Optimal estimation of SU(d) using exact and approximate 2-designs

TL;DR

This paper demonstrates that using entangled states and 2-designs can significantly improve the estimation accuracy of SU(d) quantum operations, achieving a 1/n^2 error decay, and provides practical approximate designs for all dimensions.

Contribution

It introduces optimal estimation strategies for SU(d) using exact and approximate 2-designs, showing entanglement enhances precision and providing feasible input states when exact designs are unknown.

Findings

Entangled states achieve 1/n^2 error decay in estimation.

Spherical 2-designs enable optimal input state design.

Approximate 2-designs can approach optimal performance.

Abstract

We consider the problem of estimating an SU(d) quantum operation when n copies of it are available at the same time. It is well known that, if one uses a separable state as the input for the unitaries, the optimal mean square error will decrease as 1/n. However it is shown here that, if a proper entangled state is used, the optimal mean square error will decrease at a 1/n^2 rate. It is also shown that spherical 2-designs (e.g. complete sets of mutually unbiased bases and symmetric informationally complete positive operator valued measures) can be used to design optimal input states. Although 2-designs are believed to exist for every dimension, this has not yet been proven. Therefore, we give an alternative input state based on approximate 2-designs which can be made arbitrarily close to optimal. It is shown that measurement strategies which are based on local operations and classical communication between the ancilla and the rest of the system can be optimal.