Generation of tunable entanglement and violation of a Bell-like   inequality between different degrees of freedom of a single photon
  We demonstrate a scheme to generate noncoherent and coherent correlations, i.e., a tunable degree of entanglement, between degrees of freedom of a single photon. Its nature is analogous to the tuning of the purity (first-order coherence) of a single photon forming part of a two-photon state by tailoring the correlations between the paired photons. Therefore, well-known tools such as the Clauser-Horne-Shimony-Holt (CHSH) Bell-like inequality can also be used to characterize entanglement between degrees of freedom. More specifically, CHSH inequality tests are performed, making use of the polarization and the spatial shape of a single photon. The four modes required are two polarization modes and two spatial modes with different orbital angular momentum. 
Adam Valles, Vincenzo D'Ambrosio, Martin Hendrych, Michal Micuda,   Lorenzo Marrucci, Fabio Sciarrino, Juan P. Torres
quant-ph