  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.
