Measures of macroscopicity for quantum spin systems

TL;DR

This paper explores how to quantify macroscopicity in large quantum spin systems, proposing Fisher information as a measure and clarifying distinctions among different types of macroscopic quantum states.

Contribution

It introduces Fisher information as a new measure for macroscopicity and clarifies the hierarchy and distinctions among various measures and types of macroscopic quantum states.

Findings

Fisher information is a suitable measure for macroscopicity.

A hierarchy among existing measures is established.

Distinction between macroscopic quantum states and superpositions is emphasized.

Abstract

We investigate the notion of "macroscopicity" in the case of large quantum spin systems and provide two main results. First, we motivate the Fisher information as a measure for the macroscopicity of quantum states. Second, we compare the existing literature of this topic. We report on a hierarchy among the measures and we conclude that one should carefully distinguish between "macroscopic quantum states" and "macroscopic superpositions", which is a strict subclass of the former.