Photon Frequency Conversion in High-$Q$ Superconducting Resonators: Axion Electrodynamics, QED & Nonlinear Meissner Radiation

High-Q superconducting resonators have been proposed and developed as detectors of light-by-light scattering mediated by the hypothesized axion or virtual electron-positron pairs in quantum electrodynamics - the Euler-Heisenberg (EH) interaction. Photon frequency and mode conversion is central to the scheme for detecting such rare events. Superconducting resonators are nonlinear devices. The Meissner screening currents that confine the electromagnetic fields to the vacuum region of a superconduc...