Magnetic trapping of calcium monohydride molecules at millikelvin temperatures

Recent advances1,2,3,4,5 in the magnetic trapping and evaporative cooling of atoms to nanokelvin temperatures have opened important areas of research, such as Bose–Einstein condensation and ultracold atomic collisions. Similarly, the ability to trap and cool molecules should facilitate the study of ultracold molecular physics and collisions6; improvements in molecular spectroscopy could be anticipated. Also, ultracold molecules could aid the search for electric dipole moments of elementary particles7. But although laser cooling (in the case of alkali metals1,8,9) and cryogenic surface thermalization (in the case of hydrogen10,11) are currently used to cool some atoms sufficiently to permit their loading into magnetic traps, such techniques are not applicable to molecules, because of the latter's complex internal energy-level structure. (Indeed, most atoms have resisted trapping by these techniques.) We have reported a more general loading technique12 based on elastic collisions with a cold buffer gas, and have used it to trap atomic chromium and europium13,14. Here we apply this technique to magnetically trap a molecular species—calcium monohydride (CaH). We use Zeeman spectroscopy to determine the number of trapped molecules and their temperature, and set upper bounds on the cross-sectional areas of collisional relaxation processes. The technique should be applicable to many paramagnetic molecules and atoms.