  In a recent letter [Phys. Rev. Lett. 94, 100501 (2005)], we presented a
scheme for generating pure entangled states of spatial qudits ($D$-dimensional
quantum systems) by using the momentum transverse correlation of the parametric
down-converted photons. In this work we discuss a generalization of this
process to enable the creation of mixed states. With the technique proposed we
experimentally generated a mixture of two spatial qubits.
