Methods for Obtaining Which Path Information in a Quantum Eraser Experiment In all of the quantum eraser experiments I have heard of, polarization is used as a means for identifying the path of a photon. Is this the only method, or do other methods exist for obtaining which path information? If so, what are they?
 A: Yes, polarization is currently the only means of "tagging" light with which-way information in the quantum eraser experiment. This shouldn't seem strange, though. For the experiment is based precisely on the similar experiment conducted by Fresnel and Arago in which they made that remarkable discovery about polarized light: namely, that perpendicularly polarized light brought together shows no interference. 
As a matter of fact, a more recent discovery that circularly polarized clockwise and counter-clockwise light shows the same non-interference has fashioned us with an additional way to perform the experiment with polarizers.
People often have a hard time grasping what's so significant about the quantum eraser experiment; that is, given that it's based on discoveries already made about polarized light. Its significance lies in the fact that it establishes a priority of the quantum explanation over the original classical explanation. That is to say, it purports that what Fresnel and Arago consciously discovered in the non-interference of light is the effect rather than the cause. 
And what then is the cause? Well in Fresnel's and Arago's case, when they attached a horizontal polarizer over one slit of the double-slit, and a vertical polarizer over the other slit, and observed no interference on the back screen, what they really discovered, i.e. though they were unaware of it, was a perfect and very rare means of marking or imbuing each photon that traversed the slits with distinctive information that recorded through which slit it traveled. That, according to the quantum explanation, was what actually caused the photons to behave differently, to give up their wave-like behavior, to travel each through only one slit, and to not interfere on the other side.
