The picture shows ytterbium ions in an ion Coulomb crystal, taken with an EMCCD camera (electron multiplying CCD camera). The ionized atoms fluoresce in the laser light; the distance between the ions is approx. 10µm to 20 µm.
(a) Radial symmetry: At a high radial trap enclosure, the ionized atoms line up like a string of pearls. The radial potential felt by the positively charged particles is depicted at the bottom left.
(b) Mirror symmetry: If the strength of the radial enclosure changes, the ions become free to avoid each other, and the crystal adopts a new, energetically more favourable structure. Hereby, two energetically equivalent configurations can be chosen from. Areas which do not communicate with each other decide independently of each other on a new orientation. Topological defects occur where two areas which have made different "decisions" collide. The black-and-white images show fluorescing ytterbium ions.