On the tightness of correlation inequalities with no quantum violation

TL;DR

This paper investigates the structure of quantum correlations and Bell inequalities with no quantum violation, revealing that some well-known non-violation examples are not tight and exploring the geometric properties of these inequalities.

Contribution

It introduces a bound on the quantum value of linear games, shows non-local computation games are not tight Bell inequalities, and relates non-contextual inequalities to cut polytopes, advancing understanding of quantum correlation faces.

Findings

Non-local computation games do not form tight Bell inequalities.

A bound on quantum value of linear games based on game matrix norms.

Existence of tight non-contextuality inequalities with no quantum violation.

Abstract

We study the faces of the set of quantum correlations, i.e., the Bell and noncontextuality inequalities without any quantum violation. First, we investigate the question whether every proper (tight) Bell inequality for two parties, other than the trivial ones from positivity, normalization and no-signaling can be violated by quantum correlations, i.e., whether the classical Bell polytope or the smaller correlation polytope share any facets with their respective quantum sets. To do this, we develop a recently derived bound on the quantum value of linear games based on the norms of game matrices to give a simple sufficient condition to identify linear games with no quantum advantage. Additionally we show how this bound can be extended to the general class of unique games, illustrating it for the case of three outcomes. We then show as a main result that the paradigmatic examples of correlation Bell inequalities with no quantum violation, namely the non-local computation games do not constitute tight Bell inequalities, not even for the correlation polytope. We also extend this to an arbitrary prime number of outcomes for a specific class of these games. We then study the faces in the simplest CHSH Bell scenario of binary dichotomic measurements, and identify edges in the set of quantum correlations in this scenario. Finally, we relate the non-contextual polytope of single-party correlation inequalities with the cut polytope $CUT(\nabla G)$, where $G$ denotes the compatibility graph of observables in the contextuality scenario and $\nabla G$ denotes the suspension graph of $G$. We observe that there exist tight non-contextuality inequalities with no quantum violation, and furthermore that this set of inequalities is beyond those implied by the Consistent Exclusivity principle.