An idea from 1931
In 1931 the physicist Hans Bethe proposed that particles in certain one-dimensional quantum systems could join into collective states. Those states later took his name. Unlike molecules, Bethe strings are not held together by chemical bonds. The particles stay bound because of the way they interact, and the states can only exist in one dimension.
For most of a century they were mainly a theoretical idea. A team at the University of Innsbruck has now created and detected them in a gas of ultracold atoms, working with theory groups at the University of Amsterdam and the Technical University of Munich. The work was published in Nature Communications.
Cold atoms in narrow tubes
The experiment, led by Hanns-Christoph Naegerl, started with a cloud of caesium atoms cooled to within a few billionths of a degree of absolute zero. The researchers then separated the cloud into several thousand very narrow tubes. Inside each tube the atoms can move in only one direction, which creates the one-dimensional setting the strings need.
The team can also tune how strongly the atoms interact. When they changed the interactions from repulsive to attractive, the atoms bound together. Rather than collapsing into a single group, they formed bound states of several different sizes, and some of the larger clusters held six or more particles.
Collisions that leave the strings intact
The hard part was proving the particles were truly bound. One test let the strings expand while still confined to their tubes. As they expanded, the strings met and collided, yet the bound structures survived. “This is a remarkable feature of the strings: they can collide without breaking apart,” said lead author Milena Horvath.
A second test removed the confinement and let the atoms spread through three-dimensional space. Because Bethe strings exist only in one dimension, the bound states broke apart, and the energy that had held them turned into motion. The atoms then spread faster, and that extra energy gave researchers a clear signature of the strings.
The group says the system offers a controllable new platform for studying quantum many-body physics. Bethe strings had been seen before in solid-state magnets, but cold atoms let researchers adjust geometry, density and interaction strength with high precision.