Nature Commun. 4, 2290 (2013); Nature Commun. 4, 2291 (2013)

Topological defects can form in a system that is undergoing a symmetry-breaking phase transition at a finite rate. This scenario — described by the Kibble–Zurek mechanism — may explain the origin of the large-scale structure in the early Universe. The same mechanism was found at work in liquid crystals, superfluid helium and superconductors, but the predicted power-law scaling for the defect density was hard to confirm. Using strings of trapped ions, Stefan Ulm and colleagues, and Karsten Pyka and co-workers, observe the formation of defects as expected from the Kibble–Zurek theory.

The two groups use laser-cooled ions that are confined by electric potentials to linear strings, known as Coulomb crystals. The individual positions of the ions can be accurately monitored through their fluorescence, revealing the crystal configuration. Increasing the confinement strength forces the ion string to undergo a structural phase transition to a zig-zag. During this transition defects appear in the form of kinks in the zig-zag. The rate at which kinks are created depends on the quench of the confining potential, and follows the power law predicted by the Kibble–Zurek mechanism.