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.
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