Entanglement and conservation of orbital angular momentum in spontaneous   parametric down-conversion
  We show that the transfer of the plane wave spectrum of the pump beam to the fourth-order transverse spatial correlation function of the two-photon field generated by spontaneous parametric down-conversion leads to the conservation and entanglement of orbital angular momentum of light. By means of a simple experimental setup based on fourth-order (or two-photon) interferometry, we show that our theoretical model provides a good description for down-converted fields carrying orbital angular momentum. 
S. P. Walborn, A. N. de Oliveira, R. S. Thebaldi, and C. H. Monken