Reaching for New Levels of Precision with the First Molecular Fountain

There’s an old saying that’s popular in science departments:

If it moves, it’s biology.
If it smells, it’s chemistry.
If it doesn’t work, it’s physics.

While not exactly right, the saying does have some truth. After all, if you want to see, measure, detect, or explain something that hasn’t been done before, it’s not like there’s a step-by-step manual to follow. Physicists often have to design and build custom equipment. There is a lot of engineering and troubleshooting, and like Edison’s lightbulb, things usually don’t work the first time around. Or the second time. But, often, persistence pays off. Hendrick Bethlem and his team know this well.

Bethlem leads a team at VU University Amsterdam that just demonstrated the first molecular fountain, an instrument capable of making extremely precise measurements. In theory, molecular fountains offer the most stringent tests of some fundamental physics theories. Their work is being published today in Physical Review Letters.

Cunfeng Cheng and Hendrick Bethlem with the molecular fountain.
Image Credit: Hendrick Bethlem.

Like atomic fountain clocks and water fountains, a molecular fountain is a device that tosses particles upward and then catches them when gravity overcomes them and they fall back down. Unlike in a water fountain though, a molecular fountain propels the particles (molecules, in this case), up through a microwave cavity, a kind of metal cylinder with microwave radiation inside. The molecules travel through the cavity twice, once on their way up and once on their way down. By analyzing how the molecules interact with the radiation, you can measure their properties very precisely. The longer the molecules interact with the microwaves, the higher the resolution.

The idea isn’t new. Molecular fountains were considered a long time ago, but this is the first time one has been brought to life. It’s a huge physics and engineering challenge. Before you can toss the molecules upward, you have to slow them down to a standstill. Then you can point them in the right direction and nudge them upward at the right speed, so that they are overcome by gravity and fall back down from the right height with the right speed. This is challenging enough to do with atoms, but molecules can vibrate and wiggle in ways that atoms can’t, making them even harder to slow down. It’s a tradeoff. Atoms are easier to work with, but molecules are more sensitive to certain physical effects and therefore make it possible to do more precise measurements.

As a graduate student, Bethlem developed a device that used electric fields to slow down polar molecules. When he started his own research group a few years later, Bethlem started work on a molecular fountain based on this device. After tests showed that this design would never be able to capture the signal from molecules falling back down, the team went back to the drawing board and came up with a new design.

This time the results seemed more promising and the team was optimistic that they would see the falling molecules. But, before they did, someone threw a wrench in the plans—more accurately, someone accidentally dropped a screw in the device. The screw destroyed some equipment and the entire fountain had to be taken apart and rebuilt. “A blessing in disguise,” Bethlem calls it, because after putting it back together they had twenty times more signal. After another year of hard work by postdoc Cunfeng Cheng and the rest of the team, they finally saw the falling molecules! This means the molecular fountain can be used to make measurements.

Unlike the grand fountains at the Bellagio, the molecular fountain is unassuming and looks like it belongs in a lab. The molecules reach a height of 100-180 millimeters, and travel between 1.4 and 1.9 meters per second. For comparison, the average air molecule at room temperature moves at close to 500 meters per second, so slowing them to less than 2 is a real feat. It might not sound like much, but the molecules are in free fall for more than 250 milliseconds—that’s really long for molecules. This span of time, while quite short, could help us understand the matter and laws that make up our physical world more precisely than ever before. If, for instance, rotating molecules were found to fall at a different rate than stationary ones, it could provide deep insights into the fundamental nature of gravity.

Kendra Redmond

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