Quantum correlations on the no-signaling boundary: self-testing and more

TL;DR

This paper investigates the geometry of quantum correlations on the no-signaling boundary, demonstrating the possibility of self-testing in new classes of correlations, and characterizing the set of quantum correlations from maximally entangled states.

Contribution

It introduces a family of quantum strategies on the no-signaling boundary, proves self-testing beyond Hardy correlations, and characterizes quantum correlations from maximally entangled states.

Findings

Self-testing is possible in all nontrivial classes beyond Hardy correlations.

All correlations from maximally entangled states are convex combinations of Bell pair strategies.

Maximal CHSH violation is achieved by any maximally entangled two-qudit state.

Abstract

In device-independent quantum information, correlations between local measurement outcomes observed by spatially separated parties in a Bell test play a fundamental role. Even though it is long-known that the set of correlations allowed in quantum theory lies strictly between the Bell-local set and the no-signaling set, many questions concerning the geometry of the quantum set remain unanswered. Here, we revisit the problem of when the boundary of the quantum set coincides with the no-signaling set in the simplest Bell scenario. In particular, for each Class of these common boundaries containing $k$ zero probabilities, we provide a $(5-k)$-parameter family of quantum strategies realizing these (extremal) correlations. We further prove that self-testing is possible in all nontrivial Classes beyond the known examples of Hardy-type correlations, and provide numerical evidence supporting the robustness of these self-testing results. Candidates of one-parameter families of self-testing correlations from some of these Classes are identified. As a byproduct of our investigation, if the qubit strategies leading to an extremal nonlocal correlation are local-unitarily equivalent, a self-testing statement provably follows. Interestingly, all these self-testing correlations found on the no-signaling boundary are provably non-exposed. An analogous characterization for the set $\mathcal{M}$ of quantum correlations arising from finite-dimensional maximally entangled states is also provided. En route to establishing this last result, we show that all correlations of $\mathcal{M}$ in the simplest Bell scenario are attainable as convex combinations of those achievable using a Bell pair and projective measurements. In turn, we obtain the maximal Clauser-Horne-Shimony-Holt Bell inequality violation by any maximally entangled two-qudit state and a no-go theorem regarding the self-testing of such states.