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 pho-tons. The measurement technique is based on a 90 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. 
Giorgio Brida, Maria Chekhova, Marco Genovese, Marco Gramegna, Leonid   Krivitsky, and Maria Luisa Rastello