  We investigated non-equilibrium atomic dynamics in a moving optical lattice
via observation of atomic resonance fluorescence spectrum. A three-dimensional
optical lattice was generated in a phase-stabilized magneto-optical trap (MOT)
and the lattice was made to move by introducing a detuning between the
counter-propagating trap lasers. A non-equilibrium steady states (NESS's) of
atoms was then established in the hybrid of the moving optical lattice and the
surrounding MOT. A part of atoms were localized and transported in the moving
optical lattice and the rest were not localized in the lattice while trapped as
a cold gas in the MOT. These motional states coexisted with continuous
transition between them. As the speed of the lattice increased, the population
of the non-localized state increased in a stepwise fashion due to the existence
of bound states at the local minima of the lattice potential. A deterministic
rate-equation model for atomic populations in those motional states was
introduced in order to explain the experimental results. The model calculations
then well reproduced the key features of the experimental observations,
confirming the existence of an NESS in the cold atom system.
