Exposure of subtle multipartite quantum nonlocality

TL;DR

This paper clarifies the subtle structure of multipartite quantum nonlocality, redefining steering to avoid paradoxical effects, and demonstrates experimental protocols revealing hidden quantum correlations in photonic systems.

Contribution

It introduces a refined definition of multipartite steering, resolving previous contradictions, and experimentally demonstrates exposure of quantum nonlocality in photonic qubits.

Findings

Redefinition of multipartite steering avoids paradoxical effects.

Protocols reveal hidden steering and Bell nonlocality in unsteerable systems.

Experimental demonstration with three photonic qubits confirms theoretical predictions.

Abstract

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.