A single photon can be in |+1> or |-1> state - each being correlated with L/R circular polarization. When we consider plane polarized light coming from a polarizer does it mean that we observe an entangled photon pair?
I am having trouble understanding your question. Are you asking whether a pair of photons that are entangled on circular polarization can continue to be entangled if the photons pass through a polarizer that forces a reorientation of each photon to vertical/horizontal polarization?
If you find that in spite of an isotropic surroundings , there is plane polarized light from a luminescent source what will you conclude ? CP violation would lead to dominance of either forms of CPL , but would it yield plane polarized light?
Your question is not quite clear. I understand the physical situation you described as following. There is a plane polarized light which can be considered as composition of two circular polarized lights. Now if you ask, can we consider this pair of circular lights being in entangled state? My answer 'no' because we can't distinguish them between each other or spatially or energy. Even if we discuss pair of photons born from process of parametric down conversion 2 and which collinear propagate in one direction we can't write for them wave function:
|psi> = (|L1>|R2>+|R1>|L2>) / Sqrt(2)
for the same resons. We should write:
|psi> = |L>|R>
But a single photon is always in |+1> or |-1> state - which corresponds to circular polarization.
What do you mean? Why a single photon is always in |+1> or |-1>? Arbitrary single photon can be in arbitrary state, for example: c1|+1> + c2|-1>. It depends on source.
But if you speak about parametric down conversion experiment then I should say that everyone from the pair of photons is not ALWAYS in |+1> or |-1> state. They are in entangled state after their birth. Only after detection one of them they will be in the definite state. One will be in |+1> another in |-1>.
If your last comment connected with my words about an indistinguishability pair of photons which collinear propagated in one direction after the birth in parametric down conversion process then I repeat my thought.
We have not two separated single photons. We have two single photons which we can't distinguish from each other. It means that we can't write indexes 1 and 2 in the equation for total wave function like in this |psi> = (|L1>|R2> + |R1>|L2>) / Sqrt(2).
Maybe I didn't understand your last comment at all. So could you please explain it in more detail.
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