Terahertz light-matter interaction beyond unity coupling strength

Achieving control over light-matter interaction in custom-tailored nanostructures is at the core of modern quantum electrodynamics [1-15]. In ultrastrongly coupled systems [5-15], excitation is repeatedly exchanged between a resonator and an electronic transition at a rate known as the vacuum Rabi frequency $\Omega_R$. For $\Omega_R$ approaching the resonance frequency $\omega_c$, novel quantum phenomena including squeezed states [16], Dicke superradiant phase transitions [17,18], the collapse of the Purcell effect [19], and a population of the ground state with virtual photon pairs [16,20] are predicted. Yet, the experimental realization of optical systems with $\Omega_R$/$\omega_c$ has remained elusive. Here, we introduce a paradigm change in the design of light-matter coupling by treating the electronic and the photonic components of the system as an entity instead of optimizing them separately. Using the electronic excitation to not only boost the oscillator strength but furthermore tailor the shape of the vacuum mode, we push $\Omega_R$/$\omega_c$ of cyclotron resonances ultrastrongly coupled to metamaterials far beyond unity. As one prominent illustration of the unfolding possibilities, we calculate a ground state population of 0.37 virtual photons for our best structure with $\Omega_R$/$\omega_c$ = 1.43, and suggest a realistic experimental scenario for measuring vacuum radiation by cutting-edge terahertz quantum detection [21,22].