Unravelling Two-Photon High-Dimensional Entanglement
  We propose an interferometric method to investigate the non-locality of high-dimensional two-photon orbital angular momentum states generated by spontaneous parametric down conversion. We incorporate two half-integer spiral phase plates and a variable-reflectivity output beam splitter into a Mach-Zehnder interferometer to build an orbital angular momentum analyzer. This setup enables testing the non-locality of high-dimensional two-photon states by repeated use of the Clauser-Horne inequality. 
A. Aiello, S. S. R. Oemrawsingh, E. R. Eliel, J.P. Woerdman