Absolute calibration of photodetectors: photocurrent multiplication versus photocurrent subtraction.

We report testing of the new absolute method of photodetector calibration based on the difference-signal measurement for two-mode squeezed vacuum by comparison with the traditional absolute method based on coincidence counting. Using low-gain parametric downconversion, we have measured the quantum efficiency of a counting detector by both methods. The difference-signal method was adapted for the counting detectors by taking into account the dead-time effect.