Abstract

Coupling motional degrees of freedom of a laser-driven Bose-Einstein condensate to the quantized field of an optical high-finesse resonator provides a clean model system for studying the driven-dissipative Dicke phase transition. In this talk, I will present recent experimental results on the observation of critical behavior and mode softening at this second-order phase transition. By monitoring the cavity output field we are able to observe in real-time the diverging atomic density fluctuations while approaching the critical point. The observed critical behavior deviates from that expected for the closed Dicke model and is in quantitative agreement with the enhanced fluctuations spectrum of the driven-dissipative system. A correlation analysis of the cavity output field reveals the critical slowdown of the system dynamics and allows us to identify an additional dissipation channel for the atomic degrees of freedom