A measure of physical reality

TL;DR

This paper introduces a tomography-based measure of the indefiniteness of observables in quantum states, linking local reality, quantum discord, and nonlocality, revealing nonlocal features beyond Bell inequalities.

Contribution

It proposes a novel quantifier for the indefiniteness of observables and establishes its relation to quantum discord and nonlocality, providing new insights into quantum correlations.

Findings

Reality can be inferred locally only if the observable is not quantumly correlated with an informer.

The measure signals nonlocality even in separable states, beyond Bell inequality violations.

Quantum discord precludes Einstein's separable realities, affecting local reality inference.

Abstract

From the premise that an observable is real after it is measured, we envisage a tomography-based protocol that allows us to propose a quantifier for the degree of indefiniteness of an observable given a quantum state. Then, we find that the reality of an observable can be inferred locally only if this observable is not quantumly correlated with an informer. In other words, quantum discord precludes Einstein's notion of separable realities. Also, by monitoring changes in the local reality upon measurements on a remote system, we are led to define a measure of nonlocality. Proved upper bounded by discordlike correlations and requiring indefiniteness as a basic ingredient, our measure signals nonlocality even for separable states, thus revealing nonlocal aspects that are not captured by Bell inequality violations.