Experimental Observation of Four-Photon Entanglement from Parametric Down-Conversion
Manfred Eibl, Sascha Gaertner, Mohamed Bourennane, Christian Kurtsiefer, Marek Żukowski, Harald Weinfurter · Physical Review Letters · 2003
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.