Polarization of photons quantum mechanically

[sol47739]

TL;DR

I have some fundamental questions about the polarization of photons from the quantum mechanical perspective.

What is it of the photon that gets polarized from a quantum mechanical perspective? In the classical perspective it is often thought that it is the oscillating electric field that gets polarized. But in the quantum case: Is it the de Broglie wave function? Or is it the spin and in case it is the spin, how is the polarizing filter able to determine which kind of spin gets through? What is the polarizing filter made of incase it determines what kind of spin of the photons get’s through?

 

[PeterDonis]

What is it of the photon that gets polarized from a quantum mechanical perspective?

The question is unanswerable because it is based on a false assumption. A photon doesn't "get polarized" by acting on some part of it. Polarization is just the spin part of the photon's quantum state.

Thinking of what is classically called "unpolarized light" is not a good way to approach photon polarization in QED. Classical "unpolarized light", in quantum terms, is (highly heuristically) just a huge number of photons with random polarizations, so the total polarization is negligible. But there is no such thing as an "unpolarized" single photon.

 

[hutchphd]

But there is no such thing as an "unpolarized" single photon.

But that is a bit too facile it seems to me. Absent some asymmetry (perhaps a filter) the description of the photon is basis dependent. In particular one can have RHCP and LHCP or X and Y .

A photon doesn't "get polarized" by acting on some part of it.

The polarization of "the photon" will produce an answer that depends upon the question. In that sense one "acts upon it".

 

[PeterDonis]

the description of the photon is basis dependent

The photon's state is not basis dependent. Its components might be, but that is true of any quantum system.

The polarization of "the photon" will produce an answer that depends upon the question.

You can change the polarization part of a photon's state by measuring its polarization, yes. But that's true of any quantum system: a measurement, unless it already happens to be in an eigenstate of the measurement operator, will change the state.

In that sense one "acts upon it".

Perhaps, but I don't think that's what the OP meant. The OP seemed to me to be thinking of polarization as something you "do" to some part of the photon. That's not correct. Polarization is, as I said, the spin part of the photon's state. Or, if you like, it's the spin degree of freedom of the photon. It's not something you "add on" to a photon by doing something to it: that spin degree of freedom is always there.