Collective resonance fluorescence in small and dense atom clouds:   Comparison between theory and experiment
  We study the emergence of a collective optical response of a cold and dense $^{87}$Rb atomic cloud to a near-resonant low-intensity light when the atom number is gradually increased. Experimental observations are compared with microscopic stochastic simulations of recurrent scattering processes between the atoms that incorporate the atomic multilevel structure and the optical measurement setup. We analyze the optical response of an inhomogeneously-broadened gas and find that the experimental observations of the resonance line shifts and the total collected scattered light intensity in cold atom clouds substantially deviate from those of thermal atomic ensembles, indicating strong light-induced resonant dipole-dipole interactions between the atoms. At high densities, the simulations also predict a significantly slower decay of light-induced excitations in cold than in thermal atom clouds. The role of dipole-dipole interactions is discussed in terms of resonant coupling examples and the collective radiative excitation eigenmodes of the system. 
S. D. Jenkins, J. Ruostekoski, J. Javanainen, S. Jennewein, R.   Bourgain, J. Pellegrino, Y. R. P. Sortais, A. Browaeys
physics.atom-ph cond-mat.quant-gas physics.optics quant-ph