  Manifestations of dipole-dipole interactions in dilute thermal gases are
difficult to sense because of strong inhomogeneous broadening. Recent
experiments reported signatures of such interactions in fluorescence
detection-based measurements of multiple quantum coherence (MQC) signals, with
many characteristic features hitherto unexplained. We develop an original open
quantum systems theory of MQC in dilute thermal gases, which allows us to
resolve this conundrum. Our theory accounts for the vector character of the
atomic dipoles as well as for driving laser pulses of arbitrary strength,
includes the far-field coupling between the dipoles, which prevails in dilute
ensembles, and effectively incorporates atomic motion via a disorder average.
We show that collective decay processes -- which were ignored in previous
treatments employing the electrostatic form of dipolar interactions -- play a
key role in the emergence of MQC signals.
