  Cavity photon resonators with ultrastrong light-matter interactions are
attracting interest both in semiconductor and superconducting systems
displaying the capability to manipulate the cavity quantum electrodynamic
ground state with controllable physical properties. Here we review a series of
experiments aimed at probing the ultrastrong light-matter coupling regime,
where the vacuum Rabi splitting $\Omega$ is comparable to the bare transition
frequency $\omega$ . We present a new platform where the inter-Landau level
transition of a two-dimensional electron gas (2DEG) is strongly coupled to the
fundamental mode of deeply subwavelength split-ring resonators operating in the
mm-wave range. Record-high values of the normalized light-matter coupling ratio
$\frac{\Omega}{\omega}= 0.89$ are reached and the system appears highly
scalable far into the microwave range.
