Researchers at Austria’s University of Innsbruck have created and observed bound clusters called Bethe strings in an ultracold atomic gas, the university announced on September 14. The work offers a controlled way to study how quantum particles behave collectively, with evidence for clusters containing more than six atoms.
The team led by Hanns-Christoph Nägerl worked with theorists in Amsterdam and Munich. Its peer-reviewed paper appeared in Nature Communications on August 29; today’s university announcement explains the experiment. Hans Bethe predicted such states in 1931, and researchers have already observed smaller strings in quantum magnets and superconducting qubits.
According to the university, the experiment cooled cesium atoms to a few billionths of a degree above absolute zero and divided them among thousands of narrow tubes. This confined their movement essentially to one direction. Changing the interactions from repulsion to attraction produced a mixture of bound clusters of different sizes.
The researchers compared two kinds of expansion to identify the strings. Within the tubes, clusters collided while remaining intact. Releasing the gas into three dimensions broke the bound states apart and produced faster expansion, which the team used to probe their binding energy. Unbound atoms gave essentially the same energy in both measurements.
The paper reports further evidence in the distribution of atomic momenta and in Tan’s contact, a measure connected to particle correlations. Its measurements agree quantitatively with generalized hydrodynamics, a theoretical framework for these systems. The advance concerns mixtures containing many strings, including large ones, extending earlier observations of small individual strings.
The university says the gas allows precise control of geometry, density and interactions. This provides a platform to investigate how these collective objects form and collide. The authors identify future opportunities to explore quantum coherence, nonlinear dynamics and thermalization, the process by which an interacting system approaches thermal equilibrium.
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