Kinetics of a single trapped ion in an ultracold buffer gas

TL;DR

This paper investigates the kinetic behavior of a single ion in an ultracold atomic gas, revealing how radiofrequency trap dynamics influence ion energy distribution and demonstrating the back-action effects on the buffer gas.

Contribution

It introduces a numerical model that captures the ion's steady state energy distribution considering trap effects and validates it with experimental data in the ultracold regime.

Findings

Ion energy distribution is governed by trap dynamics, not just buffer gas temperature.

Back-action of the ion on the ultracold gas is observable due to finite gas size.

Elastic collisions suffice to explain experimental results.

Abstract

The immersion of a single ion confined by a radiofrequency trap in an ultracold atomic gas extends the concept of buffer gas cooling to a new temperature regime. The steady state energy distribution of the ion is determined by its kinetics in the radiofrequency field rather than the temperature of the buffer gas. Moreover, the finite size of the ultracold gas facilitates the observation of back-action of the ion onto the buffer gas. We numerically investigate the system's properties depending on atom-ion mass ratio, trap geometry, differential cross-section, and non-uniform neutral atom density distribution. Experimental results are well reproduced by our model considering only elastic collisions. We identify excess micromotion to set the typical scale for the ion energy statistics and explore the applicability of the mobility collision cross-section to the ultracold regime.