Quantum theory of a polarization phase-gate in an atomic tripod   configuration
  We present the quantum theory of a polarization phase-gate that can be realized in a sample of ultracold rubidium atoms driven into a tripod configuration. The main advantages of this scheme are in its relative simplicity and inherent symmetry. It is shown that the conditional phase shifts of order $\pi$ can be attained. 
S. Rebic, D. Vitali, C. Ottaviani, P. Tombesi, M. Artoni, F.   Cataliotti and R. Corbalan