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Sunday, April 14, 2024

Researchers Made Ultracold Quantum Bubbles on the Space Station

In March 2018, researchers launched what looks like a white, cooler-sized fridge to the International Space Station. That heavy box houses a $100 million facility known as the Cold Atom Laboratory, which enables an array of atomic physics experiments to be done at freezing temperatures in the zero-g of space. With those unique conditions, scientists have now produced tiny bubbles of extremely cold gas atoms, putting them on the edge of quantum physics territory.

That achievement, only possible in microgravity and at a millionth of a degree above absolute zero, the minimum temperature of the universe, would’ve been impossible to accomplish on Earth. The team of physicists behind the milestone, who are all working remotely—that is, on the ground—published their new research in the journal Nature last week, showing that they made the ultracold bubbles with an experimental apparatus that beamed lasers into a sealed vacuum chamber to cool down gas atoms. Then they deployed magnetic fields and radio waves to cast them into hollow, egg-shaped blobs. The experiment gives insight into the quantum realm and has applications for other areas of physics too.

“It’s exciting to see the atoms take these new shapes and to see new behaviors when you turn gravity off,” says David Aveline, an author of the study and member of the collaboration working on the Cold Atom Lab, operated by NASA’s Jet Propulsion Laboratory in Pasadena, California.

Ultracold atoms of gas—in this case, of rubidium—don’t act the way they normally would at room temperature, zipping around their container like microscopic billiard balls. As the gas cools, they move slower and slower, but without the sluggish atoms turning into a liquid or solid, like a vapor would. When they’re chilled close to absolute zero, they begin clumping together, and the wavelengths associated with the gas particles get longer and begin to overlap.

At such extremely frigid temperatures, the atoms start acting weirdly. They coalesce into a substance with quantum properties, behaving both as particles and as waves. At that point, they’re basically a quantum paradox and almost like a new state of matter, called a Bose-Einstein condensate, named after the Indian and German physicists from a century ago. (Technically, the ultracold atoms need to be cooled even further to be considered a Bose-Einstein condensate, but they’re showing signs of being on the cusp of that.) In any case, while quantum phenomena usually need powerful microscopes to be observed, these bubbles can be inflated to a size much bigger than the width of a human hair.

“We’re taking neat physics effects that normally happen at the scale of atoms, and we’re making them happen in objects that are up to a millimeter in size, trying to make quantum mechanics and strange physics behavior visible to the naked eye,” says Nathan Lundblad, an atomic physicist at Bates College in Maine and lead author of the study.

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This research could have applications beyond the world of quantum physics. One reason NASA’s interested is because such work on ultracold atoms could eventually aid the development of more precise gyroscopes and accelerometers, Aveline says. Inflating a bubble of ultracold atoms could also provide insight into the extremely fast expansion of the baby universe a fraction of a second after the Big Bang.

While these physicists and their colleagues have studied ultracold atoms on Earth for decades, the planet’s gravity still tugs on the atoms, even though it’s nature’s weakest force. On the ground, if scientists try nudging the atoms into a round blob or bubble, they end up drooping, creating a concave shape more like a little contact lens. That hasn’t stopped researchers from manipulating them into other shapes, like needles, rings, and pancakes. (The geometry of atoms can matter, since an ultrathin layer of carbon can be made into graphene, for example.) But to make bubbles of ultracold gas atoms that stay spherical or ellipsoidal and don’t flatten out, they had to take gravity out of the picture. That’s where the ISS came in.

Lundblad and Aveline’s supercool experiment is just one within the Cold Atom Lab, or CAL. Unlike a research lab at a university, CAL contains hardware that enables six teams to perform a variety of experiments, sort of like a kitchen where groups of cooks can come in to make use of the ingredients and tools to prepare their own dishes. Once astronauts installed the lab, it was able to run on its own, requiring no monitoring or assistance by ISS crew. (It can occasionally be repaired or improved, like when NASA astronauts Christina Koch and Jessica Meir conducted an upgrade in 2020.)

Unlike atomic physics research on Earth, teams of scientists like Lundblad and Aveline’s have to propose and conduct their experiments from afar. “It’s like the Hubble telescope, but for atomic physicists,” Lundblad says. The researchers operate CAL remotely from JPL, sending commands and receiving data, which they then distribute to the scientists who developed the experiments. They generally run them when astronauts are sleeping, partly because CAL sits near the exercise bike on the ISS, which could ever-so-slightly shake the apparatus.

In 2018, a group of German scientists launched a similar experiment on a rocket that briefly went into space, but this is the first time anyone has attempted it in orbit. Researchers have also attempted to simulate microgravity with a vacuum chamber in a 400-foot drop tower at the University of Bremen in northern Germany. But that near-weightlessness lasts just a few seconds, and scientists can only run a few such short-lived experiments per day, as opposed to CAL, which can run some experiments multiple times a minute.

“It’s great to see a low-cost serious science experiment happening. I see a lot of biological experiments in space, but in terms of the physical sciences, I think the Cold Atom Laboratory has been fantastic,” says Barry Garraway, a quantum physicist at the University of Sussex in the UK who earlier led theoretical work on Bose-Einstein condensates and isn’t involved in CAL. (The lab isn’t exactly cheap, but it’s inexpensive compared to multibillion-dollar particle accelerators, for example.)

“For me, this has reinvigorated my interest in space,” Garraway says. “For the experiment, my interest is now about how to improve it, make it more symmetric, smooth out some of the wrinkles, and help them on the journey.”

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