  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.
