ИСТИНА

Due to many unusual manifestations of the response of materials at terahertz frequencies, the range 0.1 - 10 THz is extremely interesting for various applications. The last decades in the world have passed under active development of this range. However, so far only sources with classical radiation statistics are being created and used here. Generation of terahertz frequency (THz) fields with quantum properties and study of statistical parameters of THz radiation at the photonic level are the new fields that can provide a new understanding of the interaction of THz fields with matter and be useful for expanding optical quantum technologies 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 lecture is devoted to the methods for calculating the correlation parameters and mode composition of optical-terahertz biphoton fields, to the choice of optimal elements of the set-ups for their generation and registration, and to the analysis of the first experimental results in this area. In particular, it will be shown how the detection of only the optical part of SPDC field can provide information on the THz properties of materials, the brightness of sources of external terahertz radiation and some parameters of optical-terahertz biphotons. At the same time, for the overwhelming majority of attractive quantum applications, such as quantum ghost imaging without THz cameras, absolute calibration of the quantum efficiency of THz detectors, creation of single-photon sources of THz fields, and other problems, it is important to directly measure the optical-terahertz correlation function g(2). Recently, an experimental scheme for direct measurement of g(2) for optical-terahertz biphotons was developed and implemented. For the first time, a quantum excess over the classical correlation level has been experimentally discovered. Special procedures for measuring g(2) and for estimating the degree of correlation, used in the absence of single-photon terahertz detectors and the impossibility of using coincidence schemes, are discussed.