  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.
