Nonlocal correlations in a macroscopic measurement scenario

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) behaviour 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., Cirel'son bound 2 √ 2), but in the macroscopic limit gives rise to a value which is higher than 2 √ 2. Such correlations are therefore not considered as physical. Our study thus provides a sufficient criterion to identify some of unphysical correlations.

Geometry of the quantum set on no-signaling faces

“…However, a generalization of this principle [13] leads to tighter conditions which enables detecting some postquantum nonlocal correlations where Uffink's inequality fails. On the other hand, it is known that the set of correlations defined by Uffink's inequality is a convex set but it is not closed under wirings [25]; this implies that some correlations which do not violate Uffink's inequality can do so [25,61] by nonlocality distillation [62][63][64]. We will focus on testing the necessary condition for respecting the information causality principle, i.e., Uffink's inequality.…”