Necessary and sufficient criterion for extremal quantum correlations in the simplest Bell scenario

TL;DR

This paper introduces an analytical necessary and sufficient criterion for identifying extremal quantum correlations in the simplest Bell scenario, based on an information-theoretical guessing probability.

Contribution

It proposes a novel, analytical extremality condition for quantum correlations, linking it to a measurable guessing probability, advancing understanding of quantum nonlocality boundaries.

Findings

Bounds on guessing probability can be used to identify extremal correlations.

Numerical evidence supports the bounds as necessary and sufficient conditions.

The criterion provides a device-independent way to certify extremality.

Abstract

In the study of quantum nonlocality, one obstacle is that the analytical criterion for identifying the boundaries between quantum and postquantum correlations has not yet been given, even in the simplest Bell scenario. We propose a plausible, analytical, necessary and sufficient condition ensuring that a nonlocal quantum correlation in the simplest scenario is an extremal boundary point. Our extremality condition amounts to certifying an information-theoretical quantity; the probability of guessing a measurement outcome of a distant party optimized using any quantum instrument. We show that this quantity can be upper and lower bounded from any correlation in a device-independent way, and we use numerical calculations to confirm that coincidence of the upper and lower bounds appears to be necessary and sufficient for the extremality.