Entangled states are key elements in the field of quantum information processing. The experimental preparation, manipulation and detection of multi-photon entangled states are of great interest for implementations of quantum communication schemes, quantum cryptographic protocols, and for fundamental tests of quantum theory.
Here, we show how to create three and four photon entangled states using parametric down conversion. These quantum states are used to violate Bell inequalities, for decoherence-free quantum communication and for teleportation-cloning protocols.