  Whether the total angular momentum of the photon can be separated into spin
and orbital parts has been a long-standing problem due to the constraint of
transversality condition on its vector wavefunction. A careful analysis shows
that the situation arises from a misuse of the constraint in quantum mechanics.
To use the constraint properly, we convert the vector representation into a
two-component representation in which the wavefunction is not constrained by
any conditions. Upon doing so, we not only separate the spin conceptually from
the orbital angular momentum but also identify the Berry gauge in which the
two-component wavefunction can be canonically quantized. A corollary is that
only in one particular Berry gauge can a radiation field be canonically
quantized in terms of the plane waves. The degree of freedom to fix the Berry
gauge, which reflects a symmetry of the constraint, turns out to have
observable quantum effects.
