Abstract:
A quantum mechanical prediction born in 1931 was finally successfully realized by scientists nearly a century later.
A research team from the University of Innsbruck in Austria, the University of Amsterdam in the Netherlands, and the Technical University of Munich in Germany announced that they created and directly observed special quantum structures called "Bethe Strings" for the first time in ultracold atomic gases. This achievement not only verifies the theoretical prediction proposed by German physicist Hans Bethe nearly a century ago, but also provides a new experimental platform for studying quantum many-body systems.
Bate proposed in 1931 that in some special quantum systems, particles can form a unique bound state. These particle groups are called Bethe strings and can only exist in a one-dimensional environment, that is, a system in which particles can only move along a straight line.
Unlike traditional molecules, Bet strings do not rely on chemical bonds to maintain their structure, but remain combined entirely through quantum interactions between particles. Because of this, it has long been regarded as one of the most elegant and difficult to test predictions of quantum many-body theory.
Over the past few decades, scientists have indirectly observed phenomena related to Bethe strings in solid-state magnetic materials. But these systems lack sufficient controllability, making it difficult for researchers to precisely regulate the interactions between particles.
Now, the research team has successfully constructed a Bethe string in an ultracold atomic gas for the first time, creating an ideal environment for further studying its properties.
Sudipta Dhar, one of the study leaders, said that Bethe strings have appeared in mathematical theories describing quantum many-body systems almost a century ago. Now, researchers can not only create them in the lab, but also manipulate them, make them collide, and study their surprising stability.
To allow the Bethe strings to form, the team first cooled the cesium atoms to just a few billionths of a degree above absolute zero. They then used a special device to distribute the atoms into thousands of extremely narrow microscopic tubes.
The function of these pipes is to limit the movement direction of atoms so that particles can almost only move in a straight line, thereby creating the one-dimensional quantum environment required by the theory.
After establishing this system, the researchers adjusted the nature of the interaction between the atoms, converting the originally mutually exclusive state into mutual attraction.
Intuitively, strong attraction could cause the entire cloud of atoms to collapse. However, experiments found that the system did not collapse, but formed multiple stable bound clusters. These clusters vary in size, with some structures containing more than six particles clustered together.
The research team confirmed that these special clusters are exactly the Bethe strings predicted by Hans Bethe's 1931 theory.
This ultra-cold gas experiment has obvious advantages over previous observations in solid materials. Researchers can not only adjust the particle density, but also precisely control the system geometry and the intensity of interactions between particles, allowing them to study the formation and evolution of Beit strings with an unprecedented degree of freedom.
Scientists said that this result is not only an important verification of classical quantum theory, but also helps to deeply understand the complex behavior in one-dimensional quantum systems.
Since the development of quantum computing, quantum simulation, and future quantum communication systems all rely on the understanding of many-body quantum phenomena, Bethe string research is expected to become an important theoretical and experimental basis in related fields.
From mathematical derivation in 1931 to experimental realization in 2026, this scientific journey spanning 95 years has finally come to an end. For the quantum physics community, this is not the end, but a new starting point for exploring more complex quantum structures.
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