  Over the past decade, strong interactions of light and matter at the
single-photon level have enabled a wide set of scientific advances in quantum
optics and quantum information science. This work has been performed
principally within the setting of cavity quantum electrodynamics with diverse
physical systems, including single atoms in Fabry-Perot resonators, quantum
dots coupled to micropillars and photonic bandgap cavities, and Cooper-pairs
interacting with superconducting resonators. Experiments with single, localized
atoms have been at the forefront of these advances with the use of optical
resonators in high-finesse Fabry-Perot configurations. As a result of the
extreme technical challenges involved in further improving the multilayer
dielectric mirror coatings of these resonators and in scaling to large numbers
of devices, there has been increased interest in the development of alternative
microcavity systems. Here we show strong coupling between individual Cesium
atoms and the fields of a high-quality toroidal microresonator. From
observations of transit events for single atoms falling through the resonator's
evanescent field, we determine the coherent coupling rate for interactions near
the surface of the resonator. We develop a theoretical model to quantify our
observations, demonstrating that strong coupling is achieved, with the rate of
coherent coupling exceeding the dissipative rates of the atom and the cavity.
Our work opens the way for investigations of optical processes with single
atoms and photons in lithographically fabricated microresonators. Applications
include the implementation of quantum networks, scalable quantum logic with
photons, and quantum information processing on atom chips.
