  Recent interest in the physics of non-perturbative light-matter coupling has
led to the development of solid-state cavity quantum electrodynamics setups in
which the interaction energies are comparable with the bare ones. In such a
regime the ground state of the coupled system is predicted to contain a
population of virtual excitations which, notwithstanding having been object of
many investigations, remain still unobserved. In this paper we propose a novel
approach to the observation of virtual excitations. We consider asymmetric
systems where virtual electronic transitions lead to static polarizations. The
virtual population in the ground state can then be observed using the photonic
equivalent of a Kelvin probe. We estimate the intensity of the effect in
intersubband polariton systems and find it observable with present-day
technology.
