Scientists recently created a never-before-seen four-atom molecule — the coolest ever created.
Researchers have created a strange structure of sodium-potassium – an oddball molecule – with an ultralong chemical bond at 134 nanokelvins, or more than 134 billionths of a degree. Absolute zero. They described the ultracold object in the journal on January 31 Nature.
Ultracold systems are important for understanding quantum behavior because quantum mechanics, the laws governing subatomic particles, dominate at low temperatures. These systems allow scientists to precisely control the energy of particles to create quantum simulations, which model physics that we do not fully understand in other quantum systems. For example, studying quantum behavior in a system of ultracold molecules may one day help scientists identify the material properties needed in high-temperature superconductors.
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The problem is that there is an inherent trade-off: a very simple ultracold system may not capture the full range of behavior in interesting quantum systems. But add more complexity, and designing an effective experiment becomes trickier.
„Usually people use atoms or ions and what makes them somewhat controllable is that you have a relatively small number of quantum states.” Roman bassquantum optics researcher at the University of Groningen in the Netherlands told Live Science.
„But if I were to draw all the quantum states of a molecule, it would fill a very thick book. It's a factor of a million or more states.”
All these extra quantum states open up very interesting quantum questions, but make molecules harder to cool.
To solve that problem, in a new study, Tao ShiPhysicists from the Chinese Academy of Sciences and international collaborators used a multi-step cooling process, starting 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 moves with the laser beam (known as the Doppler effect), the atom emits slightly more energy than it absorbs, cooling itself.
„The problem with applying this technique to molecules is that there isn't just one ground state. You need thousands of laser beams and it's a very technical endeavor,” Bass said.
However, ultracold atoms are an excellent starting point for creating ultracold molecules. Using a mixture of ultracold sodium (Na) and potassium (K) atoms, Shi's team weakly attached these single particles to diatomic NaK molecules.
This is where the technical problems really started. „The problem with fusing cold atoms is that you heat them up when you do this, so you need another cooling technique, evaporative cooling,” Bass said.
For reasons no one understands, the molecules stick together under these cooling conditions, and the experimenter cannot control them precisely. This particular challenge has stumped researchers across the field for years.
But Shi's team overcame the clumping problem in diatomic NaK molecules by shining it at precisely controlled microwaves, cooled down to 134 nanokelvins.
Microwaves also have a distinct advantage when weakly coupling two NaK molecules to form a four-atom molecule of (NaK)2. „If you design microwaves correctly, what you have is not repulsive at short range, but attractive at long range,” Bass said.
Thus, this first type of four-atom molecule has a central bond 1000 times longer than the bond between sodium and potassium atoms and was formed at a temperature 3000 times cooler than the previous four-atom molecule.
The new discoveries are exciting because they „ultimately bring us to interesting places where we don't have a theoretical handle — materials for high-temperature superconductors and better lithium batteries,” Bass said.