This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: In 1931, physicist Hans Bethe predicted that, in certain one-dimensional quantum systems, particles can bind together to form multi-particle states known as Bethe strings. Unlike ordinary molecules, which are held together by chemical bonds, Bethe strings arise purely from interactions between particles and exist only in one dimension.
For decades, Bethe strings remained primarily a theoretical concept. Now, almost a century after Bethe's prediction, researchers from the University of Innsbruck, in collaboration with theory teams from the Department of Experimental Physics at the University of Amsterdam and the Technical University of Munich, have created and observed these multi-particle bound states in an ultracold gas. Their findings are published in Nature Communications.
The experiment begins with a cloud of cesium atoms cooled to temperatures only a few billionths of a degree above absolute zero. The researchers then divide the cloud into several thousand narrow, one-dimensional tubes. Inside these tubes, the atoms can move essentially only along a single direction.
The interactions between the atoms can be precisely controlled. By tuning the interactions between the atoms from repulsive to attractive, they can make the atoms bind together. Instead of simply collapsing, the atoms form bound states of different sizes, including larger clusters containing six or more particles.
The researchers then asked a simple question: How can we tell that the particles are really bound together? "One of the simplest experiments was to let the strings expand," says Milena Horvath, one of the lead authors. First, the researchers allow the atoms to expand while keeping them confined to their one-dimensional tubes.
As they move, the strings collide but remain intact. "This is a remarkable feature of the strings: they can collide without breaking apart," Horvath says. In a second step, the researchers remove the confinement and let the atoms expand freely in three dimensions.
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