For a single plane-wave photon, the spin can only have two values ± ± ℏ ℏ , which are eigenvalues of the spin operator s ^ ^ 3 . The corresponding eigenfunctions describing photons with well defined values of SAM are described as circularly polarized waves: | ± ± ⟩ ⟩ = ( 1 ± ± i 0 ) .