  The ground state of a cavity-electron system in the ultrastrong coupling
regime is characterized by the presence of virtual photons. If an electric
current flows through this system, the modulation of the light-matter coupling
induced by this non-equilibrium effect can induce an extra-cavity photon
emission signal, even when electrons entering the cavity do not have enough
energy to populate the excited states. We show that this ground-state
electroluminescence, previously identified in a single-qubit system [Phys. Rev.
Lett. 116, 113601 (2016)] can arise in a many-electron system. The collective
enhancement of the light-matter coupling makes this effect, described beyond
the rotating wave approximation, robust in the thermodynamic limit, allowing
its observation in a broad range of physical systems, from a semiconductor
heterostructure with flat-band dispersion to various implementations of the
Dicke model.
