Multiphoton entanglement and interferometry

Abstract Multiphoton entanglement is the basis of many quantum communication schemes, quantum cryptographic protocols, and fundamental tests of quantum theory. Spontaneous parametric down‐conversion is the most effective source for polarization entangled photon pairs. Here we show, that a class of entangled 4‐photon states can be directly created by parametric down‐conversion. These states exhibit perfect quantum correlations and a high robustness of entanglement against photon loss. Therefore these states are well suited for new types of quantum communication.