High-fidelity source of four-photon entanglement

A polarisation-entangled four-photon state can be generated directly by a second order parametric down-conversion process. We use this emission to characterise the properties of a four-qubit state and to analyse its entanglement based on the violation of a four-particle Bell inequality. The observed high count rates and the fidelity of the polarisation correlations are the basis for the realisation of several new multiparty quantum communications schemes, such as secure multiparty key distribution and quantum telecloning.