  Hybrid excitations, called polaritons, emerge in systems with strong
light-matter coupling. Usually, they dominate the linear and nonlinear optical
properties with applications in quantum optics. Here, we show the crucial role
of the electronic component of polaritons in the magneto-transport of a
cavity-embedded 2D electron gas in the ultrastrong coupling regime. We show
that the linear dc resistivity is significantly modified by the coupling to the
cavity even without external irradiation. Our observations confirm recent
predictions of vacuum-induced modification of the resistivity. Furthermore,
photo-assisted transport in presence of a weak irradiation field at sub-THz
frequencies highlights the different roles of localized and delocalized states.
