  Weak single-photon nonlinearities have many potential applications in quantum
computing and quantum information. Here we demonstrate a relatively simple
system for producing low-power cross-phase modulation using metastable xenon
inside a high finesse cavity. The use of a noble gas such as xenon eliminates
the contamination of the high-finesse mirrors that can occur when using alkali
metal vapors such as rubidium. Cross-phase shifts of 5 mrad with 4.5 fJ control
pulses were demonstrated. Numerical solutions of the master equation are in
good agreement with the experimental results, and they predict that cross-phase
shifts greater than 1 mrad per control photon should be achievable by reducing
the size of the cavity.
