Absolute calibration of photodetectors based on spontaneous parametric down-conversion biphoton field

A new method of absolute calibration of photodetector sensitivity based on spontaneous parametric down-conversion (SPDC) biphoton field is described. The process of SPDC is studied theoretically. The single photon detection p robability and two-photon coincidence probability are derived and the calib ration principle for photodetectors is explained. An experimental system ha s been set up. The sensitivity of a photon-counting photomultiplier tube wa s measured, and the results were compared with those obtained using convent ional method.