Direct measurement of the Wigner time-delay for the scattering of light   by a single atom
  We have implemented the gedanken experiment of an individual atom scattering a wave packet of near-resonant light, and measured the associated Wigner time-delay as a function of the frequency of the light. In our apparatus the atom behaves as a two-level system and we have found delays as large as 42 nanoseconds at resonance, limited by the lifetime of the excited state. This delay is an important parameter in the problem of collective near-resonant scattering by an ensemble of interacting particles, which is encountered in many areas of physics. 
R. Bourgain, J. Pellegrino, Y. R. P. Sortais and A. Browaeys
physics.atom-ph physics.optics quant-ph