Optical polarization conversion

From a classical perspective, linearly polarized light can be converted to circular by introducing a phase shift between the orthogonal modes.

Could someone give me an explanation of how this works from a quantum mechanics perspective, starting from the fact that each photon has angular momentum either left spin or right spin?

I suppose this means that linearly polarized light has an equal number of the left/right spinning photons, but what happens when this light is converted to right-circularly polarized? Are the left spinning photons converted into right spinning ones??

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