  We apply the Wigner function formalism to partial Bell-state analysis using
polarization entanglement produced in parametric down conversion. Two-photon
statistics at a beam-splitter are reproduced by a wavelike description with
zeropoint fluctuations of the electromagnetic field. In particular, the
fermionic behaviour of two photons in the singlet state is explained from the
invariance on the correlation properties of two light beams going through a
balanced beam-splitter. Moreover, we show that a Bell-state measurement
introduces some fundamental noise at the idle channels of the analyzers. As a
consequence, the consideration of more independent sets of vacuum modes
entering the crystal appears as a need for a complete Bell-state analysis.
