Observation of a non-equilibrium steady state of cold atoms in a moving optical lattice

Non-equilibrium dynamics expands our understanding on physical processes based on the conventional equilibrium physics. In particular, non-equilibrium steady states with continuous flow among them have drawn much interest related to various biochemical processes, biomolecular motors, and high-temperature quantum entanglement as well as Bose–Einstein condensates. Here we report observation of a non-equilibrium steady states of atoms achieved in a hybrid of a moving optical lattice and a surrounding cold atom cloud in a phase-stabilized magneto-optical trap. A part of atoms are localized and transported in the moving optical lattice and the rest are not localized in the lattice while trapped as a cold cloud of atoms. These motional states coexist with continuous transition between them. Our model calculations well reproduce the key features of the experimental observations including stepwise transitions, confirming the existence of a non-equilibrium steady state with characteristics of asymmetric simple exclusion process in the cold atom system. Non-equilibrium dynamics of cold atoms have recently attracted attention revealing unconventional phenomena. The authors report here the experimental observation of a non-equilibrium steady state in a hybrid trap composed of a magneto-optical trap and a moving optical lattice.