This paper demonstrates that entangled measurements significantly improve the efficiency of evaluating quantum observables in variational algorithms on NISQ devices, both theoretically and experimentally.

Contribution

It introduces a method leveraging entangled measurements to enhance observable evaluation efficiency while maintaining low circuit depth.

Findings

Entangled measurements outperform separable measurements in observable evaluation.

Theoretical analysis shows covariance effects impact measurement quality.

Experimental results confirm efficiency gains with entangled measurements.

Abstract

The advent of cloud quantum computing has led to the rapid development of quantum algorithms. In particular, it is necessary to study variational quantum-classical hybrid algorithms, which are executable on noisy intermediate-scale quantum (NISQ) computers. Evaluations of observables appear frequently in the variational quantum-classical hybrid algorithms for NISQ computers. By speeding up the evaluation of observables, it is possible to realize a faster algorithm and save resources of quantum computers. Grouping of observables with separable measurements has been conventionally used, and the grouping with entangled measurements has also been proposed recently by several teams. In this paper, we show that entangled measurements enhance the efficiency of evaluation of observables, both theoretically and experimentally by taking into account the covariance effect, which may affect the quality of evaluation of observables. We also propose using a part of entangled measurements for grouping to keep the depth of extra gates constant. Our proposed method is expected to be used in conjunction with other related studies. We hope that entangled measurements would become crucial resources, not only for joint measurements but also for quantum information processing.