Laser cooling of a fermionic molecule, CaD

Since the first molecule to have been directly laser cooled (SrF), this technique has been extended to other diatomic (CaF, YO, YbF, BaH, CaH, BaF) and polyatomic (SrOH, CaOH, YbOH, CaOCH₃) species. The demonstration of molecular magneto-optical traps (MOTs) opens many possibilities in quantum science and technology. To date, all directly laser cooled molecules have been bosonic. Fermionic molecules, however, are an important ingredient for ultracold chemistry, quantum simulation and precision measurement applications. Here we report the first fermionic molecule to have been laser cooled, calcium monodeuteride (CaD). We obtain rotational closure and show magnetically assisted Sisyphus cooling for a beam of CaD molecules. Laser cooling of CaD is a key step towards a MOT of a fermionic molecule. The existence of an electron spin in the ground state provides additional degrees of quantum control compared to most assembled bi-alkali molecules once in a conservative trap. Subsequently with a controlled dissociation pathway, obtaining a trapped gas of atomic deuterium could be possible for precision measurements.