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The difficulties encountered up till now in the theory of identifying thespin and orbital angular momentum of the photon stem from the approach ofdividing the angular momentum of the photon into spin and orbital parts. Herewe derive the spin of the photon from a set of two relativistic quantumequations that was first cast from the free-space Maxwell equations by Darwinin 1932. Much attention is focused on the nonlocal properties of the photonspin that are determined by the relativistic quantum constraint, one of theso-called Darwin equations. Meanwhile, we point out that for the Darwinequations to describe the quantum motion of the photon, the upper and lowerparts of the wavefunction cannot be the electric and magnetic fields as Darwinprescribed. Their nonlocal relations are investigated. The Lorentz covarianceof the Darwin equations is also proven, to the best of our knowledge, for thefirst time.

Deriving photon spin from relativistic quantum equations: Nonlocality of photon spin and relativistic quantum constraint