Collisional Cooling of Ultracold Molecules

April 8, 2020
Figure 1: a new refrigerator for molecules
Fig. 1, right: A new refrigerator for molecules.  Sodium atoms (yellow spheres) collide with sodium-lithium molecules (combined-yellow-red spheres).  The atom-molecule mixture is trapped in an optical trap whose effective edge is shown as a white rim. As the trap is loosened (depicted as a dimmer rim), the most energetic sodium atoms leave the trap, providing evaporative cooling.  The cooling is transferred to the molecules via elastic collisions.  The frost on the molecules indicates that they have reached a temperature of 200 billionths of a degree Kelvin. [Credit: Pilsu Heo at Micropicture, South Korea]

A diatomic molecule consists of two atoms, held together by a chemical bond. But these molecules are more than just a pair of atoms: if one atom is different from the other, the molecules become polar. This polarity empowers the diatomic molecules to strongly interact with each other, even at long distance. These molecules can also vibrate or rotate--something that single atoms cannot do--giving us extra-knobs to control their quantum behavior. These special features of the molecules make them important and powerful candidates for quantum computers and quantum simulators as well as a platform to study chemical reactions at the quantum level. However, there is one very important condition for molecules to satisfy in order to become useful in the quantum realm: they need to be ultracold, which means they need to be cooled down to a temperature between one millionth and one billionth of a degree (i.e., 1 micro-Kelvin to 1 nano-Kelvin) in order to amplify and precisely control the quantum phenomena.

Physicists have been successfully cooling atoms to ultracold temperatures for years but some atoms are easier to cool than others. A common technique, called sympathetic cooling, uses an easily cooled type of atom to cool down a trickier type. Sympathetic cooling is like a refrigerator: a bunch of colder particles remove heat from the sample. A key in this technique is that particles should effectively exchange energy through “good” collisions without destroying each other through “bad” ones resulting from, for instance, chemical reactions.

For many years, researchers have searched for a system where sympathetic cooling could be applied to molecules at ultracold temperatures. Using atoms as a refrigerator for molecules seemed ideal. However, as mentioned, diatomic molecules are more than just a pair of atoms. The complexity that makes them exciting for quantum computing and quantum simulation also makes molecules tend to have destructive “bad” collisions with atoms. Until now, the molecules and the refrigerator couldn’t be held together long enough to make the molecules colder.

In a paper published in Nature*, Harvard physics graduate student Hyungmok Son and colleagues from the MIT-Harvard Center for Ultracold Atoms describe their discovery of the first working refrigerator for ultracold molecules. The authors used sodium (Na) atoms  to sympathetically cool sodium-lithium (NaLi) molecules, from 2 micro-Kelvin down to 200 nano-Kelvin. The Na atoms and NaLi molecules usually undergo chemical reactions when they collide. However, there is one quantum state (a pure, spin-polarized state) in which those reactions nearly stop, which makes the result even more surprising and challenges the old assumptions about which atoms and molecules can make good refrigerator systems. Future experiments can build on this technique to bring NaLi molecules to even lower temperatures, where they can be used to simulate exotic quantum materials.

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* Hyungmok Son, Juliana J. Park, Wolfgang Ketterle, and Alan O. Jamison, "Collisional cooling of ultracold molecules," Nature 580 (08 April 2020) https://doi.org/10.1038/s41586-020-2141-z.