Superactivation of multipartite unlockable bound entanglement.

The superactivation of multipartite bound entanglement (BE) is a special protocol proposed by Shor et al. in 2003, which can distill Einstein-Podolsky-Rosen (EPR) entanglement states between two subsystems of two multipartite BE states. Here we present the first experimental realization of the superactivation of the BE state, in which two copies of the four-partite unlockable BE state in a continuous-variable regime are used. Coupling two thermal states with Gaussian noises into two submodes of an EPR entangled state on two 50-50 beam splitters respectively, the four output optical modes form a four-partite unlocklable BE state. Using two EPR entangled states, we experimentally produce two BE states first. Then through a superactivation operation involving measurements and feedback on the two BE states, an EPR entangled state is distilled out between two designated parties of the two four-partite BE states. The experiment demonstrates the superadditivity of quantum entanglement as the individual BE state cannot be distilled, only two BE states together can be distilled.