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Opposite spin of entangled photons

  1. Oct 19, 2011 #1
    Quick question: Do entangled photons have opposite spin? If so, how do we know? I mean I know that they should because of conservation of angular momentum. But Cos(0)^2 = Cos(180)^2. So it looks like they’re the same. Or am I missing something.
  2. jcsd
  3. Oct 19, 2011 #2


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    A couple of points: Type I pairs are same polarization, Type II are orthogonal (crossed). Crossed means they are perpendicular, so they are 90 degrees apart. (You were thinking 180 degrees.)
  4. Oct 19, 2011 #3
    OK so what I think you're saying is that 2 entangled photons are offset by 90 degrees so that if one makes it through a polarized lens then you can be sure the other won't. (Type II?) Is that right?

    I don't know the types but I do know that photons are bosons with spin 1. So I guess you mean that they're Type II.
  5. Oct 19, 2011 #4


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    Type I and Type II are terms for the types of PDC crystal arrangements that are most often used for creating entangled photons. With a Type I arrangement (which is actually 2 crystal arranged perpendicularly), you get identical polarizations. With Type II, you get crossed (opposite) polarizations.
  6. Oct 20, 2011 #5
    Very cool! Thank you.
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