Ultracold Tetratomic Molecules With Strange Chemical Bonds Shows Potential in Precision Measurements and Quantum Information Processing

Due to their rich internal structure, ultracold polyatomic molecules show potential in cold chemistry. However, compared with diatomic molecules, they are more complex, which poses a challenge in using conventional cooling techniques.

(Photo: Wikimedia Commons/ National Institute of Standards and Technology)

Ultracold Molecules System

Ultracold systems are essential in understanding quantum behavior because quantum mechanics dominate at low temperatures. These setups allow experts to control the energy of particles to generate quantum simulations. By understanding the quantum behavior in an ultracold molecules system, scientists can one day identify the properties required in high-temperature superconductors.

However, there is an inherent tradeoff. A too-simple ultracold system may not capture the full order of behavior in interesting quantum systems. If more complexity is added, designing an effective experiment gets even trickier.

Advanced technologies have allowed humans to use atoms or ions. Having a relatively limited number of quantum states makes these particles somewhat controllable. However, all the quantum states of a molecule are a factor of a million or so. These additional quantum states open up more interesting quantum questions but also make the molecules different to cool.

Oddball Molecule

A team of researchers led by physicist Tao Shi from the Chinese Academy of Sciences has been working to address this challenge has been working to address this challenge. They created an oddball molecule at 134 nanokelvins, or 134 billionths of a degree above absolute zero,, using a strange configuration of sodium-potassium with an ultralong chemical bond.

Shi and his collaborators used a multi-step cooling process to make the record-breaking molecules. This method utilizes laser beams fired from all directions at a moving atom, which then releases energy to return to its ground state. Based on the movement of the atom relative to the laser beams, this particle releases a little more energy than it absorbs, cooling itself as a result.

The problem with this technique is that there is not just one ground state; researchers would need thousands of laser beams and too much technical effort. Fortunately, ultracold atoms are an excellent starting point for creating ultracold molecules. Using a mixture of ultracold sodium (Na) and potassium (K) atoms, the research team weakly associated the single particles with diatomic NaK molecules.

However, this method will heat the atoms so that researchers would need another cooling technique. Under various cooling conditions, the molecules stick together, and the researcher can no longer control them precisely. This challenge has mystified scientists across the field for years.

Shi's team overcame the clumping issue in the diatomic NaK molecules by shining in precisely controlled microwaves. This radiation has the advantage of weakly associating the two NaK molecules to form a four-atom-molecule (NaK)2. The microwaves are shaped exactly right, and the resulting potential is not just repulsive at short ranges but also attractive at longer ranges.

As a result, the first-of-its-kind four-atom polyatomic molecule has a central bond that is 1000 times longer than the bond between the sodium and potassium atoms. It is also created over 3000 times colder than any previous four-atom molecule.