Kinetic Phase Transition in a Periodically Perturbed Magneto-Optically Trapped Atoms Far From Equilibrium

Over the past few decades, the behavior of far from equilibrium systems and role of the fluctuation in nonequilibrium systems have been brought to attention and a considerable number of studies have been conducted in physics, chemistry, and biology. We use the periodically perturbed magneto-optical trapped $^85$Rb atoms to investigate a noise-induced transition in nonlinear dynamical system, especially kinetic phase transition (KPT) which is similar behavior to first-order phase transition in equilibrium systems. The system is described by Duffing oscillation, and it is well-known that the Duffing oscillator undergoes the hysteresis. Within the particular range in frequency of the external driving, there is the bi-stable region which the large and small vibration amplitude coexists. We measure the occupation probability of bi-stable states directly because our system consists of many atoms in contrast to many trials for the measurement for single particle system. A critical region is shown to arise in the vicinity of the KPT point, at which the populations of two stable states are equal, including an appearance of a supernarrow peak in the spectral density of the fluctuations.