Molecules face the big chill

Molecules are seriously chilling out. Scientists report the first cooling of molecules below a previously impassable milestone. The result, in which scientists cooled molecules down to tens of millionths of a degree, is a step toward reaching the ultracold temperatures already achievable with atoms, researchers report August 28 in Nature Physics.

Scientists regularly chill atoms to less than a millionth of a degree above absolute zero (‒273.15° Celsius), even reaching temperatures as low as 50 trillionths of a degree (SN: 5/16/15, p. 4). But molecules are more difficult to cool down, as they can spin and vibrate in a variety of ways, and that motion is a form of heat.

Previously, physicists have made ultracold molecules by convincing prechilled atoms to link up (SN: 12/20/08, p. 22), but the technique works for only a few kinds of molecules. Putting the freeze on already assembled molecules has allowed scientists to chill additional types but, until now, down to only a few hundreds of millionths of degrees.

Using lasers and magnetic fields, the scientists corralled and cooled molecules inside a device called a magneto-optical trap. In the trap, molecules of calcium monofluoride are slowed — and therefore cooled — when they absorb photons from a laser. But only so much cooling is possible with this method. To go beyond what’s called the Doppler limit, the researchers adapted a method used for cooling atoms, known as Sisyphus cooling. Two lasers pointed at one another create an electromagnetic field that acts like an endless hill the molecule must climb, thereby sapping its energy and heat. With these two techniques, the molecules reached a frigid 50 millionths of a degree above absolute zero.

As the art of laser cooling advanced in recent decades, ultracold atoms rapidly became a popular research topic. Now, predicts study coauthor Michael Tarbutt, a physicist at Imperial College London, cold molecule research is “going to explode in exactly the same way that it did for cold atoms.”

Cold molecules could be useful for a variety of scientific purposes: studying how chemical reactions occur, looking for hints of new fundamental particles or simulating complex quantum materials in which many particles interact at once.

“It’s a really exciting result,” says physicist David DeMille of Yale University, who was not involved with the research. “It turns out it’s harder in almost every way to apply laser cooling and trapping to molecules, but there are many, many motivations for doing that.”