We observe polarization-entanglement between four photons produced from a single down-conversion source. The non-classical correlations between the measurement results violate a generalized Bell inequality for four qubits. The characteristic properties and its easy generation with high interferometric contrast make the observed four-photon state well-suited for implementing advanced quantum communication schemes such as multi-party quantum key distribution, secret sharing and telecloning.
@article{arxiv.quant-ph/0302042,
title = {Experimental observation of four-photon entanglement from down-conversion},
author = {M. Eibl and S. Gaertner and M. Bourennane and Ch. Kurtsiefer and M. Zukowski and H. Weinfurter},
journal= {arXiv preprint arXiv:quant-ph/0302042},
year = {2009}
}