  In this work we study the transverse spatial correlation of the pair of
photons generated via the process of spontaneous parametric frequency
down-conversion, in periodically poled non-linear crystals illuminated by a
pulsed laser beam. It is well known that the two-photon state generated in
quasi-phase matching (QPM) configurations depends explicitly on the
characteristics of the pump beam, on the crystal modulated non-linearity, and
on the detection geometry. This has allowed the development of several
techniques for controlling the biphoton spectral and spatial properties. Here
we discuss another technique for implementing the spatial entanglement
modification in QPM gratings. We show, theoretically and experimentally, that
in nearly collinear geometries, the spatial shape modulation of the pump beam
allows for the control of the biphoton transverse spatial correlation.
