Experimental Bound Entanglement in a Four-Photon State by Jonathan Lavoie, Rainer Kaltenbaek, Marco Piani, and Kevin Resch was published today in Physical Review Letters.
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.
An experimental 4-photon bound entangled state from a family of noisy Smolin states