Nano ‘refrigerator’ holds promise for molecular quantum computing

Technology News | April 9, 2020

By Rich Pell

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. We know now that it works for cooling sodium lithium molecules. Whether it will work for other classes of molecules remains to be seen.”

Alan Jamison, a professor of physics at the University of Waterloo and visiting scientist in MIT’s Research Laboratory of Electronics adds, “Sodium lithium molecules are quite different from other molecules people have tried. Many folks expected those differences would make cooling even less likely to work. However, we had a feeling these differences could be an advantage instead of a detriment.”

In their work, the researchers fine-tuned a system of more than 20 laser beams and various magnetic fields to trap and cool atoms of sodium and lithium in a vacuum chamber, down to about 2 microkelvins — a temperature that is optimal for the atoms to bond together as sodium lithium molecules. Once enough molecules were produced, the researchers shone laser beams of specific frequencies and polarizations to control the quantum state of the molecules and carefully tuned microwave fields to make atoms spin in the same way as the molecules.

“Then we make the refrigerator [the sodium atoms that surround the cloud of newly formed molecules] colder and colder,” says Hyungmok Son, a graduate student in Harvard University’s Department of Physics. “We lower the power of the trapping laser, making the optical trap looser and looser, which brings the temperature of sodium atoms down, and further cools the molecules, to 200 billionths of a kelvin.”

The researchers observed that the molecules were able to remain at these ultracold temperatures for up to one second – more than long enough for quantum computation and exploring new materials, which all can be done in small fractions of a second, say the researchers. If they can get sodium lithium molecules to be about five times colder than what they have so far achieved, say the researchers, they will have reached a so-called “quantum degenerate” regime, where individual molecules become indistinguishable and their collective behavior is controlled by quantum mechanics.

The researchers believe they have some ideas for how to achieve this, which will involve months of work in optimizing their setup, as well as acquiring a new laser to integrate into their setup.

“Our work will lead to discussion in our community why collisional cooling has worked for us but not for others,” says Son. “Perhaps we will soon have predictions how other molecules could be cooled in this way.”

For more, see “Collisional cooling of ultracold molecules.”