Quantum manipulation and measurement of single atoms in optical cavity QED

Summary form only given. Using cold atoms strongly interacting with individual photons inside a high finesse cavity, we are approaching an ideal quantum system in which both matter (internal and external degrees of freedom) and light exhibit strong quantum character and the system's coherent evolution dominates decoherence processes. With this system we have performed real-time continuous measurement of single atomic spatial trajectories. The measurement protocol involves the determination of the phase of the cavity output field, which is far-detuned from the atomic resonance to minimize the measurement back action on the center-of-mass (CM) motion. The real-time evolution of the complex field amplitude is efficiently recorded at high bandwidth and with good signal-to-noise ratio, limited respectively by the rate of coherent coupling between atom and cavity mode and the photodetection shot noise. This nearly optimal detection technique has already revealed atomic CM motion inside the cavity standing-wave field of single photons, and should eventually lead to the strong conditioning of system evolution on measurement results and the realization of quantum feedback control. Another related but independent experiment concentrates on quantum manipulation of the CM motion. Owing to the highest coherent coupling rate between cavity field and individual atoms achieved to date, the mechanical coupling between atom and cavity can be significantly larger than the kinetic energy of laser cooled atoms, even under very low cavity excitation. Indeed, evidence of mechanical light forces for intracavity photon number <1 has already been observed. The origin of this mechanical force is radically different from that in conventional laser trapping where photon scattering (coherent or incoherent) drives the atomic CM motion. The spatial variation of the standing cavity mode (and hence of the atom-field coupling), produce optical potentials capable of localizing atoms inside the cavity. With the initial evidence of prolonged atomic transit through the cavity field (by 3/spl times/) when the suitable trapping state (the lower dressed state of the atom-cavity system) was excited, we are working towards suppression of the atomic momentum diffusion to achieve long-term atomic confinement via quantum feedback.