In 1931, physicist Hans Bethe predicted that particles confined to a single line could bind together in groups, called Bethe strings, held together by their interactions alone, not by chemical bonds. Researchers led by Hanns-Christoph Nägerl at the University of Innsbruck, with theory teams in Amsterdam and Munich, have now created and observed them in an ultracold atomic gas. The results were published in Nature Communications.
To do it, the team cooled cesium atoms to a few billionths of a degree above absolute zero and split the cloud into thousands of extremely thin tubes, so the atoms could only move in one dimension. They then switched the atoms’ interactions from repulsive to attractive, and the atoms formed bound groups, some with six or more particles. To confirm they were really Bethe strings, they let the atoms expand in two different ways and compared how much energy was released when the groups stayed together versus when they broke apart.
Why it matters: Bethe strings had only been detected before in certain magnetic materials. In an ultracold gas, scientists can control exactly how many atoms there are and how strongly they interact, which makes it a much cleaner place to study how these quantum states form and behave.