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. © 2007 The American Physical Society.