How the world’s first ‘quantum tornadoes’ came to be
The atom-sized version of extreme weather.
In some of the most extreme physical conditions, physicists have created perhaps the smallest storms yet.
These “quantum tornadoes,” whipped up by quantum researchers from MIT and Harvard, are the latest demonstrations of quantum mechanics—the strange code of laws that governs the universe at its finest, subatomic scales. They’re made from little clouds of sodium atoms, swirling around at temperatures of a fraction of a degree above absolute zero.
There’s a well-established method of deep-freezing atoms to these ultra-cold depths. It starts by trapping atoms—often alkali metals—in a magnetic cage, then shooting a laser at them. It might seem odd to use lasers as a way of cooling, but they produce a beam with only one wavelength of light (in this case yellow, to match sodium’s vaporous hue). When finely tuned, they can slow down the atoms to the point where they no longer produce heat.
What might be left in the end is a Bose-Einstein condensate, a lovably esoteric state of matter where multiple atoms act as one and behave in all sorts of unimaginable quantum ways.
Bose-Einstein condensates might seem weird, and they are—but it’s a sort of weirdness that physicists are used to dealing with. They were first predicted in the 1920s, and scientists succeeded in creating one in the lab in 1995. Those efforts won their scientists the 2001 Nobel Prize in Physics.
Since then, the physics world has been abuzz with attempts to push Bose-Einstein condensates to new heights (or new lows, as they were). For instance, physicists have wondered for some time if they could get atoms frozen in this state of matter to rotate.
Researchers were interested in doing that because it followed in the footsteps of something called a quantum Hall liquid. To make a long story short, under certain quantum conditions and in a magnetic field, a cloud of electrons that normally would push each other away would instead begin mimicking each other’s properties. That would cause them to act a little like water molecules in a fluid, flowing freely.
Electrons are difficult to observe, but physicists thought that rotating a Bose-Einstein condensate in a whirlpool could make atoms behave the same way. That’s appealing, because atoms are much, much bigger than electrons.
This latest research group isn’t the only one that has tried to stir up up a vortex. The challenge, then, comes with trying to get the atoms to spin without breaking the Bose-Einstein condensate.
“It’s kind of tricky to get, essentially, this rotation under control,” says Peter Schauss, a physicist at the University of Virginia, who wasn’t part of the newest experiment. “It’s easy to rotate it somehow, but it’s hard to rotate it in a way that you don’t heat it up.”
The Harvard-MIT group took their shot by wrangling a million sodium atoms, cooling them down to 100 billionths of a kelvin above absolute zero, and corralling them inside powerful electromagnets. Then, they spun the condensate around, hoping that they could see a quantum fluid in motion.
It worked—to a point. The atoms formed a thin, needle-like structure that had the properties of the fluid they’d been seeking. The researchers published the results up to here in Science in June 2021.
But they knew they could go further. They decided to keep spinning that needle to see what happened. And that’s when they notices something extraordinary: The needle began to undulate. At first, it began to wrap itself into a corkscrew. Then, the curls broke apart, shattering the needle into a smattering of little quantum blobs, which each began to rotate. Hence, quantum tornadoes.
The researchers compare this to chaos theory. Creating these tornadoes is akin to the famous example of a butterfly’s wings flapping and causing a storm on the other side of the planet, only that the process is playing out on a quantum scale. They published their description of the vortices in Nature earlier this month.
So what comes next? As one might imagine, getting atoms to cooperate at this level is not easy. “It’s still kind of a work in progress to get more stable lasers to … run these experiments efficiently,” says Schauss. “A lot of these experiments are limited by that.”
Another challenge: The tiniest of these quantum tornadoes had 10 atoms each. But some physicists think it’s possible to go even further—to get down to a Bose-Einstein condensate with just one atom. Doing that would really help physicists watch some of the arcane equations of quantum mechanics playing out in the real world (with very sophisticated cameras, at any rate).
While scientists continue to refine their process for crafting these vortices and other shapes of ultra-cold matter, their creations might be applied to technologies like sensors. The MIT-Harvard research was funded by DARPA, which wants to use the spinning condensates to detect subtle underwater movements. But so far, subtlety has not been part of the equation.