Shannon dimensionality of quantum channels and its application to photon   entanglement
  We introduce the concept of Shannon dimensionality D as a new way to quantify bipartite entanglement as measured in an experiment. This is applied to orbital-angular-momentum entanglement of two photons, using two state analyzers composed of a rotatable angular-sector phase plate that is lens-coupled to a single-mode fiber. We can deduce the value of D directly from the observed two-photon coincidence fringe. In our experiment, D varies between 2 and 6, depending on the experimental conditions. We predict how the Shannon dimensionality evolves when the number of angular sectors imprinted in the phase plate is increased and anticipate that D = 50 is experimentally within reach. 
J.B. Pors, S.S.R. Oemrawsingh, A. Aiello, M.P. van Exter, E.R. Eliel,   G. W. 't Hooft, and J.P. Woerdman