# Buffer gas cooling of a trapped ion to the quantum regime

## TL;DR

This paper demonstrates buffer gas cooling of a trapped ytterbium ion to near the quantum regime within an ultracold lithium gas, revealing quantum collision behavior and opening new avenues for quantum control and studies of atom-ion interactions.

## Contribution

First experimental observation of quantum regime collision energies in an atom-ion mixture, showing deviation from classical theory and enabling future quantum control applications.

## Key findings

- Collision energy near the s-wave threshold (~10 μK)
- Deviation from classical Langevin collision behavior
- Observation of quantum effects in atom-ion collisions

## Abstract

Great advances in precision quantum measurement have been achieved with trapped ions and atomic gases at the lowest possible temperatures. These successes have inspired ideas to merge the two systems. In this way one can study the unique properties of ionic impurities inside a quantum fluid or explore buffer gas cooling of the trapped ion quantum computer. Remarkably, in spite of its importance, experiments with atom-ion mixtures remained firmly confined to the classical collision regime. We report a collision energy of 1.15(0.23) times the $s$-wave energy (or 9.9(2.0)~$\mu$K) for a trapped ytterbium ion in an ultracold lithium gas. We observed a deviation from classical Langevin theory by studying the spin-exchange dynamics, indicating quantum behavior in the atom-ion collisions. Our results open up numerous opportunities, such as the exploration of atom-ion Feshbach resonances, in analogy to neutral systems.