Experimental Observation of Four-Photon Entangled Dicke State with High Fidelity

We present the experimental observation of the symmetric four-photon entangled Dicke state with two excitations |D₄;{(2)}. A simple experimental setup allowed quantum state tomography yielding a fidelity as high as 0.844+/-0.008. We study the entanglement persistency of the state using novel witness operators and focus on the demonstration of a remarkable property: depending on the orientation of a measurement on one photon, the remaining three photons are projected into both inequivalent classes of genuine tripartite entanglement, the Greenberger-Horne-Zeilinger and W class. Furthermore, we discuss possible applications of |D₄;{(2)} in quantum communication.