Theory of dark-state polariton collapses and revivals
  We investigate the dynamics of dark-state polaritons in an atomic ensemble with ground-state degeneracy. A signal light pulse may be stored and retrieved from the atomic sample by adiabatic variation of the amplitude of a control field. During the storage process, a magnetic field causes rotation of the atomic hyperfine coherences, leading to collapses and revivals of the dark-state polariton number. These collapses and revivals should be observable in measurements of the retrieved signal field, as a function of storage time and magnetic field orientation. 
S. D. Jenkins, D. N. Matsukevich, T. Chaneliere, A. Kuzmich, T. A. B.   Kennedy