Title of seminar: Onset of continuous cavity-mediated collective phenomena in an atomic beam
Abstract:
An atom-cavity system is a simple yet rich open quantum system.
By adjusting the characteristic rate of each individual component, the nature of the interplay can be manipulated. Here, we present the case where the dissipation rate of a single mode of an optical cavity is larger than the decay rate of the gain medium. In this scenario, photons do not remain in the cavity mode for long, so the cavity field is primarily determined by the photons emitted and absorbed by the atomic gain medium. Contributions to the atomic decay rate include spontaneous decay into vacuum modes as well as inhomogeneous and homogeneous broadenings. This regime is often referred to as the ‘bad-cavity regime.
We demonstrate, theoretically and experimentally, that a large ensemble of atoms can drive this coupled system into the ‘strong-coupling regime’, characterized by a non-linear response to an external drive. We argue that this regime arises not from strong single atom-cavity coupling, but from the collective weak coupling of many atoms to a single cavity mode. For an inverted sample, we will show that this regime results in optical amplification of the external drive.
The experimental system consists of a beam of Sr atoms. The beam has a large vertical velocity, resulting in a short transit time for atoms passing through the cavity mode, while being cooled in the other two directions. The velocity along the cavity axis is particularly important as it determines the atom-cavity interaction phase. We show how to prepare an ensemble of either ground or excited state atoms that exhibit collective effects.