ИСТИНА

We consider a possibility of measuring the quantum efficiency of sensitive elements of terahertz detectors using quantum-correlated pairs of optical and terahertz photons. Such pairs are generated via the nonlinear-optical process of spontaneous parametric down-conversion (SPDC) in strongly frequency non-degenerate regime. Nowadays, the SPDC method of absolute calibration is successfully applied to optical single-photon detectors and CCD cameras in frequency degenerate regime, but its extension to the terahertz range requires some specific modification. Virtually none of the currently used terahertz detectors has the single-photon detection property. At the same time, our analysis of statistical distributions of the analog readings of antenna-coupled terahertz superconducting bolometer in the terahertz SPDC channel reveals the discrete nature of terahertz detection. Similar features are observed in histograms of an analog photomultiplier tube placed in the optical SPDC channel. We propose a modified SPDC quantum efficiency calibration method based on approximation of the histograms and applicable to both analog and single-photon detectors. Experimental verification of the method is carried out in the optical range using degenerate SPDC. We show a fairly good matching between the input photon numbers measured by a single-photon avalanche diode calibrated using the standard photon-counting approach, and by an analog photomultiplier tube calibrated using the proposed method. This result opens a possibility to calibrate terahertz analog detectors as well. A possible experimental scheme and the whole calibration procedure are discussed.