Evaporative cooling of bosonic molecules near a Förster resonance

Evaporative cooling of ground state molecules requires suppression of inelastic collisions. Recently, microwave shielding has been used to suppress collisional loss, which has led to the achievement of degenerate gases of polar molecules [1,2,3]. Meanwhile, static electric fields can also provide shielding near a Förster resonance and have been demonstrated in fermionic gases [4]. We demonstrate this electric field shielding scheme for bosonic molecules, suppressing two-body loss significantly and reaching high elastic-to-inelastic collision rates near the resonance.  Using this scheme, we achieve evaporative cooling of bosonic NaRb molecules. We also present a proposal for suppressing three-body loss by introducing microwaves near the Förster resonance. The scheme has the advantage of significantly reduced sensitivity to the microwave polarization.

[1] Bigagli, N. et al. Observation of Bose-Einstein condensation of dipolar molecules. Nature 631, 289-293 (2024).

[2] Shi, Z. et al. Bose-Einstein condensate of ultracold sodium-rubidium molecules with tunable dipolar interactions. arXiv:2508.20518.

[3] Schindewolf, A. et al. Evaporation of microwave-shielded polar molecules to quantum degeneracy. Nature 607, 677-681 (2022).

[4] Li, JR. et al. Tuning of dipolar interactions and evaporative cooling in a three-dimensional molecular quantum gas. Nat. Phys. 17, 1144-1148 (2021).