Tracking evaporative cooling of an atomic quantum gas in real time

Ultracold atomic gases provide a clean setting for the study of mesoscopic systems, which are characterized by the importance of fluctations of their constituent particles. However, detection of ultracold atomic gases is typically destructive, precluding repeated measurements on the same sample. In our experiment, we overcome this limitation by utilizing the enhanced light-matter coupling in a high-finesse optical cavity. We use a non-invasive measurement scheme to record real-time traces of the atom number dynamics in a mesoscopic quantum gas undergoing evaporative cooling. Extracting two-time correlation functions from our measurements, we reveal the non-linear dynamics of the evaporating gas. This allows for exploring the intriguing interplay between atom number and temperature as well as their fluctuations. Furthermore, by closing a classical feedback loop, we demonstrate the preparation of atomic ensembles with sub-poissonian shot-to-shot atom number fluctuations. Our results provide a novel testbed for observing thermodynamics and transport phenomena in mesosopic cold atomic gases and pave the way for cavity-assisted feedback stabilization of atom number and temperature in atomic quantum simulators.