Observation of dark state polariton collapses and revivals
  By time-dependent variation of a control field, both coherent and single photon states of light are stored in, and retrieved from, a cold atomic gas. The efficiency of retrieval is studied as a function of the storage time in an applied magnetic field. A series of collapses and revivals is observed, in very good agreement with theoretical predictions. The observations are interpreted in terms of the time evolution of the collective excitation of atomic spin wave and light wave, known as the dark-state polariton. 
D. N. Matsukevich, T. Chaneliere, S. D. Jenkins, S.-Y. Lan, T.A.B.   Kennedy, and A. Kuzmich