  Different from the usual conclusion that the separation of the photon angular
momentum into orbital and spin parts is physically meaningless, the orbital and
spin angular momenta are demonstrated in the first-quantization framework not
to satisfy the standard commutation relation. It is shown on the basis of the
transversality condition that the spin is aligned with the propagation
direction so that only the helicity can be the intrinsic degree of freedom. It
is also shown on the same basis that only in the so-called intrinsic reference
system does the helicity behave intrinsic. The intrinsic reference system of
the photon is determined by the "action" of a gauge potential, the Berry
potential, on the helicity of the photon. The Berry potential is fixed by a
vector-valued degree of freedom, called the Berry degree of freedom. Because
only the position of the photon in its intrinsic reference system is
canonically conjugate to the momentum, the intrinsic reference system itself is
endowed with quantum effects that depend on the Berry degree of freedom as well
as the helicity. This not only explains the so-called spin Hall effect of light
but also helps to understand why the total angular momentum cannot be generally
split into helicity-independent orbital and helicity-dependent spin parts.

Quantum Physics