  We have observed the three-photon correlation in nonclassical light sources
by using an indirect measurement scheme based on the dead time effect of
photon-counting detectors. We first developed a general theory which enables us
to extract the three-photon correlation from the two-photon correlation of an
arbitrary light source measured with detectors with finite dead times. We then
confirmed the validity of our measurement scheme in experiments done with the
cavity-QED microlaser operating with a large intra-cavity mean photon number
exhibiting both sub- and super-poissoniaq photon statistics. The experimental
results were in a good agreement with the theoretical expectation. Our
measurement scheme provides an alternative approach for N-photon correlation
measurement employing (N-1) detectors and thus a reduced measurement time for a
given signal-to-noise ratio, compared to the usual scheme requiring N
detectors.
