By cooling molecules down to ultracold temperatures - at which point molecular activity should slow to a crawl - scientists can precisely control their quantum behavior. This could enable researchers, say the scientists, to use molecules as complex bits for quantum computing - tuning individual molecules like tiny knobs to carry out multiple streams of calculations at a time.
Scientists have previously super-cooled atoms, but doing the same for molecules - which are more complex in their behavior and structure - is a much bigger challenge. However the MIT researchers say using their approach they have been able to cool molecules of sodium lithium down to 200 billionths of a Kelvin - just a hair above absolute zero.
They did so by applying "collisional cooling" - a standard technique used to cool down atoms by immersing them with other, colder atoms. Previously, when used to try to supercool molecules, this technique has proved unsuccessful: when molecules collided with atoms, they exchanged energy in such a way that the molecules were heated or destroyed in the process, called "bad" collisions.
However, MIT researchers found that if sodium lithium molecules and sodium atoms were made to spin in the same way, they could avoid self-destructing, and instead engage in "good" collisions, where the atoms took away the molecules’ energy, in the form of heat, as intended. This required the use of precise control of magnetic fields and an intricate system of lasers to choreograph the spin and the rotational motion of the molecules, say the researchers. As result, the atom-molecule mixture had a high ratio of good-to-bad collisions and was cooled from 2 microkelvins down to 220 nanokelvins.
"Collisional cooling has been the workhorse for cooling atoms," says Nobel Prize laureate Wolfgang Ketterle, the John D. Arthur professor of physics at MIT. "I wasn't convinced that our scheme would work, but since we didn't know for sure, we had to try it.