Generation of Photon Pairs in the Light-Matter Ultrastrong Coupling Regime: From Casimir Radiation to Stimulated Raman Adiabatic Passage

The ultrastrong coupling regime of light-matter interaction is achieved when the coupling strength is a significant fraction of the natural frequencies of the noninteracting parts. Physics in this regime has recently attracted great interest, both theoretically and experimentally being a fruitful platform to test fundamental quantum mechanics in a new non-perturbative regime, and for applications to quantum technologies.Here we discuss the generation of photon-pair states, which is a distinctive feature of this new regime, and interesting new dynamicsl effects both in optomechanics and in circuit-QED architectures