A versatile construction of Bell inequalities for the multipartite scenario

TL;DR

This paper introduces a flexible method to construct Bell inequalities for multipartite quantum systems, enabling detection of various levels of nonlocality and establishing a link between entanglement and nonlocality.

Contribution

The authors develop a versatile technique to generate families of Bell inequalities from known seeds, applicable to any number of parties, to identify genuine multipartite nonlocality.

Findings

Constructed Bell inequalities detect genuine multipartite nonlocality.

Analytical proof that certain entangled states violate these inequalities.

Numerical evidence shows violation for all genuine multipartite entangled states of 3 and 4 qubits.

Abstract

Local measurements acting on entangled quantum states give rise to a rich correlation structure in the multipartite scenario. We introduce a versatile technique to build families of Bell inequalities witnessing different notions of multipartite nonlocality for any number of parties. The idea behind our method is simple: a known Bell inequality satisfying certain constraints, for example the Clauser-Horne-Shimony-Holt inequality, serves as the $seed$ to build new families of inequalities for more parties. The constructed inequalities have a clear operational meaning, capturing an essential feature of multipartite correlations: their violation implies that numerous subgroups of parties violate the inequality chosen as seed. The more multipartite nonlocal the correlations, the more subgroups can violate the seed. We illustrate our construction using different seeds and designing Bell inequalities to detect $m$-way nonlocal multipartite correlations, in particular, $genuine$ $multipartite$ $nonlocal$ correlations -- the strongest notion of multipartite nonlocality. For one of our inequalities we prove analytically that a large class of pure states that are genuine multipartite entangled exhibit genuine multipartite nonlocality for any number of parties, even for some states that are almost product. We also provide numerical evidence that this family is violated by all genuine multipartite entangled pure states of three and four qubits. Our results make us conjecture that this family of Bell inequalities can be used to prove the equivalence between genuine multipartite pure-state entanglement and nonlocality for any number of parties.