Laser cooling of atoms enables a great deal of ultracold physics. In its most widespread form, called Doppler cooling, a sample of atoms is irradiated from all sides with laser light whose frequency is tuned just below an atomic resonance. Each atom preferentially absorbs photons that are blueshifted into resonance—that is, the ones that oppose the atom’s motion. The atom then reradiates the light in a random direction and returns to its ground state. Repeating that optical cycle some tens of thousands of times can cool the atomic sample to below 1 mK.
Cooling of molecules, particularly molecules comprising more than one type of atom, to ultracold temperatures could expand the potential for studies of ultracold chemistry or even for a quantum computer that uses molecules as qubits. Doppler cooling is tricky to apply to molecules—for a long time it was believed to be close to impossible—so researchers focused on...
