Observation of individual tracer atoms in an ultracold dilute gas

TL;DR

This paper reports the first direct observation of individual tracer atoms colliding with ultracold gases, revealing nonequilibrium dynamics and validating a generalized Langevin model with velocity-dependent friction.

Contribution

It provides experimental evidence of single collisions in an ultracold gas and demonstrates the applicability of a generalized Langevin equation to describe tracer atom dynamics.

Findings

Single collisions detected in ultracold gas

Spatial distribution matches generalized Langevin model

Velocity-dependent friction observed over various Knudsen numbers

Abstract

Understanding the motion of a tracer particle in a rarefied gas is of fundamental and practical importance. We report the experimental investigation of individual Cs atoms impinging on a dilute cloud of ultracold Rb atoms with variable density. We study the nonequilibrium relaxation of the initial nonthermal state and detect the effect of single collisions which has eluded observation so far. We show that after few collisions, the measured spatial distribution of the light tracer atoms is correctly described by a generalized Langevin equation with a velocity-dependent friction coefficient, over a large range of Knudsen numbers.