Bipartite nonlocality with a many-body system

TL;DR

This paper investigates the limitations of detecting bipartite quantum nonlocality in many-body systems with collective measurements, and proposes methods to overcome these restrictions for Bell inequality violations.

Contribution

It provides numerical evidence that first-order moment observables cannot reveal nonlocality in large ensembles, and offers a strategy to achieve Bell violations when higher-order measurements are used.

Findings

Bell inequalities cannot be violated with only first-order moments in large ensembles

A method to achieve Bell violations with split many-body systems when higher-order moments are accessible

Highlights the challenges in device-independent detection of quantum correlations in many-body systems

Abstract

We consider a bipartite scenario where two parties hold ensembles of $1/2$-spins which can only be measured collectively. We give numerical arguments supporting the conjecture that in this scenario no Bell inequality can be violated for arbitrary numbers of spins if only first order moment observables are available. We then give a recipe to achieve a significant Bell violation with a split many-body system when this restriction is lifted. This highlights the strong requirements needed to detect bipartite quantum correlations in many-body systems device-independently.