Radio-frequency magneto-optical trap of ⁸⁷Rb

Recent progress on direct laser cooling and trapping of ultracold molecules shows this method’s potential to reach quantum degeneracy. However, the limited molecule number and density so far achievable and short blackbody-limited lifetime still present difficulties for further collisional cooling and reaching this regime.These problems can be mitigated by sympathetic cooling with ⁸⁷Rb.

Molecular laser cooling schemes operate on "type-II" transitions with polarization dark states. Radio-frequency magneto-optical traps (RF MOTs), in which the magnetic field gradient and laser polarization are flipped rapidly and synchronously, are usually used for such transitions. Here we demonstrate an RF MOT of ⁸⁷Rb, which paves the way for co-trapping with directly-cooled molecules. We operate the Rb RF MOT on the D2 line, and load it from a 2D MOT. We fully characterize the RF MOT, and also compare it to a standard DC MOT loaded from the same source. We find that the RF MOT captures similar numbers of atoms but at 2.3 times lower density. We also applied Λ-enhanced gray molasses after the RF MOT, to achieve temperatures of ≈10 μK with only 35% atom loss. We also detail our plan to co-trap ⁸⁸Sr¹⁹F and ⁸⁷Rb to study collisions and sympathetic cooling.