Order preserving maps on quantum measurements

TL;DR

This paper investigates the structure of quantum measurements under the post-processing order, introducing order-preserving maps to simplify the analysis of measurement incompatibility and improving existing criteria.

Contribution

It introduces a new approach using order-preserving maps to analyze quantum measurements, extending and optimizing Zhu's Fisher information-based incompatibility criterion.

Findings

The Fisher information map is an order morphism into positive semidefinite matrices.

The incompatibility criterion is improved by incorporating outcome number constraints.

The construction is generalized to other ordered vector spaces and shown to be optimal among quadratic maps.

Abstract

We study the partially ordered set of equivalence classes of quantum measurements endowed with the post-processing partial order. The post-processing order is fundamental as it enables to compare measurements by their intrinsic noise and it gives grounds to define the important concept of quantum incompatibility. Our approach is based on mapping this set into a simpler partially ordered set using an order preserving map and investigating the resulting image. The aim is to ignore unnecessary details while keeping the essential structure, thereby simplifying e.g. detection of incompatibility. One possible choice is the map based on Fisher information introduced by Huangjun Zhu, known to be an order morphism taking values in the cone of positive semidefinite matrices. We explore the properties of that construction and improve Zhu's incompatibility criterion by adding a constraint depending on the number of measurement outcomes. We generalize this type of construction to other ordered vector spaces and we show that this map is optimal among all quadratic maps.