%0 Journal Article %T Experimental realization of a measurement conditional unitary operation at single photon level and application to detector characterization %A M. Genovese %A G. Brida %A M. Chekhova %A M. Gramegna %A L. Krivitsky %A S. Kulik %A M. L. Rastello %J Physics %D 2004 %I arXiv %R 10.1117/12.561416 %X Our last experimental results on the realization of a measurement-conditional unitary operation at single photon level are presented. This gate operates by rotating by $90^o$ the polarization of a photon produced by means of Type-II Parametric Down Conversion conditional to a polarization measurement on the correlated photon. We then propose a new scheme for measuring the quantum efficiency of a single photon detection apparatus by using this set-up. We present experimental results obtained with this scheme compared with {\it traditional} biphoton calibration. Our results show the interesting potentiality of the suggested scheme. %U http://arxiv.org/abs/quant-ph/0409055v1