Аннотация:Quantum effect of parametric down-conversion is studied in a strongly frequency non-degenerate regime, when pairs of correlated optical and terahertz photons are generated along with parametric amplification and frequency conversion of thermal fluctuations and external radiation at terahertz frequencies. The terahertz photon fluxes emitted due to parametric down-conversion are registered. Direct incoherent nonlinear-optical detection is demonstrated for continuous wave sources at 0.2-0.4 THz.