Experimental study of small ensembles of atoms in a microwave cavity

We report results of an experimental study of the evolution of small ensembles of atoms in a microwave cavity. The experiment employs an atomic beam of calcium Rydberg atoms and a split superconducting cavity operating at 35 GHz. At the ambient temperature of 2 K, the mean blackbody photon number is 0.8. Selective field ionization allows us to monitor simultaneously the populations of the initial and final states. The time evolution of the atomic system is probed by ‘‘Stark switching,’’ i.e., shifting the atomic resonance away from the cavity resonance by applying an electric field. The collective oscillations of energy between ensembles of atoms and a cavity with Q>${10}^7$ are studied for one to several hundred atoms. The results agree well with theory for ∼50 or more atoms, but some discrepancies are observed for smaller numbers of atoms. Possible experimental sources of these discrepancies are discussed.