Photon-changing collisions between microwave-shielded NaCs molecules

Double microwave shielding has enabled the observation of Bose-Einstein condensation of polar molecules through strong suppression of two-body loss and the elimination of three-body recombination. We have analyzed the nature of the remaining two-body losses. We find that those losses have the character of photon-changing collisions: colliding molecules mediate the exchange of photons between the two microwave fields. This process transfers the energy difference between the two fields into kinetic energy of the molecules. These photon-accelerated molecules can collide with the remaining ultracold molecules while escaping the trap, leaving behind a tiny fraction of their kinetic energy. In this work, we build a semi-analytic model of this process, called secondary heating, and find it agrees well with experimental measurements on trapped samples of NaCs molecules. Finally, in this talk, I look at implications for experiments going forward.