Linear Polarization of Single Photon? 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?  
 A: 
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?

No. It is perfectly possible to assign polarization states to single photons. There is no need for ensembles in order to talk about polarization.

If so, would this mean that would could then talk about individual unpolarized photons?

Yes, you can indeed talk about individual unpolarized photons, i.e. single-photon states of the electromagnetic field which do not have a well-defined polarization. This is not a pure state, though: it is a maximally mixed state, within the span of two different modes that are identical except for having orthogonal polarizations.

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?

No, this is not the case. Polarized light won't interfere, and neither will unpolarized light. This holds for both classical light and single-photon sources.

When the light intensity was high, the classical explanation would be that light from one polarization was interfering with light from another polarization.

No, that's incorrect. In unpolarized light there is no coherence between the two orthogonal polarization components. The polarization-rotation element does take the light polarized along $x$ and puts it in yuxtaposition with the original $y$-polarized light from the other arm, but since there is no coherence between the two, they won't interfere.
