On The Measurement of Photon Flux in Parametric Down Conversion

We report the measurement of the photons flux produced in parametric down-conversion, performed in photon counting regime with actively quenched silicon avalanche photodiodes as single photon detectors. Measurements are done with the detector in a well defined geometrical and spectral situation. By comparison of the experimental data with the theory, a value for the second order susceptibilities of the non linear crystal can be inferred.