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.
@article{arxiv.quant-ph/9810035,
title = {Observation of three-photon Greenberger-Horne-Zeilinger entanglement},
author = {Dik Bouwmeester and Jian-Wei Pan and Matthew Daniell and Harald Weinfurter and Anton Zeilinger},
journal= {arXiv preprint arXiv:quant-ph/9810035},
year = {2009}
}