Direct measurement of the correlation function of optical–terahertz biphotons

The second-order correlation function which characterizes the quantum correlation between the optical and terahertz photons generated under spontaneous parametric down-conversion has been measured for the first time. A scheme of experiment based on the analysis of analog signals from terahertz and optical detectors has been proposed. Various approaches to exclude the effect of electronic and thermal noise on the measured correlation level have been discussed. The results make it possible to expand quantum optical technologies such as quantum calibration of detectors, fabrication of single-photon sources, and imaging with single-pixel detectors to the terahertz range.