Collisions of ultracold molecules in bright and dark optical dipole traps

TL;DR

This study investigates light-induced collisional loss in ultracold fermionic molecules trapped in a repulsive optical potential, revealing universal loss behavior and challenging existing theoretical models.

Contribution

It demonstrates the first experimental test of light-induced collisional loss at very low intensities and provides a benchmark that contradicts current theoretical predictions.

Findings

Results are consistent with universal loss at low light intensities.

Experimental loss rates are at least two orders of magnitude higher than theoretical predictions.

The study challenges existing models of molecular collision dynamics.

Abstract

Understanding collisions between ultracold molecules is crucial for making stable molecular quantum gases and harnessing their rich internal degrees of freedom for quantum engineering. Transient complexes can strongly influence collisional physics, but in the ultracold regime, key aspects of their behavior have remained unknown. To explain experimentally observed loss of ground-state molecules from optical dipole traps, it was recently proposed that molecular complexes can be lost due to photo-excitation. By trapping molecules in a repulsive box potential using laser light near a narrow molecular transition, we are able to test this hypothesis with light intensities three orders of magnitude lower than what is typical in red-detuned dipole traps. This allows us to investigate light-induced collisional loss in a gas of nonreactive fermionic $^{23}$Na$^{40}$K molecules. Even for the lowest intensities available in our experiment, our results are consistent with universal loss, meaning unit loss probability inside the short-range interaction potential. Our findings disagree by at least two orders of magnitude with latest theoretical predictions, showing that crucial aspects of molecular collisions are not yet understood, and provide a benchmark for the development of new theories.