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 photoexcitation. 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 $^{23}\mathrm{Na}^{40}\mathrm{K}$ 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.
Highlights
Diatomic molecules lie at an intriguing boundary in our current understanding of quantum physics
We have demonstrated that 23Na 40K molecules exhibit near-universal two-body loss even under conditions of very low light intensity
This was enabled by loading molecules into an optical box trap with extremely low residual light intensity
Summary
Diatomic molecules lie at an intriguing boundary in our current understanding of quantum physics. Dipolar bialkali dimers are a interesting class of molecules, both because they can be associated from ultracold atoms and because they offer large permanent dipole moments, making them an ideal choice for the study of dipolar quantum many-body systems For this reason, a variety of ground-state bialkali dimers have been experimentally investigated, including the fermionic species 40K 87Rb and 23Na 40K. We realize trapping conditions of permanently very low light intensity in a blue-detuned optical box trap and compare the results to those from standard, red-detuned optical dipole traps Under such low-intensity conditions, the lifetime of molecules should be significantly increased, as most sticky complexes are not excited by photon scattering and can decay back into diatomic molecules. Our findings give a joint lower bound on the lifetime and photoexcitation rate of sticky complexes which disagrees by at least two orders of magnitude with state-of-the-art theoretical predictions
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