  We study the squeezing of output quadratures of an electro-magnetic field
escaping from a resonator coupled to a general quantum system with arbitrary
interaction strengths. The generalized theoretical analysis of output squeezing
proposed here is valid for all the interaction regimes of cavity-quantum
electrodynamics: from the weak to the strong, ultrastrong, and deep coupling
regimes. For coupling rates comparable or larger then the cavity resonance
frequency, the standard input-output theory for optical cavities fails to
calculate the correct output field-quadratures and predicts a non-negligible
amount of output squeezing, even if the system is in its ground state. Here we
show that, for arbitrary interactions and cavity-embedded quantum systems, no
squeezing can be found in the output-field quadratures if the system is in its
ground state. We also apply the proposed theoretical approach to study the
output squeezing produced by: (i) an artificial two-level atom embedded in a
coherently-excited cavity; and (ii) a cascade-type three-level system
interacting with a cavity field mode. In the latter case the output squeezing
arises from the virtual photons of the atom-cavity dressed states. This work
extends the possibility of predicting and analyzing continuous-variable optical
quantum-state tomography when optical resonators interact very strongly with
other quantum systems.
