# Does entangled polarized photon get through filter?

Gold Member
Suppose we have a creation point C where two entangled photons A en B are produced. 1m in the direction of A is a vertically polarized filter V. 2m in the direction of B is a horizontal polarized filter H.

Now, I understand that when photon A strikes filter V, it either gets blocked or gets through at a 50/50 rate. If, however, photon A gets through the filter, it becomes vertically filtered, and so is photon B. So photon B then gets blocked by filter H.

Now I wonder what happens if photon A gets blocked by filter V. Does photon B remain unpolarized, thereby retaining the 50/50 percent chance of getting blocked by filter H? So, is it possible that both photons get blocked at any given time?

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DrChinese
Gold Member
No, their relationship guarantees that one will get through and one will not.

By the way: there is no experimental sense in which the order of the photons encountering their respective polarizing filters matters. Whether it is A's being 1 meter and B's being 2 meters away, or vice versa, outcomes appear the same.

bhobba
Nugatory
Mentor
Now I wonder what happens if photon A gets blocked by filter V. Does photon B remain unpolarized, thereby retaining the 50/50 percent chance of getting blocked by filter H? So, is it possible that both photons get blocked at any given time?
No. A's interaction with the polarizer is: absorbed if horizontally polarized, transmitted if vertically polarized. That's sufficient to determine the polarization of both A and B after the interaction.

I have a problem with this thread: OP seems to assume initially unpolarized entangled photons (allthough the title of this thread seems to suggest otherwise), but the answers assume a kind of definite either vertical or horizontal polarization, don't they ?

Gold Member
Yes, my mistake (the title).

Nugatory
Mentor
I have a problem with this thread: OP seems to assume initially unpolarized entangled photons (allthough the title of this thread seems to suggest otherwise), but the answers assume a kind of definite either vertical or horizontal polarization, don't they ?
They only acquire that definite polarization when one of them encounters the polarizing filter, and then when we say "horizontal" or "vertical" we mean with respect to angle of the polarizer.

Any polarization state can be written as a superposition of horizontal and vertical polarization about any axis; interaction with a polarizer on a given axis causes the superposition to collapse to one of those two states.