  We present one- and two-photon diffraction and interference experiments
involving parametric down-converted photon pairs. By controlling the divergence
of the pump beam in parametric down-conversion, the diffraction-interference
pattern produced by an object changes from a quantum (perfectly correlated)
case to a classical (uncorrelated) one. The observed diffraction and
interference patterns are accurately reproduced by Fourier-optical analysis
taking into account the quantum spatial correlation. We show that the relation
between the spatial correlation and the object size plays a crucial role in the
formation of both one- and two-photon diffraction-interference patterns.
