Towards a degenerate gas of bosonic molecules

Ultracold molecules typically have short lifetimes due to inelastic collisions, motivating recent efforts to engineer repulsive barriers between molecules. Microwave shielding of both bosonic and fermionic molecules has been demonstrated and has enabled the creation of degenerate polar molecular gases [1–3]. In parallel, static electric fields have been used to shield fermionic molecules [4]. Here, we apply a static electric field to bosonic NaRb molecules above a Förster resonance and observe a dramatic suppression of two-body loss. In this regime, molecules thermalize sufficiently rapidly to allow evaporative cooling. Using evaporative cooling, we increase the molecular phase-space density by more than an order of magnitude. However, theoretical considerations suggest that the system will ultimately be limited by three-body loss due to the presence of a bound state [5]. To overcome this limitation and reach molecular degeneracy, we propose a scheme that combines a microwave field with a static electric field above the Förster resonance to suppress three-body loss. Notably, this shielding method is insensitive to the polarization of the microwave field.

[1] Bigagli, N. et al. Nature 631, 289-293 (2024).

[2] Shi, Z. et al. arXiv:2508.20518.

[3] Schindewolf, A. et al. Nature 607, 677-681 (2022).

[4] Li, JR. et al. Nat. Phys. 17, 1144-1148 (2021).

[5] Mukherjee, B. and Hutson J. M., Controlling collisional loss and scattering lengths of ultracold dipolar molecules with static electric fields. Phys. Rev. Research 6, 013145 (2024).