  We present a quantum description of a planar microcavity photon mode strongly
coupled to a semiconductor intersubband transition in presence of a
two-dimensional electron gas. We show that, in this kind of system, the vacuum
Rabi frequency $\Omega\_R$ can be a significant fraction of the intersubband
transition frequency $\omega\_{12}$. This regime of ultra-strong light-matter
coupling is enhanced for long wavelength transitions, because for a given
doping density, effective mass and number of quantum wells, the ratio
$\Omega\_R/\omega\_{12}$ increases as the square root of the intersubband
emission wavelength. We characterize the quantum properties of the ground state
(a two-mode squeezed vacuum), which can be tuned {\it in-situ} by changing the
value of $\Omega\_R$, e.g., through an electrostatic gate. We finally point out
how the tunability of the polariton quantum vacuum can be exploited to generate
correlated photon pairs out of the vacuum via quantum electrodynamics phenomena
reminiscent of the dynamic Casimir effect.
