Electromagnetically induced transparency of an interacting cold Rydberg   ensemble
  We study electromagnetically induced transparency (EIT) of a weakly interacting cold Rydberg gas. We show that the onset of interactions is manifest as a depopulation of the Rydberg state and numerically model this effect by adding a density-dependent non-linear term to the optical Bloch equations. In the limit of a weak probe where the depopulation effect is negligible, we observe no evidence of interaction induced decoherence and obtain a narrow Rydberg dark resonance with a linewidth of <600 kHz, limited by the Rabi frequency of the coupling beam 
K. J. Weatherill, J. D. Pritchard, R. P. Abel, M. G. Bason, A. K.   Mohapatra and C. S. Adams