A quantum prediction from 1931 just came to life
Nearly a century after physicist Hans Bethe predicted them, scientists have created and directly observed unusual quantum states known as “Bethe strings” in an ultracold gas. By cooling cesium atoms to near absolute zero and trapping them in thousands of tiny one-dimensional tubes, researchers were able to make groups of atoms bind together without ordinary chemical bonds. These strange clusters could contain six or…
Key points
- Scientists created and observed Bethe strings, quantum structures Hans Bethe predicted in 1931.
- The team used ultracold cesium atoms confined to thousands of narrow one-dimensional tubes.
- Collisions and expansion tests showed the bound states survive collisions but break apart in three dimensions.
Researchers have created and observed Bethe strings, unusual quantum structures first predicted by physicist Hans Bethe in 1931, using ultracold atoms. The experiment was led by quantum physicist Hanns-Christoph Nägerl and involved theory teams from the University of Amsterdam and the Technical University of Munich. The results were published in Nature Communications.
Bethe strings are collective states in which particles in certain one-dimensional quantum systems bind together through their interactions rather than chemical bonds. For most of the past century they remained a theoretical idea.
How the experiment worked
The team cooled a cloud of cesium atoms to within a few billionths of a degree above absolute zero, then split it into several thousand extremely narrow tubes. Inside each tube, the atoms can move only along one direction, providing the one-dimensional setting Bethe strings require. By shifting the atoms' interactions from repulsive to attractive, the researchers made them bind into clusters of several sizes, some containing six or more particles.
This is a remarkable feature of the strings: they can collide without breaking apart
— Milena Horvath
Testing the bound states
To confirm the particles were truly bound, the researchers let the strings expand while trapped in their tubes. The strings collided with one another but survived. In a second experiment, the atoms expanded freely in three-dimensional space, where Bethe strings cannot exist, causing the bound states to break apart and release extra energy as motion. Comparing the two expansions let the team identify the strings; unbound particles showed essentially the same energy in both measurements.
What researchers said
Milena Horvath, a lead author, said the strings' ability to collide without breaking apart is a remarkable feature. Sudipta Dhar, another lead author, said Bethe strings were predicted almost a century ago and can now be created, manipulated and probed in the laboratory. Lead theorist Alvise Bastianello said the work opens new possibilities for studying how these collective quantum objects form and interact.
