A team of physicists led by Prof Martin Zwierlein from Massachusetts Institute of Technology has cooled molecules of sodium-potassium (NaK), each consisting of a single sodium and potassium atom, to a temperature of 500 nanokelvins – very close to absolute zero and over a million times colder than interstellar space.
In this artist’s illustration, the sodium-potassium molecule is represented with frozen spheres of ice merged together: the smaller sphere on the left represents a sodium atom, and the larger sphere on the right is a potassium atom. Image credit: Jose-Luis Olivares / MIT.
Every molecule is composed of individual atoms that are bonded together to form a molecular structure. The simplest molecule, resembling a dumbbell, is made up of two atoms connected by electromagnetic forces.
Prof Zwierlein and his colleagues sought to create ultracold molecules of sodium-potassium. However, due to their many degrees of freedom cooling molecules directly is very difficult. Atoms, with their much simpler structure, are much easier to chill.
As a first step, the physicists used lasers and evaporative cooling to cool clouds of individual sodium and potassium atoms to near absolute zero. They then essentially glued the atoms together to form ultracold molecules, applying a magnetic field to prompt the atoms to bond – a mechanism known as a ‘Feshbach resonance.’
The team found that the ultracold molecules of sodium-potassium exhibited very strong dipole moments and were relatively long-lived and stable, resisting reactive collisions with other molecules.
“While molecules are normally full of energy, vibrating and rotating and moving through space at a frenetic pace, the ultracold molecules have been effectively stilled – cooled to average speeds of centimeters per second and prepared in their absolute lowest vibrational and rotational states,” explained Prof Zwierlein, senior author of the paper reporting the results in the journal Physical Review Letters.
“We are very close to the temperature at which quantum mechanics plays a big role in the motion of molecules,” he added.
“So these molecules would no longer run around like billiard balls, but move as quantum mechanical matter waves. And with ultracold molecules, you can get a huge variety of different states of matter, like superfluid crystals, which are crystalline, yet feel no friction, which is totally bizarre. This has not been observed so far, but predicted. We might not be far from seeing these effects, so we’re all excited.”
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Jee Woo Park et al. Ultracold Dipolar Gas of Fermionic 23Na40K Molecules in Their Absolute Ground State. Phys. Rev. Lett. 114, 205302; doi: 10.1103/PhysRevLett.114.205302
