Full observation of single-atom dynamics in cavity QED

We report the use of broadband heterodyne spectroscopy to perform continuous measurement of the interaction energy E-int between one atom and a high-finesse optical cavity, during individual transit events of approximate to 250 mu s duration. We achieve a fractional sensitivity approximate to 4 x 10(-4)/root Hz to variations in E-int/(h) over bar within a measurement bandwidth that covers 2.5 decades of frequency (1-300 kHz). Our basic procedure is to drop cold cesium atoms into the cavity from a magnetooptic trap while monitoring the cavity's complex optical susceptibility with a weak probe laser. The instantaneous value of the atom-cavity interaction energy, which in turn determines the coupled system's optical susceptibility, depends on both the atomic position and (Zeeman) internal state. Measurements over a wide range of atom-cavity detunings reveal the transition from resonant to dispersive coupling, via the transfer of atom-induced signals from the amplitude to the phase of light transmitted through the cavity. By suppressing all sources of excess technical noise, we approach a measurement regime in which the broadband photocurrent may be interpreted as a classical record of conditional quantum evolution in the sense of recently developed quantum trajectory theories.