Quantum caustics and the hierarchy of light cones in quenched spin chains

The authors demonstrate that the light cones which spread information in spin chains are examples of quantum caustics, i.e. quantum versions of a class of phenomena that includes ship's wakes and rainbows. Caustics form of a hierarchy, described by catastrophe theory, which gives a natural explanation of the emergence of universal scaling wave functions dressing light cones: the simplest is the Airy function while more general situations lead to Pearcey and Hyperbolic Umbilic functions. The presence of space-time vortices inside light cones is also predicted.