Experimental bound entanglement in a four-photon state.

Bound entanglement is central to many exciting theoretical results in quantum information processing, but has thus far not been experimentally realized. In this work, we consider a one-parameter family of four-qubit Smolin states. We experimentally produce these states in the polarization of four optical photons produced from parametric down-conversion. Within a range of the parameter, we show that our states are entangled and undistillable, and thus bound entangled. Using these bound-entangled states we demonstrate entanglement unlocking.