Cavity QED experiments in the Caltech Quantum Optics group explore the interactions of single atoms and single photons within an optical cavity.  Specifically, our research accesses the strong coupling regime, in which a single atom can have a profound effect on a cavity's resonant mode, and the field associated with less than a single photon can saturate the atomic response.  In order to meet the requirements of strong coupling, we construct small (~ 40 micron), high finesse (~ 500,000) optical resonators, place them in an ultra-high vacuum environment, then position a single cesium atom within the standing wave mode supported by the cavity.

But how do you put a single atom inside a cavity -- and keep it there?  Initial cavity QED experiments in our group utilized a thermal beam of atoms which traversed the cavity mode.  In the past ten years, we have taken advantage of laser cooling and trapping techniques to achieve new levels of control over atom-cavity interactions.  We collect a cold, dense cloud of cesium atoms in a magneto-optical trap (MOT), cool the atoms further to ~ tens of microKelvin, then release the atoms from the trap.  As pictured to the left, the cesium atoms fall onto a high finesse cavity within a vacuum chamber.  The large white areas are mirror substrates, while the gap between mirrors which forms the cavity (in this case, only 8 microns wide) cannot be seen at this resolution. (For scale, the diameter of the mirrors at the narrow 'notch' is 1mm.)  Because of the narrowness of the gap, only a few atoms per drop interact with the cavity mode.  In our most recent experiments, we catch the atoms inside a dipole trap within the cavity and confine them there for a few seconds.