# Quantum eraser input polarization

gespex
Hi all,

I'm a bit confused about the polarization state of the photons after the BBO. I am aware that they are polarized orthogonally, but are they polarized according to a certain axis?

The reason I'm asking is that one article I read seems to imply that that's not the case: i.e. the polarizations are orthogonal, but they may still be at any angle. However, Wikipedia states that the QWP produces circularly polarized photons. But if I'm not mistaken, a QWP only produces circularly polarized photons if the angle of the QWP is exactly 45 degrees relative to the photon's polarization. Otherwise it will produce elliptically polarized photons, or even linear if the incoming angle is exactly that of the QWP.

So which of the two is true? By the looks of it they are always parallel to either the x or the y axis, but it can be either for either particle. Is that true?

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Mentor
There is one polarization direction where QWP1 gives clockwise polarized light and QWP 2 gives counterclockwise polarized light (and therefore no interference). Incoming light perpendicular to that direction will give the opposite: CCW at QWP1 and CW at QWP 2 (and again no interference).
You can write a linear polarization as superposition of those.

What about a different axis to look at?
Consider light, polarized along the fast axis of QWP1: It will stay polarized, and the same happens at QWP2, but they get a relative phase shift of 1/4 wave and the interference pattern moves a bit relative to a regular double slit.
Now, consider light polarized along the slow axis of QWP1: It is similar to above, but the phase shift and the shift of the interference pattern is in the other direction - you get light where the other part is dark and vice versa. And the sum of both is "no interference" again.

Quantum magic :D.

Gold Member
Hi all,

I'm a bit confused about the polarization state of the photons after the BBO. I am aware that they are polarized orthogonally, but are they polarized according to a certain axis?

Good question, and the answer is both yes and no. It depends on the type of crystal used. Usually called Type I and Type II, here is what you have:

Type I: Photon pair comes out polarized in a known direction parallel to each other, ie HH>
Type II: Photon pair comes out polarized in a superposition of HV> and VH>

Type II is suitable for a Bell test as is. Type I requires 2 crystals oriented orthogonally to get a HV> + VH> superposition.

Going back to your question: they do always come out as H or V and so the QWP can do its thing with no problem.