Molecules given the big chill

Enlarge / Sisyphus (right) doomed forever to push a rock up a hill. His name has been borrowed for a technique of successive application of light to cool molecules to fractions of a degree above absolute zero.

Bibi Saint-Pol

At extremely cold temperatures, the quantum nature of matter expresses itself in unique collective behaviors. These include superconductivity, superfluidity, and Bose-Einstein condensation, where collections of particles act as a single quantum system. Most of these dramatic effects have been achieved with collections of atoms. Molecules could potentially display even more interesting behaviors, since they have fundamental asymmetries and long-range interactions among their atoms.

Up until now, these same properties have made it difficult to bring molecules down to the same temperatures as individual atoms. But a new experiment cooled populations of roughly a million fluoromethane molecules to a fraction of a degree above absolute zero. Martin Zeppenfeld and colleagues used a combination of three different forms of light to extract energy from the molecules inside an electrical trap, a trick known as Sisyphus cooling. The assembly maintained this ultracold state for up to 27 seconds, long enough to reveal unique quantum phenomena—including possibly Bose-Einstein condensation of molecules.

Individual atoms are fairly symmetrical in terms of their electric charge distribution, at least under ordinary conditions. This means their interactions are fairly short-ranged: two atoms must be brought relatively close before the internal distribution of electrons and protons can influence each other. On the other hand, molecules—most notably water—may be strongly polar, with a significant accumulation of negative charge on one side of the molecule balanced by positive charge on the other. This occurs even though the molecule as a whole is still electrically neutral.

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