Observation of three-photon Greenberger-Horne-Zeilinger entanglement

We present the experimental observation of polarization entanglement for th ree spatially separated photons. Such states of more than two entangled par ticles, known as Greenberger-Horne-Zeilinger (GHZ) states, play a crucial r ole in fundamental tests of quantum mechanics versus local realism and in m any quantum information and quantum computation schemes. Our experimental a rrangement is such that we start with two pairs of entangled photons and re gister the photons in a way that any information as to which pair each phot on belongs to is erased. After detecting a trigger photon, the registered e vents at the detectors for the remaining three photons exhibit the desired GHZ correlations. [S0031-9007(98)08348-3].