Quantum manipulation and measurement of single atoms in optical cavity QED

Using laser-cooled atoms strongly coupled to a high finesse optical cavity, we have performed real-time continuous measurements of single atomic trajectories in terms of the interaction energy (E/sub int/) with the cavity. Individual transit events reveal a shot-noise limited measurement (fractional) sensitivity of 4/spl times/10/sup -4///spl radic/Hz to variations in E/sub int///spl planck/ within a bandwidth of 1300 kHz. The strong coupling of atom and cavity leads to a maximum interaction energy greater than the kinetic energy of an intracavity laser-cooled atom, even under weak cavity excitation. Evidence of mechanical light forces for intracavity photon number <1 has been observed. The quantum character of the nonlinear optical response of the atom-cavity system is manifested for the trajectory of a single atom.