ИСТИНА

Generation of terahertz frequency (THz) fields with quantum properties and study of statistical parameters of THz radiation at the photonic level can provide a new understanding of the interaction of THz fields with matter and be useful for expanding optical quantum technologies, such as quantum field sensing, imaging, spectroscopy, photometry, for the THz range. Quantum-correlated pairs of photons of optical and terahertz ranges (“optical - terahertz biphotons”), generated under spontaneous parametric down-conversion (SPDC) in a strongly frequency non-degenerate regime, are first exciting examples of non-classical radiation matching the terahertz gap. The prospects and challenges of generation and detection of optical-terahertz biphotons are analyzed theoretically, using the generalized Klyshko-Kirchhoff approach, and experimentally, by studying the SPDC fields generated under pulsed laser pumping of nonlinear Mg:LiNbO3 crystal. Analysis of frequency-angular distributions of optical photons generated at the Stokes idler frequency shifts 0.2–5 THz at different crystal temperatures in the range from 300 K to 4.2 K enabled to predict the temperature behavior of the total number of THz idler photons, the temperature variation of the optical-terahertz biphoton function, and to study the contributions of classical thermal and pure quantum field fluctuations to parameters of biphotons. It was shown that detecting only optical part of SPDC can provide information on THz properties of matter without direct detection of THz waves. However, for a vast majority of attracting quantum applications such as ghost imaging without THz cameras, absolute calibration of quantum efficiency of THz detectors, and other tasks, the direct measurement of optical-terahertz correlation function g(2) is important. An experimental scheme for direct measuring of g(2) for the optical–THz biphotons has been designed and implemented recently. Terahertz radiation of type-0 SPDC in cooled down to 4.8 K Mg:LiNbO3 was detected with a superconducting NbN bolometer operating in an analog detection mode. A special procedure was proposed for evaluating g(2) in the absence of single-photon THz detectors and impossibility of using coincidence circuits. A quantum excess over the classical level of correlations between optical and terahertz fields was detected experimentally for the first time.