Observation of single-photon superradiance and the cooperative Lamb   shift in an extended sample of cold atoms
  We report direct, time-resolved observations of single-photon superradiance in a highly extended, elliptical sample of cold 87Rb atoms. The observed rapid decay rate is accompanied by its counterpart, the cooperative Lamb shift. The rate of the strongly directional decay, and the associated shift, scale linearly with the number of atoms and show a strong dependence on the sample shape, demonstrating the collective nature of the observed quantities. 
S.J. Roof, K.J. Kemp, M.D. Havey, and I.M. Sokolov
physics.atom-ph quant-ph