  Optical parametric down-conversion is a common source for the generation of
non-classical 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. By changing the heralded
collection efficiency, we control the level of correlations between the two
modes. Clear evidence for photon-number correlations are 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.
