Azimuthal entanglement and multichannel Schmidt-type decomposition of   non-collinear biphotons
  Purely azimuthal entanglement is analyzed for noncollinear frequency-degenerate biphoton states. The degree of azimuthal entanglement is found to be very high, with the Schmidt parameter $K$ on the order of the ratio of the pump waist to its wavelength. A scheme is suggested for partial realization of this high entanglement resource in the form of a multichannel Schmidt-type decomposition. 
M.V. Fedorov
quant-ph