Generating mixtures of spatial qubits
  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. 
G. Lima, F.A. Torres-Ruiz, L. Neves, A. Delgado, C. Saavedra and S.   Padua