Nearly a century after Nobel laureate Hans Bethe predicted their existence, scientists have created and observed unusual quantum structures known as "Bethe strings" using ultracold atoms. The experiment, led by quantum physicist Hanns-Christoph Nägerl, provides researchers with a highly controllable way to explore these unusual quantum many-body states. In 1931, physicist Hans Bethe proposed that particles in certain quantum systems restricted to one dimension could join together into collective states now called Bethe strings.
These structures differ fundamentally from familiar molecules. Rather than being connected through chemical bonds, the particles remain bound because of their interactions with one another, and the resulting states can exist only in one dimension. For much of the past century, Bethe strings were primarily a theoretical idea.
Researchers at the University of Innsbruck have now created and detected these multiparticle bound states in an ultracold gas, working with theory teams from the Department of Experimental Physics at the University of Amsterdam and the Technical University of Munich. The results were published in Nature Communications. Creating Bethe Strings Near Absolute Zero To produce the unusual quantum states, the researchers started with a cloud of cesium atoms cooled to within only a few billionths of a degree above absolute zero.
They then separated the cloud into several thousand extremely narrow tubes. Within each tube, the atoms are effectively restricted to moving along a single direction, creating the one-dimensional environment required for Bethe strings to exist. The researchers can also precisely adjust how strongly the atoms interact.
By changing those interactions from repulsive to attractive, the team caused the atoms to bind together. Rather than simply collapsing into one group, the atoms formed bound states of several different sizes. Some of the larger clusters contained six or more particles.
The next challenge was demonstrating that these particles were truly bound together. "One of the simplest experiments was to let the strings expand," says Milena Horvath, one of the lead authors. The researchers first allowed the atoms to spread out while keeping them trapped inside their one-dimensional tubes.
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