  Bright, entangled multiphoton sources based on atom photon interactions are
an essential requirement in the realization of several quantum information and
quantum computation schemes based on photonic quantum systems. Here, we
experimentally demonstrate a four photon polarization entangled GHZ state
obtained from Doppler broadened atomic ensembles of 87Rb atoms. Owing to
collective two photon coherence in the Doppler broadened cascade-type atomic
system, our setup enables the generation of robust four photon GHZ states with
a fidelity of 0.82 and a measured four photon average coincidence rate of 0.58
Hz. We believe that the generation of such bright and stable multiphoton GHZ
states from atomic media is an important step toward realizing photonic quantum
computation and practical quantum networks based on atom photon interactions.
