Physicists have used a rotating microwave field to cool a gas made up of polar molecules to
For their experiments, researchersused a gas consisting of sodium and potassium molecules, which were kept in an optical trap using laser radiation. The physicists cooled the gas using evaporative cooling, a technique that is applied to individual atoms.
Vacuum chamber in which supercold gas is generated. Image: Max Planck Society
The principle of operation of this method is based oncollision of particles in a magnetic trap. Individual atoms, colliding with each other, transfer part of their kinetic energy. Over time, individual atoms become much more energetic than others, and they leave the trap, reducing the energy of the system and the temperature of the group of atoms remaining in it.
Polar molecules are characterized by uneventhe distribution of electric charge, the scientists explain. Unlike free atoms, they can rotate, vibrate, attract or repel each other. They behave like tiny magnets and can stick together preventing cooling.
To overcome this limitation, researchersused a specially prepared electromagnetic field, which serves as an energy shield for molecules and does not allow them to adhere and stick together. Under the influence of the field, if two molecules come too close to each other, they can exchange kinetic energy, but at the same time they line up in such a way that they repel each other and move quickly in different directions.
Sodium laser generating yellow light for laser cooling and visualization of sodium atoms. Image: Max Planck Society
To create a microwave field with the requiredproperties, the researchers placed a helical antenna under an optical trap containing a gas of sodium-potassium molecules. In this experimental setup, the molecules began to collide much more frequently, on average about 500 times per molecule.
As a result, after only a third of a second, the temperature reached about 21 nK, which is well below the critical "Fermi temperature", the limit below which quantum effects determine the behavior of the gas.
The researchers believe that the new cooling technology will allow the creation and study of various quantum forms of matter that were previously predicted theoretically.
Cover photo: Artistic image of a microwave refrigerator for gasSource: Max Planck Society
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