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Journal of Experimental and Theoretical Physics Letters

Direct Measurement of the Correlation Function of Optical–Terahertz Biphotons

A.A. Leontyev, Kirill A. Kuznetsov, Pavel A. Prudkovskii, Daniil A. Safronenkov, G. Kh. Kitaeva

Published Nov 1, 2021• 8 citations
Publisher

Abstract

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.

DOI
10.1134/s0021364021220082

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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.
We study conditions for the generation of strongly correlated photons of two extremely different frequency ranges, optical and terahertz, via the spontaneous parametric down-conversion effect in a nonlinear medium. Analytical expressions for the correlation function are derived using equations of the generalized Kirchhoff law for the second-order field moments with account of both classical thermal and quantum vacuum field fluctuations, possible inherent absorption of the medium at terahertz frequencies, and angular divergences of signal and idler radiation caused by a transverse limited spatial profile of the Gaussian pump beam. Associated with the current absence of terahertz photon counting detectors and coincidence schemes, the possible experimental limitations in measuring the second-order optical-terahertz field correlations are studied. Numerical calculations of noncollinear eee-geometry parametric down-conversion in a Mg:LiNbO3\mathrm{Mg}:\mathrm{LiNb}{\mathrm{O}}_{3}Mg:LiNbO3​ crystal demonstrate angular characteristics, particular features of the temperature and spectral behavior of the second-order quantum correlation function of optical-terahertz biphotons.
An experimental scheme and data processing approaches are proposed for measuring by analog photo detectors the normalized second-order correlation function of the biphoton field generated under spontaneous parametric down-conversion. Obtained results are especially important for quantum SPDC-based technologies in the long-wave spectral ranges, where it is difficult to use the single-photon detector at least in one of the two biphoton channels. The methods of discrimination of analog detection samples are developed to eliminate the negative influence of the detection noises and get quantitatively true values of both the correlation function and the detector quantum efficiency. The methods are demonstrated depending on whether two single-photon avalanche photo detectors are used in both SPDC channels, or at least one single-photon detector is replaced by a photo-multiplier tube which cannot operate in the photon counting mode.
A method to measure the correlation function of a biphoton field, which does not require single-photon detectors and photocount coincidence schemes, has been proposed and experimentally approved. It is based on the statistical analysis of current data from analog detectors. On an example of the operation of a scheme with analog photomultiplier tubes in the signal and idler channels of degenerate parametric down conversion, it has been shown that this new approach allows one to obtain correlation function values coinciding with the results of the standard photon counting technique. It has been shown how the absolute values of photon numbers and radiation power in the signal channel of the down conversion scheme are determined from the measured correlation function. Analog detection can expand the spectral and dynamic ranges of electromagnetic radiation for the application of quantum optic technologies. A method for the reference-free measurement of the spectral sensitivity of an analog photosensor module has been demonstrated as the first example of application.