  Strong saturated absorption at nanowatt power levels has been demonstrated
using metastable xenon in a high-finesse optical cavity. The use of metastable
xenon allows a high quality factor of Q=2*10^8 to be achieved at relatively
high atomic densities without any contamination or damage to the optical
surfaces, which is often a problem when using high-density rubidium or other
alkali atoms. This technique provides a relatively straightforward way to
produce nonlinearities at the single-photon level with possible applications in
quantum communications and computing.
