Orbital angular momentum of photons in noncollinear parametric downconversion

Conservation of the orbital angular momentum (OAM) of light in photon downconversion has been observed experimentally in nearly collinear phase-matching geometries, where the pump, signal and idler photons propagate along almost the same direction [Nature 412 (2001) 313]. However, here we predict that such paraxial measurements conducted with entangled photons in noncollinear geometries are not expected to comply with OAM conservation in the above sense. Under proper conditions, the strength of such apparent anomaly approaches 100%. We discuss the physical origin of the effect and suggest experimental schemes where it can be verified.