Cavity quantum electrodynamics provides the setting for quantum control of strong interactions between a single atom and one photon. Many such atomg cavity systems interacting by coherent exchanges of single photons could be the basis for scalable quantum networks. However, moving beyond current proof-of-principle experiments involving just one or two conventional optical cavities requires the localization of individual atoms at distances ≲100 nm from a resonatorg surface. In this regime an atom can be strongly coupled to a single intracavity photon while at the same time experiencing significant radiative interactions with the dielectric boundaries of the resonator. Here, we report using real-time detection and high-bandwidth feedback to select and monitor single caesium atoms located ̃100 nm from the surface of a microtoroidal optical resonator. Strong radiative interactions of atom and cavity field probe atomic motion through the evanescent field of the resonator and reveal both the significant role of Casimirg Polder attraction and the manifestly quantum nature of the atomg cavity dynamics.
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