Photon's spin and orbital angular momentum

[intervoxel]

"Locally, the spin density S is an intrinsic (i.e. origin-independent) quantity, which is associated with the local ellipticity of the polarization of light. In turn, the orbital AM density L=r x P0 is a manifestly extrinsic (origin-dependent) and is produced by the corresponding canonical (orbital) momentum density P0."

Can anyone explain the phrase "which is associated with the local ellipticity of the polarization of light", please?

 

[rigetFrog]

Am I correct in assuming it's the word "local" that's got your knickers in twist?

 

[UltrafastPED]

Spin is a property of the photon, and usually modeled as left handed or right handed - from various combinations and phase delays you can build up any of the standard light polarizations, of which elliptical is the most general case.

Orbital angular momentum (OAM) is the property of the light beam as a whole; most light beams have zero OAM, but it is possible to generate angular momentum in the beam by how it is generated - the standard measurement uses the axis of the propagating beam as the origin for the OAM calculation.

OTOH intrinsic photon spin is independent of the measurement axis - you can only "see" a photon that is coming towards you, and the spin measured at the detector will be LH or RH without reference to any external axis.