  The feasibility to generate correlated photon pairs at variable frequencies
is investigated. For this purpose, we consider the interaction of an
off-resonant laser field with a two-level system possessing broken inversion
symmetry. We show that the system generates non-classical photon pairs
exhibiting strong intensity-intensity correlations. The intensity of the
applied laser tunes the degree of correlation while the detuning controls the
frequency of one of the photons which can be in the THz-domain. Furthermore, we
observe the violation of a Cauchy-Schwarz inequality characterizing these
photons.
