Toward laser-cooled molecules

Laser cooling of atoms enables a great deal of ultracold physics. In one of its forms, called Doppler cooling, a sample is irradiated from all sides with light tuned just below an atomic resonance. Photons that oppose an atom’s motion are shifted into resonance and absorbed, diminishing the atom’s momentum. The atom then reradiates the light in a random direction and returns to its ground state. Repeating that cycle tens of thousands of times can cool the sample below 1 mK. But applying the same technique to molecules is complicated, since rotational and vibrational degrees of freedom give them a multitude of low-lying states into which they can decay. Exciting each state with a separate laser is prohibitively difficult, but allowing molecules to accumulate in any state not excited by a laser breaks the cycle and ends the cooling. Now, Yale University’s David DeMille and colleagues have demonstrated a possible solution to that dilemma. Focusing on strontium monofluoride, one of several molecules that nearly always return to the vibrational level from which they were excited, and exploiting quantum mechanical selection rules to limit the rotational levels the molecules can access, the researchers achieved about 150 absorption–emission cycles per SrF molecule using just two lasers. They estimate that increasing the molecule–laser interaction time will be sufficient to demonstrate cooling of initially slow molecules, and adding a third laser will give them the 10⁵ cycles needed for slowing and cooling their entire sample. (E. S. Shuman et al., Phys. Rev. Lett. 103, 223001, 2009.) —Johanna Miller