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.
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