Local orthogonality as a multipartite principle for quantum correlations

TL;DR

This paper introduces local orthogonality, a multipartite principle that refines our understanding of quantum correlations beyond bipartite constraints, revealing non-quantum correlations in complex systems.

Contribution

It defines local orthogonality as a new multipartite principle and demonstrates its effectiveness in distinguishing quantum from non-quantum correlations.

Findings

Local orthogonality is equivalent to no-signaling for two parties.

It is more restrictive than no-signaling for multiple parties.

Some supra-quantum correlations violate local orthogonality.

Abstract

In recent years, the use of information principles to understand quantum correlations has been very successful. Unfortunately, all principles considered so far have a bipartite formulation, but intrinsically multipartite principles, yet to be discovered, are necessary for reproducing quantum correlations. Here, we introduce local orthogonality, an intrinsically multipartite principle stating that events involving different outcomes of the same local measurement must be exclusive, or orthogonal. We prove that it is equivalent to no-signaling in the bipartite scenario but more restrictive for more than two parties. By exploiting this non-equivalence, it is then demonstrated that some bipartite supra-quantum correlations do violate local orthogonality when distributed among several parties. Finally, we show how its multipartite character allows revealing the non-quantumness of correlations for which any bipartite principle fails. We believe that local orthogonality is a crucial ingredient for understanding no-signaling and quantum correlations.