Absolute quantum efficiency measurements by means of conditioned polarization rotation

We propose a new scheme for measuring the quantum efficiency of photon counting detectors by using correlated photons. The measurement technique is based on a 90/spl deg/ rotation of the polarization of one photon member of a correlated pair produced by parametric down-conversion, conditioned on the detection of the other correlated photon after polarization selection. We present experimental results obtained with this scheme.