Nearly a century after the physicist Hans Bethe described them on paper, unusual quantum structures called Bethe strings have been created and directly observed in an ultracold gas.
In 1931 Bethe proposed that particles in certain one-dimensional quantum systems could bind together into collective states. Unlike molecules, the particles are not joined by chemical bonds: they stay together because of their mutual interactions, and such states can exist only in one dimension. Bethe strings had been seen before in solid-state magnetic materials; the new work puts them in a far more controllable setting.
The team led by Hanns-Christoph Nägerl at the University of Innsbruck, with theory groups at the University of Amsterdam and the Technical University of Munich, cooled a cloud of cesium atoms to within billionths of a degree of absolute zero and split it into several thousand very narrow tubes. Inside each tube the atoms can move in essentially one direction, providing the required one-dimensional geometry, and the strength of their interaction can be tuned.
By switching the interactions from repulsive to attractive, the researchers made the atoms bind into clusters of varying size, some containing six or more particles. To prove the particles were truly bound, they let the strings expand: still confined to their tubes, the clusters ran into one another and collided without falling apart — a striking property, the authors note. When the confinement was removed and the atoms could spread into three dimensions, the strings broke apart, converting their binding energy into motion. Comparing the two expansions gave a clean signature: three-dimensional expansion carried extra energy only when bound states had been present.
The results, published in Nature Communications, give physicists a tunable platform for studying quantum many-body states that had largely existed as a beautiful piece of mathematics.
