  Quantum light-matter interfaces, based upon ensembles of cold atoms or other
quantum emitters, are a vital platform for diverse quantum technologies and the
exploration of fundamental quantum phenomena. Most of our understanding and
modeling of such systems are based upon macroscopic theories, wherein the atoms
are treated as a smooth, quantum polarizable medium. Although it is known that
such approaches ignore a number of microscopic details, such as the granularity
of atoms, dipole-dipole interactions and multiple scattering of light, the
consequences of such effects in practical settings are usually mixed with
background macroscopic effects and difficult to quantify. In this work we
demonstrate a time-domain method to measure microscopically-driven optical
effects in a background-free fashion, by transiently suppressing the
macroscopic dynamics. With the method, we reveal a microscopic dipolar
dephasing mechanism that generally limits the lifetime of the optical spin-wave
order in a random gas. Theoretically, we show the dephasing effect emerges from
the strong resonant dipole interaction between close-by atomic pairs.
