  We investigate the role played by quantum operator ordering in the
correlations that characterize two-photon polarization Bell measurements. The
Clauser-Horne-Shimony-Holt (CHSH) criterion is investigated in the normal
ordering imposed by the photodetection theory and in the symmetric ordering
that constitutes the standard prescription for building Hermitian operators
from products of noncommuting observables. The two approaches are obtained in a
single theoretical framework, where operator ordering is directly associated
with the representation used for the density matrix.Moreover, this discussion
can be recast in terms of the contribution given by the vacuum fluctuations to
the detected signals. We also envisage possible detection schemes sensitive to
these fluctuations with recent technological developments.
