Observation of Microwave Shielding of Ultracold Molecules

Ultracold gases of high-density polar molecules have been observed to suffer rapid 2-body losses due to a variety of inelastic mechanisms ranging from chemical reactions to trap-light induced losses. Gaining control over collisional properties is a necessary step towards evaporative cooling of polar molecules. Here we demonstrate the suppression of inelastic collisional loss between two calcium monofluoride (CaF) molecules in a merged optical tweezer trap. High power circular polarized microwaves are used to engineer a repulsive interaction in 3D (i.e. for all collision trajectories). This effective repulsive shield suppresses the inelastic loss rate by a factor of six, in agreement with coupled channel calculations, which also predict an increased elastic cross section. The demonstrated microwave shielding shows a possible route to the creation of long-lived, dense samples of ultracold molecules through evaporative cooling.