Observation of three-photon Greenberger-Horne-Zeilinger entanglement
  We present the experimental observation of polarization entanglement for three spatially separated photons. Such states of more than two entangled particles, known as GHZ states, play a crucial role in fundamental tests of quantum mechanics versus local realism and in many quantum information and quantum computation schemes. Our experimental arrangement is such that we start with two pairs of entangled photons and register one photon in a way that any information as to which pair it belongs to is erased. The registered events at the detectors for the remaining three photons then exhibit the desired GHZ correlations. 
Dik Bouwmeester, Jian-Wei Pan, Matthew Daniell, Harald Weinfurter,   Anton Zeilinger