  We have measured antinormally ordered Hanbury-Brown--Twiss correlations for
coherent states of electromagnetic field by using stimulated parametric
down-conversion process. Photons were detected by stimulated emission, rather
than by absorption, so that the detection responded not only to actual photons
but also to zero-point fluctuations via spontaneous emission. The observed
correlations were distinct from normally ordered ones as they showed excess
positive correlations, i.e., photon bunching effects, which arose from the
thermal nature of zero-point fluctuations.
