On the quantumness of multiparameter estimation problems for qubit systems

TL;DR

This paper investigates the quantumness of multiparameter estimation in qubit systems by evaluating a measure of incompatibility, R, and compares it with bounds to understand its effectiveness in characterizing quantum estimation problems.

Contribution

It introduces and evaluates the quantumness measure R for various qubit estimation problems, comparing it with established bounds to assess its usefulness.

Findings

R often equals the difference between bounds, indicating its effectiveness.

In some models, R overestimates incompatibility due to loose bounds.

Results support R as a useful tool for characterizing quantum estimation incompatibility.

Abstract

The estimation of more than one parameter in quantum mechanics is a fundamental problem with relevant practical applications. In fact, the ultimate limits in the achievable estimation precision are ultimately linked with the non-commutativity of different observables, a peculiar property of quantum mechanics. We here consider several estimation problems for qubit systems and evaluate the corresponding quantumness R, a measure that has been recently introduced in order to quantify how much incompatible are the parameters to be estimated. In particular, R is an upper bound for the renormalized difference between the (asymptotically achievable) Holevo bound and the SLD Cram\'er-Rao bound (i.e. the matrix generalization of the single-parameter quantum Cram\'er-Rao bound). For all the estimation problems considered, we evaluate the quantumness R and, in order to better understand its usefulness in characterizing a multiparameter quantum statistical model, we compare it with the renormalized difference between the Holevo and the SLD-bound. Our results give evidence that R is a useful quantity to characterize multiparameter estimation problems, as for several quantum statistical model it is equal to the difference between the bounds and, in general, their behaviour qualitatively coincide. On the other hand, we also find evidence that for certain quantum statistical models the bound is not in tight, and thus R may overestimate the degree of quantum incompatibility between parameters.