Direct observation of atom-ion non-equilibrium sympathetic cooling

TL;DR

This study directly observes atom-ion sympathetic cooling dynamics, revealing both efficient cooling and complex non-equilibrium behaviors, including forward-scattering collisions beyond traditional models, in a controlled experimental setup.

Contribution

First direct measurement of atom-ion sympathetic cooling dynamics capturing non-equilibrium effects and scattering behaviors beyond Langevin models.

Findings

Efficient cooling achieved in a few collisions.

Identification of non-equilibrium heating and cooling regimes.

Detection of forward-scattering collisions beyond Langevin predictions.

Abstract

Sympathetic cooling is the process of energy exchange between a system and a colder bath. We investigate this fundamental process in an atom-ion experiment where the system is composed of a single ion, trapped in a radio-frequency Paul trap, and prepared in a coherent state of ~200 K and the bath is an ultracold cloud of atoms at {\mu}K temperature. We directly observe the sympathetic cooling dynamics with single-shot energy measurements during one, to several, collisions in two distinct regimes. In one, collisions predominantly cool the system with very efficient momentum transfer leading to cooling in only a few collisions. In the other, collisions can both cool and heat the system due to the non-equilibrium dynamics of the atom-ion collisions in the presence of the ion-trap's oscillating electric fields. While the bulk of our observations agree well with a molecular dynamics simulation of hard-sphere (Langevin) collisions, a measurement of the scattering angle distribution reveals forward-scattering (glancing) collisions which are beyond the Langevin model. This work paves the way for further non-equilibrium and collision dynamics studies using the well-controlled atom-ion system.