JSD made a very interesting and thoughtful posting about the
unitarity of polarizer operations. I have probably mentioned this
before, but I can't resist bringing it up again:
A Faraday rotator sandwiched between two linear polarizers oriented
at 45 degrees to each other makes what is sometimes called an
"optical diode." That is, it only permits light (of a particular
wavelength) to pass through it in one direction.
So if I put an object A on the "input" side and another object B on
the "output" side, more radiation flows from A to B, right? If so, a
spontaneous temperature difference will exist between them, which I
can use to run a perpetual heat engine!