Four-dimensional Quantum Key Distribution using Position-Momentum and   Polarization Correlations
  We demonstrate experimentally that it is possible to prepare and detect photon pairs created by spontaneous parametric down-conversion which exhibit simultaneous position-momentum and polarization correlations that are adequate to implement a four-dimensional key distribution protocol. 
M. P. Almeida, S. P. Walborn and P. H. Souto Ribeiro