Scientists recently created a never-before-seen four-atom molecule — the coldest of its kind ever made.
Researchers created the oddball molecule — a strange configuration of sodium-potassium with an ultralong chemical bond — at 134 nanokelvin, or just 134 billionths of a degree above absolute zero. They described the ultracold material Jan. 31 in the journal Nature.
Ultracold systems are crucial to understand quantum behavior because quantum mechanics, the rules governing subatomic particles, dominate at low temperatures. These setups also let scientists precisely control the energy of particles to create quantum simulations, which model other quantum systems with physics we don't fully understand. For instance, studying the quantum behavior in a system of ultracold molecules could one day help scientists identify the material properties needed in high-temperature superconductors.
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The problem is that there's an inherent tradeoff: an ultracold system that is too simple may not capture the full array of behavior in interesting quantum systems. But add more complexity, and designing an effective experiment gets trickier.
"Usually people use atoms or ions and what makes them somewhat controllable is the fact that you have a relatively limited number of quantum states," Roman Bause, a quantum optics researcher at the University of Groningen in the Netherlands, told Live Science.
"But if I draw all the quantum states of a molecule, it will fill quite a thick book. It's a factor of a million or so more states."
All these additional quantum states open up more interesting quantum questions, but also make the molecules difficult to cool.
To solve that problem, in the new study, Tao Shi, a physicist at the Chinese Academy of Sciences, and international collaborators used a multi-step cooling process, beginning with laser cooling to create the record-breaking molecules.
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This cooling method uses laser beams fired from all directions at a moving atom. The atom absorbs light and enters an excited quantum state, then immediately releases energy to return to its ground state. But, because of how the atom is moving relative to the laser beams (known as the Doppler effect), the atom releases a little more energy than it absorbs, cooling itself.
"The problem with using this technique for molecules is that there's not just one ground state. You would potentially need thousands of laser beams and it's just too much technical effort," Bause said.
However, ultracold atoms are an excellent starting point to build ultracold molecules. Using a mixture of ultracold sodium (Na) and potassium (K) atoms, Shi's team weakly associated these single particles into diatomic NaK molecules.
This is where the technical difficulties really started. "The problem with associating cold atoms is you heat them while doing this so then you need another cooling technique, evaporative cooling," Bause said.
For reasons no one quite understands, under these cooling conditions the molecules stick together and the experimenter can no longer precisely control them. This particular challenge has stumped researchers across the field for years.
But, by shining in precisely controlled microwaves, Shi's team overcame the clumping issue in the diatomic NaK molecules as they were cooled down to 134 nanokelvin.
The microwaves also had a unique advantage when getting the two NaK molecules to weakly associate and form one four-atom-molecule of (NaK)2. "If you shape the microwaves exactly right, what you have is a potential that's not just repulsive at short ranges but it's also attractive at longer ranges," Bause said.
As such, this first-of-its-kind four-atom molecule has a central bond 1000 times longer than the bond between the sodium and potassium atoms and was created at a temperature more than 3000 times colder than any previous four-atom molecule.
The new finds are exciting because they "will ultimately bring us to interesting places where we currently have no theoretical handle — high temperature superconductors and materials for better lithium batteries for example," Bause said.
