Spontaneous Scattering of Raman Photons from Cavity-QED Systems in the Ultrastrong Coupling Regime

We show that spontaneous Raman scattering of incident radiation can be observed in cavity-QEDsystems without external enhancement or coupling to any vibrational degree of freedom. Raman scatteringprocesses can be evidenced as resonances in the emission spectrum, which become clearly visible as thecavity-QED system approaches the ultrastrong coupling regime. We provide a quantum mechanicaldescription of the effect, and show that ultrastrong light-matter coupling is a necessary condition for theobservation of Raman scattering. This effect, and its strong sensitivity to the system parameters, opens newavenues for the characterization of cavity QED setups and the generation of quantum states of ligh