Direct observation of the degree of correlations using photon-number- resolving detectors

Optical parametric down-conversion is a common source for the generation of nonclassical correlated photonic states. Using a parametric down-conversion source and photon-number-resolving detectors, we measure the two-mode photon-number distribution of up to 10 photons for different degrees of correlation. The degree of correlation is controlled by collecting different spatial and spectral single modes for each polarization and varying the amount of spectral and spatial overlap between them. Clear evidence for photon-number correlations is presented despite detector imperfections such as low detection efficiency and other distorting effects. Two criteria, derived directly from the raw data, are shown to be good measures for the degree of correlation. Additionally, using a fitting technique, we find a connection between the measured photon-number distribution and the degree of correlation of the reconstructed original two-mode state. These observations are only possible as a result of the detection of high photon number events.