Measurement of the quantum efficiency of analog detectors in the parametric down-conversion field

Approaches to the standard-free calibration of the quantum efficiency of a wide class of analog detectors based on the measurement of statistical characteristics of fields generated by parametric down-conversion are analyzed. General expressions are obtained for the noise reduction factor of the difference photocurrent and for the covariance of photocurrents in the signal and idler channels that take into account the possibility of strong fluctuations in the amplitudes of single-photon response functions of the detectors used. It is shown that the measurement of the noise reduction factor of the difference photocurrent using detectors that cannot operate in the photon counting mode is on its own insufficient to directly characterize the level of two-mode squeezing in the down-conversion field and to determine the quantum efficiency of photosensitive elements without additional calibration procedures. A method to determine the quantum efficiency of such detectors based on measuring the dependence of the normalized covariance of photocurrents on the parametric gain is proposed.