Ultrastrong light-matter coupling beyond unity coupling strength

We introduce a paradigm change in the design of THz light-matter coupled systems by treating the electronic and photonic components on equal footing instead of optimizing them separately. Exploiting both cavity and electronic excitation to confine the vacuum mode, we achieve ΩR/ωc=1.43 for cyclotron resonances ultrastrongly coupled to metamaterials. Under this condition, the vacuum ground state exhibits a record population of 0.37 virtual photons, massively facilitating the envisaged detection of quantum vacuum radiation by diabatic switching of ΩR/ωc