Nature 491, 570–573 (2012)

Atoms and molecules behave very differently near absolute zero than they do at everyday temperatures. Techniques for cooling atoms and diatomic molecules to such low temperatures have therefore opened an entire new field of physics. Martin Zeppenfeld and co-workers now demonstrate a cooling procedure that can be applied to molecules of three or more atoms, and use it to cool fluoromethane (CH3F) molecules to 29 mK.

Usually atoms are cooled using lasers, which requires that the atoms have a particular energy structure. But the requisite transitions do not exist in polyatomic molecules. Instead, Zeppenfeld et al. tailored the electric field in which the CH3F molecules were trapped to adjust the energy levels of the molecules, making laser-cooling possible.

The process is known as 'Sisyphus cooling', because it needs to be continuously repeated to attain low temperatures — just as the king from Greek mythology was cursed to eternally roll a boulder up a hill, only for it to roll back down again.