I've heard it said (a while ago) that we don't usually speak of a single photon being linearly polarized or unpolarized, but rather we can speak of an ensemble of photons that are linearly polarized or unpolarized.
Is this still the current thinking in Quantum Optics?
If a Mach-Zehnder interferometer had one path that included an optical element that rotated the polarization by 90 degrees, then I think polarized light going through this filter wouldn't interfere, but unpolarized light would. Is this the case?
When the light intensity was high, the classical explanation would be that light from one polarization was interfering with light from another polarization.
But when the intensity is low enough that there are single photons passing through this interferometer, I suspect there would still be interference effects.
If so, would this mean that would could then talk about individual unpolarized photons?