# Collisional Cooling of Ultracold Molecules

## TL;DR

This paper demonstrates collisional cooling of ultracold NaLi molecules via collisions with ultracold Na atoms, achieving temperatures as low as 220 nK and increasing phase-space density, paving the way for quantum degeneracy.

## Contribution

First successful demonstration of collisional cooling of ultracold molecules using atom-molecule collisions, with high elastic to inelastic collision ratio and significant phase-space density increase.

## Key findings

- Achieved molecular temperatures down to 220 nK.
- Elastic to inelastic collision ratio exceeds 50.
- Increased phase-space density by a factor of 20.

## Abstract

Since the original work on Bose-Einstein condensation, quantum degenerate gases of atoms have allowed the quantum emulation of important systems from condensed matter and nuclear physics, as well as the study of novel many-body states with no analog in other fields of physics. Ultracold molecules in the micro- and nano-Kelvin regimes promise to bring powerful new capabilities to quantum emulation and quantum computing, thanks to their rich internal degrees of freedom compared to atoms. They also open new possibilities for precision measurement and the study of quantum chemistry. Quantum gases of atoms were made possible by collision-based cooling schemes, such as evaporative cooling. For ultracold molecules, thermalization and collisional cooling have not been realized. With other techniques such as supersonic jets and cryogenic buffer gases, studies have been limited to temperatures above 10 mK. Here we show cooling of NaLi molecules at micro- and nano-Kelvin temperatures through collisions with ultracold Na atoms, both prepared in their stretched hyperfine spin states. We find a lower bound on the elastic to inelastic collision ratio between molecules and atoms greater than 50 -- large enough to support sustained collisional cooling. By employing two stages of evaporation, we increase the phase-space density (PSD) of the molecules by a factor of 20, achieving temperatures as low as 220 nK. The favorable collisional properties of a Na and NaLi mixture show great promise for making deeply quantum degenerate dipolar molecules and suggest the potential for such cooling in other systems.