Minimum number of input states required for quantum gate characterization

TL;DR

This paper presents an algebraic framework showing that only two specific input states are needed to estimate the average fidelity of a quantum gate, significantly reducing the complexity of quantum device characterization.

Contribution

It introduces a minimal set of input states for quantum gate fidelity estimation, including practical sets for bounds, simplifying quantum device assessment.

Findings

Two input states suffice for fidelity estimation

A minimal set includes a totally mixed thermal state

Practical reduced sets enable bounds on fidelity

Abstract

We derive an algebraic framework which identifies the minimal information required to assess how well a quantum device implements a desired quantum operation. Our approach is based on characterizing only the unitary part of an open system's evolution. We show that a reduced set of input states is sufficient to estimate the average fidelity of a quantum gate, avoiding a sampling over the full Liouville space. Surprisingly, the minimal set consists of only two input states, independent of the Hilbert space dimension. The minimal set is, however, impractical for device characterization since one of the states is a totally mixed thermal state and extracting bounds for the average fidelity is impossible. We therefore present two further reduced sets of input states that allow, respectively, for numerical and analytical bounds on the average fidelity.