Saturated absorption at nanowatt power levels using metastable xenon in   a high-finesse optical cavity
  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. 
G. T. Hickman, T. B. Pittman, and J. D. Franson
quant-ph physics.optics