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Low temperature record set – MPQ researchers pave the way for new forms of quantum matter
Insight into the main vacuum chamber of the NaK molecule experiment (Photo: mpq.mpg.de)
Researchers at the Max Planck Institute for Quantum Optics (MPQ) http://mpq.mpg.de have developed a new microwave cooling technique for molecular gases. This makes it possible to cool polar molecules down to a few nanokelvins, which is 21 billionths of a degree above absolute zero.
evaporative cooling used
Scientists have thus set a new low-temperature record and paved the way for new forms of quantum matter that were previously experimentally inaccessible. They used a gas of sodium-potassium (NaK) molecules trapped in an optical trap by laser light. The so-called evaporative cooling was used to cool the gas.
“This method works on the same principle as a cup of hot coffee,” says MPQ researcher Shin-Yu Luo. In coffee, water molecules constantly collide and exchange part of their kinetic energy. If two particularly high-energy molecules collide, one of them may be fast enough to escape the coffee – it pops out of the cup. The second molecule has less energy left. In this way the coffee cools slowly.
energy screen brings success
In the same way, a gas can be cooled down to a few nanokelvins—a billionth of a degree from absolute zero—at minus 273.15 °C. However: “If the gas consists of molecules, these must be additionally stabilized at very low temperatures,” Luo says. To prevent this, the researchers used a trick: the additional use of a specially formulated electromagnetic field, which acts as an energetic shield for the molecules—and keeps them from clumping together and colliding.
“We created this energy screen using a strong, rotating microwave field. This field causes the molecules to spin at a high frequency,” explains Luo’s colleague Andreas Schindewolf. If two molecules get too close, they can exchange kinetic energy – but at the same time they align themselves in such a way that they repel and quickly move away from each other.
To create a microwave field with the necessary properties, the researchers placed a helical antenna under an optical trap containing a gas composed of sodium-potassium molecules. As Shin-Yu Luo reports, “the rate at which the molecules interconnected was reduced by more than an order of magnitude.” In addition, under the influence of the field, a strong and far-reaching electrical interaction developed between the molecules.