  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.
