  Correlations between photons are interesting for a number of applications and
concepts in metrology in particular for resolution improvements in different
methods of quantum imaging. Since Fock-states of N-photons of wavelength lambda
in interference schemes acquire N-times the phase shift of single photons these
states can appear as if they had a de Broglie wavelength of lambda/N. A
biphoton beam diffracted at a blazed grating shows this reduced de Broglie
wavelength. This experiment can be seen as a purification of biphotons of a
certain correlation strength on the one hand. On the other hand the evaluation
of the one-photon and two-photon rate distributions in the Fraunhofer far field
of the grating allows for an analysis of the spatial correlation between the
photons. An experimental demonstration of these ideas tested for a biphoton
beam generated by parametric down conversion (PDC) is reported. We demonstrate
in addition that the existence of higher order spatial modes is important to
observe strong spatial correlations and to observe the photonic de Broglie
wavelength of lambda/2 for biphotons.
