Strong interactions of single atoms and photons near a dielectric boundary

Cavity quantum electrodynamics provides the setting for quantum control of strong interactions between a single atom and one photon. Many such atom-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 less than or similar to 100 nm from a resonator's 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 similar to 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 Casimir-Polder attraction and the manifestly quantum nature of the atom-cavity dynamics.