Qualitative equivalence between incompatibility and Bell nonlocality

Shiv Akshar Yadavalli, Nikola Andrejić, Ravi Kunjwal

Abstract: Measurements in quantum theory can fail to be jointly measurable. Like entanglement, this incompatibility of measurements is necessary but not sufficient for violating Bell inequalities. The (in)compatibility relations among a set of measurements can be represented by a joint measurability structure, i.e., a hypergraph whose vertices denote measurements and hyperedges denote all and only compatible sets of measurements. Since incompatibility is necessary for a Bell violation, the joint measurability structure on each wing of a Bell experiment must necessarily be nontrivial, i.e., it must admit a subset of incompatible vertices. Here we show that, for any nontrivial joint measurability structure with a finite set of vertices, there exists a quantum realization with a set of measurements that enables a Bell violation, i.e., given that Alice has access to this incompatible set of measurements, there exists a set of measurements for Bob and an entangled state shared between them such that they can jointly violate a Bell inequality. Hence, a nontrivial joint measurability structure is not only necessary for a Bell violation, but also sufficient.