# Connection between spin angular momentum of a photon and circular polarization of light

Do photons have spin angular momentum only if they are part of a circularly-polarized beam?

I suspect that every photon always has spin angular momentum, but in most cases they have a superposition of the two possible spin states, so the light seems linearly-polarized, except for the case of circularly-polarized light, where all photons express decoherence of the superposition, and they get the same spin state, which is either (+1) for all or (-1) for all.

Second question: two photons created together in such a way that they acquired quantum entanglement, are obliged to have a spin of the two states, but not their superposition? Does this mean they can't belong in a beam of linearly polarized light?

Thank you very much in advance.

• "Do photons have spin angular momentum only if they are part of a circularly-polarized beam?" Spin angular momentum, a.k.a. polarisation, is a degree of freedom of photons. Circular polarisation is one possible state of polarisation. Linear polarisation is another one. Unless you mean something else with "spin angular momentum" here? You also mention "decoherence", which suggests you are thinking about a specific type of experimental scenario. What kind of source of decoherence are you referring to? – glS Feb 2 at 18:48

Everything is quantum in the world. The photon is more fundamental than EM wave. So your suspicions are correct that linearly polarised states are superposition of the two spin states. And the circularly polarised states are the spin states.

Since a photon is a spin $$1$$ particle, spin measurements will lead to either $$+1$$ or $$-1$$ state (no $$0$$). Thus entanglement only makes sense in these spin states. And not the superposition.
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