title: Experimental observation of four-photon entanglement from parametric down-conversion.
abstract: We observe polarization entanglement between four photons produced from a single down-conversion source. The nonclassical 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 multiparty quantum key distribution, secret sharing, and telecloning.
authors.affiliation: Max-Planck-Institut für Quantenoptik, D-85748 Garching, Germany.
authors.forename: Manfred
authors.initials: M
authors.lastname: Eibl
authors.forename: Sascha
authors.initials: S
authors.lastname: Gaertner
authors.forename: Mohamed
authors.initials: M
authors.lastname: Bourennane
authors.forename: Christian
authors.initials: C
authors.lastname: Kurtsiefer
authors.forename: Marek
authors.initials: M
authors.lastname: Zukowski
authors.forename: Harald
authors.initials: H
authors.lastname: Weinfurter