Fractional quantum Hall edge polaritons

It is commonly believed that light cannot couple to the collective excitations of the fractional quantum Hall effect (FQHE). This assumption relies on Kohn's theorem that states that electron-electron interactions decouple from homogeneous electromagnetic fields due to Galilean invariance. Here, we demonstrate that light-matter coupling beyond the dipole approximation circumvents Kohn's theorem, and enables the coupling of cavity photons to the plasmonic edge modes of the FQHE. We derive the coupling using the FQHE bulk-boundary correspondence and predict the formation of experimentally detectable plasmon polaritons. In conjunction with recent experiments, we find that a single cavity mode leaves the system's topological protection intact. Interestingly, however, a multimode cavity mediates plasmon backscattering and effectively transforms the edges of the two-dimensional (2D) FQHE into a 1D wire. Such cavity-meditated nonlocal backscattering bodes the breakdown of the topological protection in the regime of ultrastrong photon-plasmon coupling. Our analytical framework and findings pave the way for investigating the topological order of the FQHE via optical spectroscopic probes as well as provide opportunities to control FQHE edge excitations using light.