Absolute standardless calibration of photodetectors based on quantum two-photon fields

A new method of absolute calibration of photodetectors based on a nonclassical effect in nonlinear optics is discussed. The combined influence of a monochromatic flux of pump photons and quantum vacuum noise on a nonlinear crystal with no center of symmetry results in the appearance of rigorously correlated pairs of photons with continuous spectral distribution in the spontaneous parametric scattering of light (spontaneous parametric downconversion) process. The presence of an optical field in a broad spectral range created by the two-photon states (biphotons) makes it possible to develop a new method of measuring the spectral distribution of the absolute value of the quantum efficiency of photodetectors, both in photon counting and in analog regimes, that does not use any calibrated standard light sources (étalons). The feasibility of the method is demonstrated on different types of photomultiplier tube.