A quantum prediction from 1931 just came to life

Nearly a century after physicist Hans Bethe predicted them, scientists have created and directly observed unusual quantum states known as “Bethe strings” in an ultracold gas. By cooling cesium atoms to near absolute zero and trapping them in thousands of tiny one-dimensional tubes, researchers were able to make groups of atoms bind together without ordinary chemical bonds. These strange clusters could contain six or more particles and even collide with one another without falling apart.

A quantum prediction from 1931 just came to life. 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.