Exposure of subtle multipartite quantum nonlocality

The celebrated Einstein-Podolsky-Rosen quantum steering has a complex structure in the multipartite scenario. We show that a naively defined criterion for multipartite steering allows, like in Bell nonlocality, for a contradictory effect whereby local operations could create steering seemingly from scratch. Nevertheless, neither in steering nor in Bell nonlocality has this effect been experimentally confirmed. Operational consistency is reestablished by presenting a suitable redefinition: there is a subtle form of steering already present at the start, and it is only exposed-as opposed to created-by the local operations. We devise protocols that, remarkably, are able to reveal, in seemingly unsteerable systems, not only steering, but also Bell nonlocality. Moreover, we find concrete cases where entanglement certification does not coincide with steering. A causal analysis reveals the crux of the issue to lie in hidden signaling. Finally, we implement one of the protocols with three photonic qubits deterministically, providing the experimental demonstration of both exposure and super-exposure of quantum nonlocality.

Automated summary

The study finds that there may be a contradictory effect in the standard definition of multipartite steering, where entanglement can be created in the system through local operations but has not been experimentally confirmed. By redefining this concept, operational consistency is reestablished, and hidden entanglement properties are revealed. In addition, there are protocols that can reveal entanglement and Bell nonlocality in seemingly unsteerable systems.