Cooperative non-equilibrium phase transition in a dilute thermal atomic gas

Abstract: We demonstrate a cooperative first-order non-equilibrium phase transition in an optically-driven atomic ensemble. The long-range correlation arises due to dipole-dipole interactions between Rydberg atoms. The macroscopic phase transition is observed in the optical domain as intrinsic optical bistability displaying hysteresis and critical slowing down. In the emitted fluorescence, the phase transition occurs between states characterized by single-atom decay and many-body superradiant decay. The demonstration of non-equilibrium phenomena in a controlled, non-destructive environment contributes important experimental evidence to the study of many-body dynamics in complex systems.