Nearly a century after physicist Hans Bethe predicted an unusual form of quantum behaviour, scientists have finally recreated it in an ultracold atomic gas. The breakthrough offers a new way to investigate how particles interact under extraordinary conditions.

Researchers at the University of Innsbruck in Austria successfully created and observed structures known as Bethe strings, in which groups of atoms remain bound together without conventional chemical bonds.

Published in Nature Communications in October 2026, the discovery provides a powerful experimental platform for exploring quantum physics and understanding the behaviour of complex systems.

A Prediction Dating Back To 1931

In 1931, German physicist Hans Bethe developed a mathematical description of particles interacting within certain one-dimensional quantum systems.

His calculations predicted that these particles could form collective bound states, now known as Bethe strings. Unlike ordinary molecules, which are held together by chemical bonds, these structures arise from quantum interactions between particles confined to one dimension.

Although researchers had previously detected evidence of Bethe strings in magnetic materials, creating and directly investigating them in a controllable gas had remained a major experimental challenge. The new work brings this longstanding theoretical prediction into a laboratory environment where scientists can manipulate the particles with remarkable precision.

How Scientists Created Quantum Strings

The research team, led by quantum physicist Hanns-Christoph Nägerl, began with a cloud of cesium atoms cooled to just a few billionths of a degree above absolute zero. At such extremely low temperatures, the atoms move slowly enough for researchers to investigate their quantum behaviour. Scientists then divided the cloud into thousands of exceptionally narrow tubes, restricting the atoms to movement along a single direction.

This arrangement created the one-dimensional conditions necessary for Bethe strings to form. By carefully adjusting the interactions between atoms from repulsive to attractive, the researchers encouraged them to assemble into bound clusters. Some structures contained six or more atoms, demonstrating that relatively large groups could remain connected through quantum interactions.

Why These Atomic Clusters Are Remarkable

One of the most surprising properties of Bethe strings is their ability to survive collisions. To investigate this behaviour, researchers allowed the atomic clusters to spread through their narrow tubes. As the structures moved, they encountered one another and collided. Remarkably, the clusters remained intact.

Lead researcher Milena Horvath explained that this unusual stability was among the most striking features observed during the experiments. The result demonstrates how quantum systems can behave very differently from familiar objects, where collisions may cause structures to break apart.

What Happened When The Atoms Were Released?

The scientists conducted another experiment to confirm that the particles had genuinely formed bound states. This time, they removed the one-dimensional confinement and allowed the atoms to expand freely through three-dimensional space. Because Bethe strings depend on their restricted geometry, the structures broke apart when released.

The energy previously associated with their binding was converted into additional motion, causing the atoms to spread more rapidly. By comparing expansion in one dimension with expansion in three dimensions, researchers identified the extra energy associated with the bound clusters. This provided compelling experimental evidence that Bethe strings had formed.

Why The Discovery Matters For Physics

The breakthrough gives scientists a highly controllable system for studying quantum many-body physics, which examines how large groups of interacting particles behave collectively. Researcher Sudipta Dhar explained that the experiment makes it possible to create, manipulate and collide structures that had long been understood primarily through mathematical models.

The work involved collaborations with theoretical physicists at the University of Amsterdam and the Technical University of Munich. Lead theorist Alvise Bastianello suggested that the findings open new opportunities to investigate how collective quantum structures form and interact.

What Could Scientists Discover Next?

Researchers can now explore how Bethe strings behave under different particle densities, interaction strengths and experimental conditions.

Such investigations may improve understanding of complex quantum systems and test predictions that have previously been difficult to examine experimentally. Although practical technological applications remain uncertain, the discovery represents an important advance in fundamental physics.

Almost 95 years after Bethe first described these unusual states, scientists can now create them, observe their behaviour and investigate their remarkable stability in a carefully controlled laboratory setting.