Scientists Realize Bethe Strings Nearly a Century After Their Prediction

After nearly 100 years since Hans Bethe first theorized their existence, physicists have successfully created and observed Bethe strings, unique quantum structures formed by ultracold atoms. This groundbreaking experiment, led by quantum physicist Hanns-Christoph Nägerl, allows for detailed studies of quantum many-body systems, offering insights previously limited to theoretical discussions.

Theoretical Foundations and Experimental Realization

In 1931, Bethe proposed that particles confined to a one-dimensional quantum system could form collective states known as Bethe strings. These aren't conventional molecules; they don't rely on chemical bonds but are instead held together through interactions among the particles, existing strictly in a one-dimensional framework. Until now, this concept remained largely hypothetical, with no experimental verification.

Now, researchers at the University of Innsbruck, collaborating with teams from the University of Amsterdam and the Technical University of Munich, have made this theoretical idea a reality. Their findings, recently published in Nature Communications, demonstrate the ability to create multiparticle bound states in a carefully controlled ultracold gas environment.

Creating Bethe Strings with Ultracold Atoms

The team began their work by cooling a cloud of cesium atoms to just a few billionths of a degree above absolute zero. They then manipulated this cloud into thousands of extremely narrow tubes, drastically restricting the movement of the atoms to one dimension, which is essential for the formation of Bethe strings. By precisely tuning the interactions between atoms, they could shift from repulsive to attractive forces and observe the formation of bound states, sometimes comprising six or more atoms.

To confirm that these atoms were indeed forming bound states, researchers designed an experiment to test the stability of the strings. "One of the simplest experiments was to let the strings expand," explained Milena Horvath, a lead author of the study. By allowing the atoms to expand while constrained within the tubes, the strings could collide without disintegrating, highlighting their unique collisional stability. "This is a remarkable feature of the strings: they can collide without breaking apart," Horvath added.

Observing Bound States and Their Properties

The team conducted a second experiment where they released the atoms from confinement into three-dimensional space. In doing so, the conditions for Bethe strings collapsed, and the individual atoms began to disperse. This transition illustrated how the energy that once held the particles together was transformed into kinetic energy, causing them to spread apart faster and offering a vital comparative measurement against the earlier, constrained observations.

The two experimental setups yielded strikingly different energy profiles. When the particles were unbound due to repulsive interactions, they behaved consistently across both testing environments. However, with Bethe strings in play, the three-dimensional expansion introduced extra energy upon the disbandment of the bound states, solidifying the experimental evidence for Bethe strings.

Implications for Quantum Physics

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

While there have been previous detections of Bethe strings in solid-state magnetic systems, this latest research showcases their realization in an ultracold atomic environment. This setting allows for unprecedented control over system parameters, providing a fertile ground for exploring the interactions and formation of these fascinating quantum structures.

According to lead theorist Alvise Bastianello, this research significantly broadens opportunities for investigating the dynamics of collective quantum objects. Such findings could steer future studies on many-body quantum phenomena and the underlying principles governing these types of systems.

The research was supported by funding from various scientific organizations, underscoring the collaborative effort in advancing our understanding of quantum mechanics. Milena Horvath belongs to the FWF doctoral program Atoms, Light, and Molecules (DK-ALM).

Materials provided by University of Innsbruck. Note: Content may be edited for style and length.