Do all entangled states violate local realism?
Despite considerable research efforts the relation between the quantum entanglement and non-locality is largely unexplored. Among the open questions is: Which quantum states of composite systems are entangled and which of those are non-local? Understanding this relation is not only of importance for fundamental research, but also in the context of quantum information processing. For certain tasks, such as quantum communication complexity problems, distillation of entanglement or device-independent quantum key distribution, entangled states are useful only to the extent that they exhibit nonlocal correlations.
It is known that all pure bipartite entangled states violate the Bell inequalities. For mixed states, however, the relation between entanglement and non-locality is much subtler. In 1989 Werner constructed a family of bipartite mixed states, which, while being entangled, yield outcomes that allow a local realistic description. We found that for three or more particles even the relation between pure entanglement and locality becomes subtle. We constructed a family of multipartite pure entangled states, which satisfy all Bell’s inequalities with two measurement settings per observer. All these strongly suggest that entanglement and non-locality are different concepts.
M. Zukowski, C. Brukner, W. Laskowski & M. Wiesniak
Do All Pure Entangled States Violate Bell's Inequalities for Correlation Functions?
