Unraveling microscopic optical dipolar interactions

Historically, the immense microscopic complexity of the quantum interactions between atoms and light required that we develop simpler macroscopic theories. A major simplification of these theories is the assumption that the atomic medium appears smooth from the standpoint of light, despite the fact that the atoms are intrinsically granular quantum objects. This simplification leaves open the possibility that fundamental phenomena associated with granularity are actually missed in our current understanding, or that microscopic correlations are a quantum resource that are not being properly utilized. Here, we experimentally demonstrate a technique that allows for such microscopic quantum correlations to build up in an atomic medium, and then be efficiently measured by imprinting them onto an outgoing optical field. These measurements reveal a previously unknown density-dependent dephasing mechanism for optical excitations, which should place fundamental limitations on quantum optical technologies. More broadly, this work opens up the possibility to bring the tools of nuclear magnetic resonance (NMR) to the manipulation of optical excitations in atomic media.