Quantum correlations are stronger than all nonsignaling correlations produced by n-outcome measurements

TL;DR

This paper demonstrates that quantum correlations with more outcomes can surpass all nonsignaling correlations from fewer outcomes, revealing a fundamental property of quantum measurements not shared by general probabilistic theories.

Contribution

It proves the existence of stronger quantum correlations with more outcomes than any n-outcome measurement, and shows these cannot be simulated by local selection from n-outcome measurements.

Findings

Quantum correlations with m>n outcomes can be stronger than any n-outcome nonsignaling correlations.

Some strong correlations predicted by the theory are observed in existing experiments.

A modified quantum theory with at most n-outcome measurements is proposed.

Abstract

We show that, for any n, there are m-outcome quantum correlations, with m>n, which are stronger than any nonsignaling correlation produced from selecting among n-outcome measurements. As a consequence, for any n, there are m-outcome quantum measurements that cannot be constructed by selecting locally from the set of n-outcome measurements. This is a property of the set of measurements in quantum theory that is not mandatory for general probabilistic theories. We also show that this prediction can be tested through high-precision Bell-type experiments and identify past experiments providing evidence that some of these strong correlations exist in nature. Finally, we provide a modified version of quantum theory restricted to having at most n-outcome quantum measurements.