New method takes big atom groups closer to long-sought temperatures

By Alexandra Witze

Web edition: August 13, 2012

Stopping big molecules cold in their tracks just got a little easier, thanks to a new trick that nudges them closer to ultracold temperatures.

Physicists have previously chilled atoms and pairs of atoms to fractions of a degree above absolute zero — a lab feat that has revealed new physics and chemistry of matter. Now, German scientists have started to do the same for molecules made of many atoms.

The work is “a real important step forward towards realizing cold and ultracold molecular samples,” says Guido Pupillo, a physicist with the French National Center for Scientific Research and the University of Strasbourg.

Supercold temperatures can bring superhot discoveries. Early studies, for instance, showed how ultracold atoms can lose their individual identities and behave instead as a single quantum entity. Later work allowed scientists to manipulate how two ultracold atoms join together.

But the laser cooling techniques used so far don’t work well for bigger molecules with complex energy levels. A team led by Martin Zeppenfeld of the Max Planck Institute for Quantum Optics in Garching, Germany, decided to explore a new method, called optoelectrical cooling, that circumvents the energy-level problem.

“Our method eliminates the primary hurdle in producing ultracold polyatomic molecules,” the scientists wrote online July 31 at arXiv.org. Zeppenfeld declined to discuss the work until it is published in a peer-reviewed journal.

The researchers took about a million molecules of CH₃F, each of which, as the name implies, contains one carbon atom, one fluorine atom and three hydrogen atoms. The scientists put the molecules in a device that had two separate regions, each with its own constant electric field. Nudging a molecule from one region to the next caused it to spit out a photon, releasing kinetic energy in the process. The team sent the molecule through the cycle repeatedly, losing a little more energy on each pass and thus cooling down.

Over time the scientists chilled the molecules from 390 millionths of a degree above absolute zero to 29 millionths of a degree above absolute zero. That’s still a long way from the billionths of a degree achieved with ultracold atoms and pairs of atoms, but it’s a step in the right direction.

Cooling the molecules down below 29 millionths of a degree will require a new lab set-up in Germany, the team says. But what they’ve reported so far should allow other researchers to develop new applications for big cold molecules.