Bounding the average gate fidelity of composite channels using the unitarity

TL;DR

This paper develops bounds on the average gate fidelity of composite quantum channels using unitarity, enhancing the precision of error characterization in quantum devices through efficient experimental protocols.

Contribution

It introduces a tighter bound on circuit fidelity based on unitarity, improving error estimation accuracy over existing methods.

Findings

Provides an optimal bound on total circuit fidelity from component fidelities.

Derives a tighter bound incorporating unitarity, applicable under coherence information.

Enhances precision in estimating individual gate errors in randomized benchmarking.

Abstract

There is currently a significant need for robust and efficient methods for characterizing quantum devices. While there has been significant progress in this direction, there remains a crucial need to precisely determine the strength and type of errors on individual gate operations, in order to assess and improve control as well as reliably bound the total error in a quantum circuit given some partial information about the errors on the components. In this work, we first provide an optimal bound on the total fidelity of a circuit in terms of component fidelities, which can be efficiently experimentally estimated via randomized benchmarking. We then derive a tighter bound that applies under additional information about the coherence of the error, namely, the unitarity, which can also be efficiently estimated via a related experimental protocol. This improved bound smoothly interpolates between the worst-case quadratic and best-case linear scalings for composite error channels. As an application we show how our analysis substantially improves the achievable precision on estimates of the error rates of individual gates under interleaved randomized benchmarking, enabling greater precision for current experimental methods to assess and tune-up control over quantum gate operations.