Scientists in Innsbruck create Bethe strings from ultracold atoms — ScienceDaily

Researchers at the University of Innsbruck in Austria created and observed unusual multipartite quantum states — Bethe strings — in an ultracold gas. Their existence was predicted in 1931 by physicist Hans Bethe. The experiment also involved theoretical groups from the University of Amsterdam and the Technical University of Munich, ScienceDaily reports. The results were published in the journal Nature Communications.

Atoms in one-dimensional tubes

For the experiment, the scientists cooled a cloud of cesium atoms to a temperature just a few billionths of a degree above absolute zero. They then divided it into several thousand extremely narrow tubes, in which the atoms could effectively move in only one direction.

The one-dimensional environment is precisely a necessary condition for the existence of Bethe strings. Unlike conventional molecules, these structures are not held together by chemical bonds: the particles remain together due to interactions between them. The researchers changed the nature of the interaction between the atoms from repulsive to attractive, after which bound states of different sizes formed. Some clusters contained six or more particles.

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Testing through collisions and expansion

To confirm that the particles had indeed formed bound structures, the team conducted two versions of an experiment involving gas expansion. In the first, the atoms remained in the one-dimensional tubes, expanded and collided with one another. The Bethe strings remained intact after the collisions.

In the second version, the confinement was removed, and the atoms expanded freely in three-dimensional space. Since Bethe strings can exist only in one dimension, they broke apart under these conditions. The energy that held the particles in a bound state was converted into motion, causing the atoms to fly apart faster. With repulsive interactions, when the particles were not bound, nearly identical energy was measured in both expansion variants. In the presence of strings, additional energy from their breakup was recorded during three-dimensional expansion.

Previously, Bethe strings had been experimentally observed in magnetic solid-state systems. An ultracold atomic gas makes it possible to precisely control the geometry of the system, particle density and their interactions, allowing researchers to study the formation and collisions of such collective quantum objects.