In 1931, physicists were trying to use quantum mechanics to explain why some materials are magnetic. Then, a young physicist named Hans Bethe looked at a single row of electrons. Each electron's spin can point up or down. Bethe worked out the math for how such a row behaves and found that if a few spins in the row flip, they can stick together and travel down the row as a single unit.
Now, physicists have found evidence of these clusters in a line of ultracold atoms. The findings were published Aug. 29 in the journal Nature Communications.
Researchers have spotted small Bethe strings before, in magnetic crystals and superconducting quantum circuits — but those experiments captured strings one at a time.
To make the "necklace" the researchers had to create a Bose-Einstein condensate, a special phase of matter. (Image credit: blackdovfx via Getty Images)
It's tricky to get atoms to attract each other. In three dimensions, a gas made of attracting atoms tends to collapse as the atoms clump together and form molecules. That process releases energy that kicks atoms out of the experiment, so the gas quickly collapses and disappears. But when they're in a single line, the atoms behave differently.
To build their "necklaces," the team started with tens of thousands of cesium atoms cooled into a Bose-Einstein condensate — a state of matter in which atoms chilled to near absolute zero share a single quantum state. Laser beams then divided the condensate into thousands of thin tubes, each of which held up to a few dozen atoms in a line.
The team couldn't see the strings directly. Instead, they let the atoms go and measured how fast the cloud spread. When the atoms could expand only along their tubes, any strings stayed intact. When the atoms were freed in all three dimensions, the strings broke apart and released their binding energy as motion. The researchers found that the 3D clouds spread with about three times as much energy as the one-dimensional ones did.
Nägerl was careful not to call this proof. "That some theory fits to your data is not a proof that something exists," he said; it's just more evidence to support it.
A short-lived state
"Transiently you create these Bethe string states," Nägerl said. "But I don't think they are long-lived."
Related storiesHe has an idea for future experiments. Physicists recently learned to shield ultracold molecules with microwaves, which keeps the molecules from colliding at close range. Strings made from shielded molecules might last long enough to be studied directly. But Nägerl stressed that the idea is untested. "I've had nobody do a theory check on my claims now, but I think intuitively this should work," he said.
For Nägerl, Bethe strings are one step in understanding how quantum systems with many particles behave and settle down over time. His long-term goal is an experiment in which physicists control both how every particle moves and how its spin flips, letting them assemble model materials piece by piece, "like a LEGO type," he said. His team has begun the next step: using lasers to control the spins of the atoms in its one-dimensional gas. He described Bethe strings as "a little diversion." But because the strings form whenever the atoms attract, they can't be ignored.
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