Optical resonance shifts in the fluorescence of thermal and cold atomic   gases
  We show that the resonance shifts in the fluorescence of a cold gas of rubidium atoms substantially differ from those of thermal atomic ensembles that obey the standard continuous medium electrodynamics. The analysis is based on large-scale microscopic numerical simulations and experimental measurements of the resonance shifts in a steady-state response in light propagation. 
S. D. Jenkins, J. Ruostekoski, J. Javanainen, R. Bourgain, S.   Jennewein, Y. R. P. Sortais, A. Browaeys
physics.atom-ph cond-mat.quant-gas physics.optics quant-ph