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

On angular momentum of photons: the role of transversality condition in quantum mechanics