FOUR PHOTON POLARIZATION ENTANGLEMENT TESTS AND APPLICATIONS
Mohamed Bourennane, Manfred Eibl, Sascha Gaertner, Christian Kurtsiefer, Harald Weinfurter, Adán Cabello, Otfried Gühne, P. Hyllus, Dagmar Bruß, Maciej Lewenstein, Anna Sanpera · International Journal of Quantum Information · 2004
Multipartite entangled states are key elements for quantum information processing. Here we experimentally investigate the particular properties of a polarization-entangled four-photon state, which can be generated directly by second order parametric down-conversion. The perfect correlations and the invariance under local transformations enable one to encode one qubit of quantum information in a decoherence-free subspace and thus to communicate it safely over noisy quantum channels. Furthermore, we present an experimental method to detect genuine fourpartite entanglement using entanglement witness operators. The implementation of such operators requires only of few local polarization measurements but uniquely proves the genuine multipartite entanglement.