In Ballentine's Quantum Mechanics book, as part of a discussion of pure states vs non-pure (mixed) states, he says Polarized monochromatic light produced by a laser can approximate a pure state of the electromagnetic field. Unpolarized monochromatic radiation and black body radiation are examples of nonpure states of the electromagnetic field. I think that he is referring to the classical EM field and using "state" in a more broad sense than quantum states, though I could be wrong about that. So in what sense is unpolarized light not a pure state? I would think that, just as superpositions of pure state quantum wave functions give another pure state, that superpositions of the EM field would also give "pure states". That is, any solution of Maxwell's equations would be considered a pure state. But really I am muddled on this concept.