title: Spatial symmetry and conservation of orbital angular momentum in spontaneous parametric down-conversion.
abstract: Directly contradictory arguments coexist regarding the conservation rule of orbital angular momentum in spontaneous parametric down-conversion. We analytically show how this rule is decided by spatial symmetry. We discover that the down-converted photon pairs can carry non-negligible extrinsic orbital angular momentum in the degrees of relative-movement freedom due to spatial symmetry breaking, leading to nonconservation of total orbital angular momentum in type-II down-conversion. Also, we demonstrate that the traditional technique does not measure the extrinsic orbital angular momentum.
authors.affiliation: Center for Photonic Communication and Computing, EECS Department, Northwestern University, Evanston, IL 60208-3118, USA. sfeng@ece.northwestern.edu
authors.forename: Sheng
authors.initials: S
authors.lastname: Feng
authors.forename: Prem
authors.initials: P
authors.lastname: Kumar