# Engineering excited-state interactions at ultracold temperatures

## TL;DR

This paper demonstrates a method to control and engineer excited-state interactions in ultracold atom-ion collisions, overcoming natural suppression effects and enabling detailed study of these interactions at ultracold temperatures.

## Contribution

The authors develop a novel laser dressing technique to remove suppression of inelastic collisions, allowing precise engineering of excited-state interactions in ultracold atom-ion systems.

## Key findings

- Suppression of inelastic collisions at low energies observed.
- Laser dressing enables excitation of collision complexes at specific separations.
- Method provides a new way to study and control ultracold excited-state interactions.

## Abstract

Using a recently developed method for precisely controlling collision energy, we observe a dramatic suppression of inelastic collisions between an atom and ion (Ca + Yb$^+$) at low collision energy. This suppression, which is expected to be a universal phenomenon, arises when the spontaneous emission lifetime of the excited state is comparable to or shorter than the collision complex lifetime. We develop a technique to remove this suppression and engineer excited-state interactions. By dressing the system with a strong catalyst laser, a significant fraction of the collision complexes can be excited at a specified internuclear separation. This technique allows excited-state collisions to be studied, even at ultracold temperature, and provides a general method for engineering ultracold excited-state interactions.