Observation of Brownian Motion of a Bose-Einstein Condensate

TL;DR

This paper reports the experimental observation of classical Brownian motion in a Bose-Einstein condensate, demonstrating how the condensate's center-of-mass undergoes random momentum kicks akin to a classical particle.

Contribution

The study provides the first experimental observation of Brownian motion in a BEC and develops a phenomenological theory explaining the observed dynamics.

Findings

Center-of-mass exhibits Brownian motion due to Langevin forces

The BEC behaves as a classical rigid body under certain conditions

The phenomenological model matches experimental data quantitatively

Abstract

We report on the experimental observation of classical Brownian motion in momentum space by a Bose-Einstein condensate (BEC) of Rubidium atoms prepared in a hexagonal optical lattice. Upon suddenly increasing the effective atomic mass, the BEC as a whole behaves as a classical rigid body with its center-of-mass receiving random momentum kicks by a Langevin force arising from atom loss and interactions with the surrounding thermal cloud. Physically, this amounts to selective heating of the BEC center-of-mass degree of freedom by a sudden quench, while with regard to the relative coordinates, the BEC is stablized by repulsive atomic interactions, and its internal dynamics is suppressed by forced evaporative cooling induced by atom loss. A phenomenological theory is developed that well explains the experimental data quantitatively.