Scientists observe Bethe strings in ultracold gas, confirming 1931 prediction

Nearly a century after physicist Hans Bethe predicted their existence, scientists have created and directly observed unusual quantum structures known as "Bethe strings" using ultracold atoms. The experiment, led by quantum physicist Hanns-Christoph Nägerl at the University of Innsbruck, provides researchers with a highly controllable way to explore these unusual quantum many-body states. The results were published in Nature Communications.

Creating Bound States in One Dimension

In 1931, Hans Bethe proposed that particles in certain quantum systems restricted to one dimension could join together into collective states now called Bethe strings. These structures differ fundamentally from familiar molecules. Rather than being connected through chemical bonds, the particles remain bound because of their interactions with one another, and the resulting states can exist only in one dimension.

To produce these states, the researchers started with a cloud of cesium atoms cooled to within only a few billionths of a degree above absolute zero. They then separated the cloud into several thousand extremely narrow tubes. Within each tube, the atoms are effectively restricted to moving along a single direction, creating the one-dimensional environment required for Bethe strings to exist.

By changing the interactions between the atoms from repulsive to attractive, the team caused the atoms to bind together. Rather than simply collapsing into one group, the atoms formed bound states of several different sizes. Some of the larger clusters contained six or more particles.

Testing Stability Through Collisions

To demonstrate that these particles were truly bound together, the researchers performed two versions of an expansion experiment. In the first version, they allowed the atoms to spread out while keeping them trapped inside their one-dimensional tubes. During this expansion, the strings encountered one another and collided, yet the bound structures survived.

"This is a remarkable feature of the strings: they can collide without breaking apart," says Milena Horvath, one of the lead authors.

In the second version, the scientists removed the confinement and allowed the atoms to expand freely through three-dimensional space. Because Bethe strings can exist only in one dimension, releasing the atoms into three dimensions caused the bound states to fall apart. The energy that had kept the particles bound was then transformed into motion, making the atoms spread apart faster.

When the particles were unbound, as they were when the interactions were repulsive, both expansion measurements produced essentially the same energy. When Bethe strings were present, however, the three-dimensional expansion contained extra energy released as the bound states broke apart. This comparison provided a clear way to identify the presence of Bethe strings.

A New Laboratory for Quantum Physics

Bethe strings have previously been detected experimentally in solid-state magnetic systems. The new work places these unusual quantum bound states in a very different environment: an ultracold atomic gas where researchers can control the system's geometry, particle density, and interactions with exceptional precision.

"Bethe strings were predicted almost a century ago as part of a beautiful mathematical description of quantum many-body systems," says Sudipta Dhar, another lead author. "Now we can create them in the laboratory, manipulate them and make them collide and probe their remarkable collisional stability."

Lead theorist Alvise Bastianello notes that this environment "opens new possibilities for studying how these collective quantum objects form and interact."

Why it matters

The direct observation of Bethe strings in a controllable ultracold gas validates a theoretical prediction from 1931 and provides a new platform for studying quantum many-body physics. Unlike solid-state systems, the ultracold atomic gas allows for precise manipulation of particle interactions and geometry, enabling scientists to probe the formation and stability of these unique quantum states in ways previously not possible.

---

Sources

• sciencedaily.com: A quantum prediction from 1931 just came to life | ScienceDaily
• ua.news: Innsbruck scientists create Bethe strings from ultracold atoms | UA.NEWS

This article was drafted with AI assistance and checked against the sources above. Company claims are reported as claims. Cover image is AI-generated.