In the CaF laser cooling experiment, we seek to study ultracold molecules by first loading molecules into a magneto-optical trap (MOT) and then transferring these molecules to an optical trap for further cooling. Interesting later experiments could involve studying atom-molecule or molecule-molecule collisions, as well as using the ultracold sample of diatomic molecules for quantum simulation and quantum computation, or a path finder for precision measurement experiments.

Loading Molecules

The first step of this experiment is to load a magneto-optical trap (MOT) with the diatomic radical calcium monofluoride (CaF) using a buffer-gas beam source (for details on buffer-gas cells see [1-3]). We first ablate a solid precursor of atomic Ca with a pulsed Nd:YAG laser. We simultaneously flow sulfur hexafluoride (SF6) into the buffer-gas cell, leading to a chemical reaction which produces CaF. The hot molecular gas then thermalizes with ~1 K Helium buffer-gas and is extracted into a beam. The molecular beam has an average forward velocity of 50-60 m/s out of our two-stage cell. While such velocities are low enough to load conventional atomic MOTs (see our previous work on lanthanide atoms), the estimated capture velocity for a MOT of CaF is less than 10 m/s. A slowing stage is thus required to bring a sufficient number of molecules to below the capture velocity. We use a slowing technique for this beam deceleration, as was demonstrated in our recent paper [14]. An additional challenge to trapping molecules is the existence of magnetic dark states in molecules, which arise due to the fact that we trap the molecules on a transition with “inverted” angular momentum structure. We address this problem by switching the polarization and the magnetic field of the MOT very rapidly (~1 MHz) to depopulate those dark states (so called RF MOT). Finally, over a million molecules are loaded into the MOT.

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