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

TL;DR

This study demonstrates the existence of a non-equilibrium steady state in cold atoms within a moving optical lattice, revealing coexistence and transition between localized and non-localized atomic states.

Contribution

It introduces a rate-equation model that accurately explains the atomic population dynamics in a moving optical lattice, confirming the non-equilibrium steady state.

Findings

Atoms exhibit localized and non-localized states coexisting in the moving lattice.

Population of non-localized atoms increases stepwise with lattice speed.

The rate-equation model reproduces experimental population behaviors.

Abstract

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.