Polymeric molecules, pure and mixed states

While it's still August, I'll quote again from friendly Frank J. Blatt's physics textbook, this time concerning one particular type of polarizer.  I bought the textbook at a yard sale at 4110 Avenue F in Austin, Texas in August 1997.  Or maybe it was 4112 Ave F.  Anyway, I like Blatt's writing:

The most common polarizer in use today is a synthetic dichroic material invented by E. H. Land. Polaroid is produced by stretching a sheet of polyvinyl alcohol, thereby aligning the long hydrocarbon chains of the polymeric molecules. The sheet is then impregnated with iodine, which attaches itself to the polymers and results in good electrical conduction along the chains, but allows little conduction perpendicular to them. Electromagnetic waves whose E vector is parallel to the polymers are then strongly absorbed by this material.

The E vector represents the electric field's direction and strength. 

Now a bit more from Gordon Baym on the mysteries of quantum mechanics:

There are really two levels of probability in quantum mechanics. The first, which is called the pure case, is when we have a system that is in a definite state (often called a pure state). Then the behavior of this system in a given experiment is governed by the probability amplitude rules we have been discussing. The second case is when the system can be in any of several states, with certain probabilities. This case is called the mixed case, and we say that the photon is in a mixed state. Then one must calculate the results one expects in a given experiment for each of the separate states that can be present, and take the weighted average of the result over these, as we have done ...   In averaging over the various states that can be present, one uses the ordinary classical probability rules; there is no possibility of interference occurring between these states.

Now, I'm not saying it makes any sense, yet.  But I learn stuff from copying it down, so by typing the quote, I will I hope remember its content.  I'll have more to say on pure and mixed states later.  To begin with, however, you can see that purity involves those delicate phase relationships and the possibility of interference, and mixed-ity involves only ordinary classical probabilities, with no chance of quantum mechanical interference.  Quantum superposition and interference are thus closely related.

And speaking of closely related:  what kicked off this whole polarization discussion was the mention by Emil Wolf of the close relationship of polarization and coherence (of light), and the fact that I had been thinking something similar myself before Wolfie wrote about it.