Study of nonlocal correlations in macroscopic measurement scenario

TL;DR

This paper investigates the behavior of nonlocal correlations in macroscopic limits, identifying which correlations are unphysical based on their violation of Bell inequalities when scaled up.

Contribution

It characterizes no-signaling correlations in the 2-2-2 scenario that appear classical microscopically but become non-physical macroscopically, providing criteria to identify unphysical correlations.

Findings

Correlations can violate Bell inequalities more strongly in the macroscopic limit.

Some correlations are unphysical because they exceed quantum bounds macroscopically.

The study offers a criterion to distinguish physical from unphysical correlations.

Abstract

Nonlocality is one of the main characteristic features of quantum systems involving more than one spatially separated subsystems. It is manifested theoretically as well as experimentally through violation of some local realistic inequality. On the other hand, classical behavior of all physical phenomena in the macroscopic limit gives a general intuition that any physical theory for describing microscopic phenomena should resemble classical physics in the macroscopic regime-- the so-called macro-realism. In the 2-2-2 scenario (two parties, each performing two measurements, each measurement with two outcomes), contemplating all the no-signaling correlations, we characterize which of them would exhibit classical (local-realistic) behavior in the macroscopic limit. Interestingly, we find correlations which at single copy level violate the Bell-Clauser-Horne-Shimony-Holt inequality by an amount less than optimal quantum violation (i.e. the Cirel'son bound $2\sqrt{2}$), but in the macroscopic limit gives rise to a value which is higher than $2\sqrt{2}$. Such correlations are therefore not considered as physical. Our study thus provides a sufficient criterion to identify some of unphysical correlations.