Exploring Bell Nonlocality with Extremal Non-Signaling Boxes

TL;DR

This paper characterizes extremal non-signaling boxes in bipartite Bell scenarios, revealing their foundational implications and limitations, despite being nonphysical in quantum theory, and explores their role in nonlocality and communication complexity.

Contribution

It provides a complete classification of ENS boxes in various scenarios and demonstrates their significance in foundational questions of Bell nonlocality.

Findings

ENS boxes violate exclusivity and Specker's principles with two copies

Minimal decomposition of the magic square correlation using ENS boxes

Identifies the minimal scenario where communication cannot simulate ENS boxes

Abstract

Extremal non-signaling (ENS) boxes are correlations that correspond to vertices of the non-signaling polytope of a Bell scenario. Neither quantum theory nor any theory for ideal measurements allows for ENS boxes. That is, according to quantum theory, ENS boxes are nonphysical. Still, ENS boxes are crucial for addressing a number of problems in Bell nonlocality. Here, we obtain ENS boxes in arbitrary bipartite Bell scenarios and present the complete list of ENS boxes for several unexplored scenarios. Equipped with the boxes, we revisit several foundational questions. We find that already two copies of any ENS box violate the exclusivity (or local orthogonality) and Specker's principles. We provide the minimal decomposition of the magic square correlation - the simplest known perfect correlation in nature - in terms of ENS boxes. We identify the minimal scenario in which a dit of communication (with d < 6) is insufficient to simulate ENS boxes. Our results show that the ENS boxes approach leads to new results and opens new avenues for research.