A Proposal for an Optical Implementation of an Universal Quantum Phase   Gate
  Large optical nonlinearities occurring in a coherently prepared atomic system are shown to produce phase shifts of order $\pi$. Such an effect may be observed in ultracold rubidium atoms where it could be feasibly exploited toward the realization of a polarization phase gate. 
S. Rebic, D. Vitali, C. Ottaviani, P. Tombesi, M. Artoni, F.   Cataliotti and R. Corbalan