  We utilize high-bandwidth phase quadrature homodyne measurement of the light
transmitted through a Fabry-Perot cavity, driven strongly and on resonance, to
detect excess phase noise induced by a single intracavity atom. We analyze the
correlation properties and driving-strength dependence of the atom-induced
phase noise to establish that it corresponds to the long-predicted phenomenon
of spontaneous dressed-state polarization. Our experiment thus provides a
demonstration of cavity quantum electrodynamics in the strong driving regime,
in which one atom interacts strongly with a many-photon cavity field to produce
novel quantum stochastic behavior.
