Rafael A. Molina

Light from a quantum system is like a singer's voice: by the tremor, you can tell something's off with the vocal cords. Physicists have found that in the emission spectrum of a collective spin (a group of atoms behaving like a single magnet), peculiar distortions appear when the system passes through so-called exceptional points.

Musicians in an orchestra each play their own part, not hearing the others. In a perfect crystal of tungsten disulfide on graphene, excitons, bound electron-hole pairs, behave the same way: they barely mix due to a hidden symmetry called the valley. The energy levels of such a system resemble notes sorted into separate folders — the chaos is incomplete.