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Spontaneous parametric down-conversion practicalities

  1. Jul 6, 2009 #1
    I'm interested in generating entangled photons from barium borate but have a few questions:

    1) Taking one of the virgin entangled photons- before any measurements - does it have a random polarization? Ie, if I put any polarizer in its path will 50% of photons make it through?

    2) I know that the entangled photons are emitted off the main axis (? approx 3 degrees) - can this be used to ensure that only entangled photons are used - ie without the need for a coincidence counter? Would a narrow bandwidth filter help?

    3) Could I buy a cheap blue laser from a standard electronics store and expect it to work?

    Any help would be greatly appreciated!
  2. jcsd
  3. Jul 6, 2009 #2


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    Welcome to PhysicsForums, auspaco!

    1. Yes and no. There are 2 main types of PDC crystals, called Type I and Type II.

    Most type II have the attribute you seek, which is random polarization at any angle. Technically, they are 50% V>H> and 50% H>V> which is essentially the same thing. (Not sure if the input orientation is important or not.)

    But Type I are a bit tricky. You need 2 Type I crystals, oriented at a right angle to each other. A V> input becomes a H>H> output for one crystal and an H> input becomes a V>V> output for the other (at a right angle). You must have the input stream oriented at 45 degrees to get the proper entanglement from the output pairs.

    2. Again yes and no. The off axis part will be the entangled pairs. Single photons go straight through. But coincidence counting is needed for a variety of reasons, so I doubt you will be able to avoid that.

    3. No, you must use a laser tuned to the proper wavelength for the BBO crystal. Usually you see stuff on the order of 405 nm input, which becomes 810 nm output.

    You have probably seen this, but if not here is a good article with a detail parts list:

    Thorn, Beck et al (2003): Observing the quantum behavior of light in an undergraduate laboratory

    I hope this helps.-DrC
  4. Jul 7, 2009 #3
    Thanks for the speedy reply! Very helpful :)
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