  Maximally entangled photon pairs with a spatial degree of freedom is a
potential way for realizing high-capacity quantum computing and communication.
However, methods to generate such entangled states with high quality, high
brightness, and good controllability are needed. Here, a scheme is
experimentally demonstrated that generates spatially maximally entangled photon
pairs with an orbital angular momentum degree of freedom via spontaneous
parametric down-conversion in a nonlinear crystal. Compared with existing
methods using post-selection, the technique can directly modulate the spatial
spectrum of down-converted photon pairs by engineering the input pump beam. In
addition, the relative phase between spatially entangled photon pairs can be
easily manipulated by preparing the relative phase of input pump states.
