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Quantum Entanglement Basic Experiment Question

  1. Apr 26, 2014 #1
    I am a law student and have no training whatsoever with regard to quantum mechanics, and I have been struggling to wrap my head around quantum entanglement in particular. I've been trying to find videos which would explain exactly what would occur in the following scenario, which I believe is very similar to the basic quantum eraser experiment:

    1. A photon is shot through a BBO crystal, converting the single photon into two quantum entangled photons of lower frequency and opposite spins.
    2. Photon 1 passes through a double-slit mask to a wall on which its pattern is detected.
    3. Photon 2 passes through a different double-slit mask, and a detector is observing which slit the photon is passing through as well as its direction of spin. The photon then passes through to a wall on which its pattern is detected.

    I would assume in this situation that the observance of which slit photon 2 passes through would destroy its interference pattern on the wall. I have the following questions:

    1. Does this observance of photon 2 also destroy the interference pattern of photon 1?
    2. Does measuring spin collapse a photon's wave function?
    3. If so, does the measurement of photon 2's spin collapse the wave function of photon 1 because of quantum entanglement?
    4. Or would you just get an interference pattern for photon 1, no interference pattern for photon 2, and still be able to determine the spin of photon 1 as it passes through the slits?
    Last edited: Apr 26, 2014
  2. jcsd
  3. Apr 27, 2014 #2
    If you measure one, you measure both, so... Yes, yes, yes and no.
  4. Apr 27, 2014 #3


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    Welcome to PhysicsForums, st29! These are some great questions. This is actually a more sophisticated situation than might be obvious.

    Regarding 1: Actually, neither photon stream (as described above) will ever evidence interference. See Zeilinger's "Experiment and the foundations of quantum physics", figure 2, page 290:


    Were this not the case, it would be possible to use the setup for FTL signalling. There are situations where subsets of photon pairs can be made to show kinds of interference by what is called coincidence counting. However, this requires a conventional signalling channel to demonstrate.

    Regarding 2: these photons can be entangled on multiple bases. It is technically possible to measure entanglement on one basis without affecting entanglement on another basis. For example: you could collapse polarization (spin) with a filter without affecting momentum (frequency) entanglement.
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