Using lasers, scientists have chilled a dipolar molecule to a temperature just a fraction of a degree above absolute zero (around minus 273 °C)--an important step in the race to generate new kinds of ultra-cold matter that could be used for everything from quantum computing to chemistry.
Edward Shuman, John Barry and David DeMille, all from Yale University in New Haven, Conn., used an old technique and several new tricks to cool molecules of strontium monofluoride (SrF) to just a few hundred microkelvin. The work represents the first time that a molecule has been laser cooled in situ, and was published online September 20 in Nature.
Regular matter is pretty cool, but for physicists, ultra-cold matter is cooler. When atoms are chilled to within just a fraction of absolute zero, they begin to obey the strange laws of quantum mechanics. Oscillations associated with their low energies can be used as ultra-sensitive accelerometers and quantum clocks, and the atoms themselves can stick together to form a quantum "super-atom," known as a Bose-Einstein condensate.
DeMille and his team wanted to try to cool molecules rather than atoms, allowing them to study the quantum-mechanical nature of chemistry. Because polar molecules are like little bar magnets--with a north and south--this property would also enable the researchers to build systems in which cold particles can interact easily, something that is difficult to do with atoms.
Cooling molecules is much trickier than chilling individual atoms. Atoms can be cooled using lasers because light particles from the laser beam are absorbed and re-emitted by the atoms, causing them to lose some of their kinetic energy. After thousands of such impacts, the atoms are chilled to within billionths of a degree above absolute zero.
But it's not so easy for molecules, says DeMille. Molecules are heavier than atoms, which makes them less responsive to laser light. And unlike atoms, molecules can store energy within the vibrations of atomic bonds and rotations, or spinning, of their entire structure. All this makes molecules harder to cool.
Others have made ultra-cold molecules by cooling atoms individually and then stitching them together, but DeMille wanted to try to chill an entire molecule in one go. His team used a number of tricks to do it. First, they chose SrF, a molecule that calculations showed was unlikely to start vibrating and thereby hinder the cooling process. They picked a color of laser light that ensured the energy absorbed by the molecules would not set them spinning. Finally, the team used a new source of molecules that pre-cooled the SrF better than had been done before.
"It worked better than we expected, and as fast as we could have possibly hoped," DeMille says. The team managed to get their SrF molecules chilled to around 300 microkelvin in one direction.
"The temperatures that he was able to achieve were not spectacularly low," says Jun Ye, a physicist at JILA in Boulder, Colo. But he adds that the ability to apply the technique to a range of molecules makes it exciting: "If laser cooling works, it potentially allows many different molecules to be put into the ultra cold regime."
DeMille says that, ultimately, the molecules could be used in a quantum computer. Their "bar-magnet" characteristics mean that they can interact with one another through their magnetic fields. That could make them useful for performing quantum calculations of the sort that might break codes and solve problems in chemistry and physics that are too complicated to solve with even the largest supercomputer.
DeMille's team will now work on reaching even lower temperatures. "Our calculations indicate it should be fine; preliminary data indicate it should be fine," he says. "But there's no substitute for actually doing it."