Physicists find evidence of atom clusters predicted by Bethe theory — Live Science
Physicists have obtained indirect evidence for the existence of so-called Bethe strings — clusters of interacting ultracold atoms arranged in a single line. The study, whose results were published in the journal Nature Communications on August 29, indicates that clusters of different sizes may have formed simultaneously in the experiment, including ones containing more than six atoms.
As Live Science reports, the mathematical description of such states originates in the 1931 work of German physicist Hans Bethe. While studying a chain of electrons, he found that several flipped spins could bind together and move as a single unit. This approach was later applied to larger particles, including atoms: according to theory, when they mutually attract, they can form clusters.
Atoms in one-dimensional tubes
The researchers cooled tens of thousands of cesium atoms into a Bose-Einstein condensate, in which the particles occupy a shared quantum state. Using laser beams, they divided the condensate into thousands of thin tubes, each containing up to several dozen atoms arranged in a line.
At first, the atoms repelled one another. The scientists then gradually reduced this repulsion to zero using a magnetic field and turned it into attraction. In one-dimensional space, atoms cannot pass each other, which makes it possible to avoid the rapid collapse of the gas characteristic of a three-dimensional environment.
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An indirect method of measurement
The team could not directly observe the Bethe strings. Instead, it compared the expansion of the atomic cloud after the particles were released. When the atoms could move only along the tubes, the clusters were expected to remain intact. After being released into three-dimensional space, they broke apart, converting binding energy into motion.
The three-dimensional clouds expanded with approximately three times more energy than the one-dimensional ones. On this basis, the researchers estimated the strength of the bond between atoms. The measurements also agreed with the predictions of generalized hydrodynamics, which describes such a gas as a mixture of particles of different types and clusters of different sizes. Co-author Hanns-Christoph Nägerl, an experimental physicist at the University of Innsbruck, stressed that the agreement between theory and data is evidence, but not conclusive proof of the existence of these states.
The strings likely do not exist for long: over time, cesium atoms form ordinary molecules. In the coldest experimental runs, the team lost about 40% of the atoms. Among the possible ways to study such states more directly, the scientists are considering protecting ultracold molecules with microwaves and quantum gas microscopy, which makes it possible to observe atoms one by one.
