Laser cooling and trapping of SrF molecules: towards a quantum gas of polar molecules

It has been nearly four decades since laser cooling techniques produced ultracold atoms, leading to rapid advances in a wide array of fields. Until recently, laser cooling had not been extended to molecules because of their complex internal structure, which precludes the realization of a true optical cycling transition. However, this complexity makes molecules potentially useful for a wide range of applications. For example, heteronuclear molecules possess permanent electric dipole moments that lead to long-range, tunable, anisotropic dipole–dipole interactions. The combination of the dipole–dipole interaction and the precise control over molecular degrees of freedom possible at ultracold temperatures makes ultracold molecules attractive candidates for use in quantum simulations of condensed-matter systems and in quantum computation. In addition, ultracold molecules could provide unique opportunities for studying chemical dynamics and for tests of fundamental symmetries.

Our group demonstrated the first laser cooling of a diatomic molecule, plus deflection and slowing of a molecular beam through radiative forces. This work is all enabled by a scheme to create a quasi-cycling transition in strontium monofluoride (SrF), which allows >10⁶ photon scattering events before the accessible molecular population decays significantly.

We subsequently demonstrated the first magneto-optical trap (MOT) for molecules, and then developed a new type of MOT–the radiofrequency (RF) MOT–that gave stronger trapping forces and enabled lower temperatures.  We recently demonstrated 3D sub-Doppler cooling of SrF to temperatures as low as ~50 microKelvin, and transfer of SrF to a conservative magnetic quadrupole trap.

We are now pursuing experiments to co-trap SrF molecules with a gas of ultracold Rb atoms, to study sympathetic cooling and atom-molecule collisions in this unexplored regime.