Feasibility of single-photon cross-phase modulation using metastable   xenon in a high finesse cavity
  Cross-phase modulation at the single-photon level has a wide variety of fundamental applications in quantum optics including the generation of macroscopic entangled states. Here we describe a practical method for producing a weak cross-phase modulation at the single-photon level using metastable xenon in a high finesse cavity. We estimate the achievable phase shift and give a brief update on the experimental progress towards its realization. A single-photon cross-phase modulation of approximately 20 milliradians is predicted by both a straightforward perturbation theory calculation and a numerical matrix diagonalization method. 
B. T. Kirby, G. T. Hickman, T. B. Pittman and J. D. Franson
quant-ph