  The spontaneous creation and persistence of ground-state coherence in an
ensemble of intracavity Rb atoms has been observed as a quantum beat. Our
system realizes a quantum eraser, where the detection of a first photon
prepares a superposition of ground-state Zeeman sublevels, while detection of a
second erases the stored information. Beats appear in the time-delayed
photon-photon coincidence rate (intensity correlation function). We study the
beats theoretically and experimentally as a function of system parameters, and
find them remarkably robust against perturbations such as spontaneous emission.
Although beats arise most simply through single-atom-mediated quantum
interference, scattering pathways involving pairs of atoms interfere also in
our intracavity experiment. We present a detailed model which identifies all
sources of interference and accounts for experimental realities such as
imperfect pre-pumping of the atomic beam, cavity birefringence, and the transit
of atoms across the cavity mode.
