Step aside, Yale scientists—you’re no longer the coolest kids on the block. After setting a world record for chilling the world’s coldest molecules last August, MIT researchers have now knocked them off the frosty podium with their frigid molecules.
With the assistance of lasers once again, the team managed to get sodium potassium gas molecules tantalizingly close to absolute zero, or 500-billionths of a degree above it, to be precise. Rather than whizzing around as molecules normally do, these stable, ultracold molecules slowed down to a snail’s pace and resisted reactive collisions with one another that could break them apart.
Last year’s efforts saw molecules of strontium monofluoride plunged to 2.5 thousandths of a degree above absolute zero, so for now the MIT team are top of the leaderboard. But scientists aren’t just chilling molecules to trump each other; it’s hoped that cooling molecules to such low temperatures will draw their normal, chaotic behavior to an end, bringing about strange and exotic states of matter that have never been seen before.
“We are very close to the temperature at which quantum mechanics plays a big role in the motion of molecules,” lead researcher Martin Zwierlein said in a statement. “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.”
As described in Physical Review Letters, the team strived to create superchilled molecules of sodium potassium, starting off with clouds of these individual atoms. Chilling atoms to ultracold temperatures is easier than attempting the same feat with molecules—two or more bonded atoms—due to their simpler structure, but that’s not the only hurdle the researchers had to overcome: Sodium and potassium don’t usually stick together to form molecules.
The team therefore had to put the individual atoms through a bonding session, which first involved cooling the clouds to close to absolute zero and then subjecting them to a magnetic field, causing them to vibrate and stick together. Although this managed to create sodium potassium molecules, the atoms were too far apart and thus the bond was weak.
To tighten it up and create a more stable molecule, the researchers employed a pair of lasers. One mirrored the frequency of the molecule’s initial vibrating state, whereas the other was set to the lowest possible state. The molecules then absorbed the energy from the lower frequency beam and emitted into the other, causing them to shake off their vibrational energy.
Ultimately, the researchers ended up with a super frosty sodium potassium molecule at its lowest vibrational and rotational state. The final temperature was just 500 nanokelvins; to put that in perspective, that’s more than a million times cooler than interstellar space. This is about one thousand times colder than direct cooling techniques have been able to achieve. Furthermore, the molecules were stable and hung around for a relatively long period of time before decaying, about 2.5 seconds.
The researchers are hopeful that this will give them a long enough window to eventually start observing exotic states of matter, but first they’ll have to dip the temperatures even lower. “Now we’re at 500 nanokelvins, which is already fantastic, we love it. A factor of 10 colder or so, and the music starts playing,” said Zwierlein.