Nearly a century after Hans Bethe’s 1931 theoretical prediction, researchers have successfully created and observed quantum structures known as Bethe strings in a laboratory setting. Led by quantum physicist Hanns-Christoph Nägerl, the international team demonstrated the existence of these unusual quantum many-body states using ultracold atoms, marking a significant milestone in quantum physics research.
Bethe strings are collective quantum states formed by particles bound together through interactions rather than chemical bonds. These structures can exist only in one-dimensional environments. For decades, they remained primarily theoretical constructs. The research team, collaborating across the University of Innsbruck, the University of Amsterdam, and the Technical University of Munich, created the strings by cooling cesium atoms to near absolute zero and confining them in narrow tubes, effectively restricting their movement to a single dimension. By manipulating the strength of atomic interactions, researchers induced the atoms to bind into clusters of varying sizes, with some containing six or more particles.
The scientists demonstrated the bound nature of these structures through collision experiments. When atoms within their one-dimensional tubes were allowed to expand, the Bethe strings collided with one another yet remained intact, a key characteristic distinguishing them from ordinary particle clusters. In contrast, when confinement was removed and atoms expanded freely into three-dimensional space, the bound states disintegrated due to the dimensional restriction. The difference in energy released during these two expansion scenarios provided clear evidence of Bethe string formation.
This experimental achievement offers researchers a new platform for studying quantum many-body physics. Unlike previous solid-state observations of Bethe strings, the ultracold atomic gas environment allows precise control of system geometry, particle density, and interaction strengths. The findings, published in Nature Communications, were supported by funding from the Austrian Science Fund, the European Union, and the UK Engineering and Physical Sciences Research Council.
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