I read that a polaroid filter is made of many long chain of molecules aligned in one direction and will only allow the vibration of light with the same alignment as the filter to be absorbed.
I always thought only certain frequency of light can be absorbed by the electrons but in this case it is the spin states that affect whether it is absorbed or not.
So the photon emitted from a light source is in superposition until the filter interact with it and cause it's wave function to collapse into only one spin state. (my own interpretation, which is wrong)
Q1: How does photon(superposition) knows whether it should be absorbed by the polaroid filter or not? either filter don't work with unpolarized light(obviously incorrect) or there's intrinsic value(also incorrect).
Q2: How does a polarized photon knows the polaroid filter's orientation?
Note: I'm now treating photon as a quantum particle instead of wave.
Best Answer
It all depends on definitions used of course, but overwhelmingly I have found that when people talk about "unpolarized" or "depolarized" they mean a classical mixture of pure quantum states. So they definitely do not mean a superposition state.
Another way to understand that a superposition is not "unpolarized", or at least that this is not a good definition, is as follows. Suppose you begin with linear polarized states in two orthogonal directions as your basis states, say $|x\rangle$ and $|y\rangle$. Now think of any superposition: it is a complex co-efficient superposition of $|x\rangle$ and $|y\rangle$. Then one can always make an orthonormal co-ordinate transformation to make the arbitrary superposition one of the new basis states. So this definition of "unpolarized" would be basis/ co-ordinate dependent!
A phenomenological description of the difference is as follows. Imagine a piece of experimental kit that can make any arbitrary unitary transformation on input light. Something like a Babinet-Soleil Compensator concatenated with a variable Faraday Rotator will do this for you. You connect its output to a linear polaroid. Now you input the light and make the following "differential diagnosis/categorization":
If there is a setting on your kit that allows all the light to pass through, then your light is in a pure, superposition state. You simply need to twiddle the knobs to find the unitary transformation to turn the pure state into a linear polarization state aligned with the polarizer. The von Neumann entropy is 0 bits per photon;
If for all possible settings on your kit, less than all the light is output, then the light is (partially) depolarized. It is totally depolarized if and only if the throughput of the system is independent of the settings: all settings attenuate the light by the same factor of $1/2$. In such a state, the von Neumann entropy is exactly one bit per photon.