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Physicists Create World’s Coolest Molecules

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Lisa Winter

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1885 Physicists Create World’s Coolest Molecules
Optical cavity of laser used in the MOT. Credit: Michael Helfenbein

Water is a pretty cool molecule, as it the basis for all life as we know it. But is it the coolest molecule out there? Heck no, say physicists at Yale, who apparently took the question a bit too literally and instead chilled strontium monofluoride (SrF) down to just above absolute zero. This made SrF quite literally the coolest molecule on the planet. The research was led by Dave DeMille and the paper was published in Nature.

The process was done with a technique known as magneto-optical trapping (MOT). The system uses lasers and magnetic fields within a vacuum chamber to trap and create an ultra cold cloud of particles, chilling the SrF down further than any molecule has been directly cooled before, at a mere 0.0025 K.


“We can start studying chemical reactions that are happening at very near to absolute zero,” DeMille said in a press release. “We have a chance to learn about fundamental chemical mechanisms.”

Previously, such extreme supercooled temperatures have only worked with single atoms, as molecules typically have too much vibration and rotation when being studied. Supercooling atoms before bonding them into molecules had been attempted before, but was not an ideal solution. However, the simple diatomic SrF essentially has only has one electron orbiting the molecule and won’t cause too much motion, which made it possible to devise an apparatus that would get the job done.

The SrF was very carefully streamed out of a cryogenic chamber, forming a beam of the molecules. Laser beams pointed at the molecule beam act like a brake, controlling where the molecules slow and eventually stop. However, it was important that the laser did not interact with the molecules too much and force them to spin. Wavelengths from the laser were carefully regulated to nine decimal places to prevent unwanted motion. The braking lasers guide the molecules to a magnetic field where other lasers are used to hold the cloud of SrF in place so it can be supercooled. The system used a total of 12 lasers to slow, trap, then cool the molecules. 

“It’s like trying to slow down a bowling ball with ping pong balls. You have to do it fast and do it a lot of times,” DeMille explained. “Quantum mechanics allows us to both cool things down and apply force that leaves the molecules levitating in an almost perfect vacuum.”


Reducing the temperature nearer to absolute zero means bringing matter closer to its ground state where it has the lowest amount of internal energy, making extremely delicate measurements much more precise. When researchers study particles under supercooled conditions, it helps make sense of how that matter behaves under normal conditions. The ability to expand upon this technique and investigate other molecules at such extremely low temperatures will also allow scientists to study certain aspects of the standard model of particle physics as well as creating particles that could ultimately be used in a quantum computer.

Lasers cool SrF down to 2.5 mK. Credit: John Barry/DeMille Group


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