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Quantum Entanglement : Spooky action at a distance

  1. Nov 2, 2010 #1
    Hi, I will like to ask this question about quantum entanglement and confirm the "weirdness", cos some info I get says entanglement aren't weird at all, while some says it does.

    Here's the question:

    If we prepare and get a bunch of entangled electrons bunch A and bunch B (A is entangled to B). We pass bunch A to Alice, and bunch B to Bob and Alice uses her Bunch of electrons to do the double slit experiment without any interferences or disturbance to the electrons, and the electrons shows a wave pattern (Q1, does entangled electrons shows wave pattern when used to do double slit experiment without doing any measurements being made?) Assuming the electrons shows a wave pattern, then Bob decided to play a trick on Alice, and measures his Bunch of B electrons, Q2. Does that screws up Alice experiment such that instead of a wave pattern, she gets an particle pattern, thereby such scenario creates spooky action at a distance ?

    Thanks for reading.
  2. jcsd
  3. Nov 2, 2010 #2


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    No. Alice cannot tell from the results of her experiment whether Bob has been doing anything with his particles. In order to find out that something "weird" has been happening, Alice and Bob have to get together, compare notes, and study the correlations between Alice's measurements and Bob's measurements.
  4. Nov 2, 2010 #3

    No, unlike normal electrons, they don't. Or at least they don't show any pattern whose physical nature takes part in the entanglement.

    Since the wave function is not separable, the available information about the electron is given by a density matrix, which contains less information than the complete wave function. The information about a possible pattern is not in the density matrix.

    The french Wikipedia describes the quantum eraser experiment of Marlan Scully, which illustrates your question :

    Particles got on the screen, in I, are entangled with the ones getting into the eraser F-G-H.
    They are in a double slit configuration, C and B being equivalent to slits.

    They don't show any interference pattern, as shown in figure 1, in the paragraph "Figures d'interférences".

    However, their position on the screen is correlated with the output way of the eraser, as shown figure 3 and 4.

    But, exactly the same way as with spins, when you look only on Alice's side, you get all possible results randomly : 50 % + and 50 % - if you deal with spins; and 25 % the first pattern (fig 3), 25 % the second (fig 4), and 50 % of other random distributions (fig 5, 6) if you deal with interferences, which add up to figure 1 or 2, where no pattern is visible.
  5. Nov 2, 2010 #4
    This is not equivalent to the slits in the double slit experiment. The reason they are different is the phase at C and B are not at all correlated. In the double slit experiment the phases are equal.
  6. Nov 2, 2010 #5
    Yes and no.
    The configuration is the same because when one photon comes from A, one photon arrives in I, and there are two possible pathes for it to follow : ABI or ACI. If B and C were mirrors, the "phases would be equal" as you say, though I've been corrected on this statement on a french forum : the phase of a photon is always undeterminated. Actually, they told me that a photon could have no quantum state because a photon was a quantum state. And since only QED can deal with photons, it was better to stick with electrons for these kind of discussions.

    But you're right about the no correlation between C and B because C and B are non-linear converters. But that's actually the situation about which gtwace is talking : flowing entangled electrons into a double slit.
  7. Nov 2, 2010 #6
    If entangled electrons does not show an interference pattern in the double slit experiment, then entanglement is not really that weird since they do not change behaviour if the other is disturbed.

    Their state/behaviour is predetermined just that it is unmeasured by us, and since probability is = 1, knowing the probability of one entangled electron will tell us instantly the probability of another.
  8. Nov 3, 2010 #7
    It's not that weird indeed. No physical law is violated at the observational level.

    But the state / behaviour is not predetermined. If it was, Bell has proven that a given inequality would be met, while it is not.

    Knowing the measurement result on one electron, we can tell the probability of each possible result for any measurement performed on the other electron. This information can't be contained in the other electron alone. It is a property of the pair and of the choice of the measurement made on the first electron.

    The fact that this probability is actually met when the other electron is measured is something that can't be explained in a local, realistic and deterministic way.
  9. Nov 3, 2010 #8
    I assumed you were asking about a case where the entangled pairs are separated into two streams and one stream is sent through the double slit. In this case there is interference regardless of what happens to the other stream.
  10. Nov 3, 2010 #9
    It is true that with the singlet state entanglement, there is no reason to suppose that interference would not occur with a double slit.

    But the experiment proposed by gtwace is different : the electrons are supposed to be entangled in such a way that if one electron takes one slit, then its entangled twin takes the the same, so that Bob can try to erase the interference pattern of Alice by forcing the determination of the slit, thanks to the EPR correlation.
  11. Nov 3, 2010 #10
    I was not referring to entangled spins. I was referring to entangled momentum/position.

    When they are entangled in this way there still will be interference. If Bob measures position this will not erase the interference.
  12. Nov 4, 2010 #11
    I don't understand... Why are there no interferences with photons in Marlan Scully's experiment ?

    And if Bob projects the quantum state in an eigenstate corresponding to one slit only, closing one of his slits, how can there be interferences ?
  13. Nov 4, 2010 #12
    The best discussion I have found of this forum of Kim and Scully's DCQE is here. https://www.physicsforums.com/showpost.php?p=2241460&postcount=8

    I'm not sure what you mean? If Bob sends his electrons through one slit he will not see double slit interference and if Alice at the same time sends her electrons through two slits she will see double slit interference.

    Bob's choice does is not affect Alice and what Alice's choice does not affect Bob. Entanglement means that if they measure the same aspect they will get correlated results (not cause and effect), if they measure different aspects then entanglement shows nothing.Take the case of entangled spins. If Alice measures the spin on the Z axis and Bob measures the spin on the Y axis their results are independent, but if they both measured on the Z axis their results would be correlated, but still Bob's result thought correlated to Alice's result has not had an affect on her result.
  14. Nov 4, 2010 #13


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    Just a reminder: generally, entangled particles do not exhibit double slit interference effects. If they did, you could find out which slit they went through. The only way to restore interference is to completely erase which slit information on one side (say Alice). This would restore the interference effect for Bob.

    Further, it is not possible to erase the which slit interference in all cases. So you need to perform coincidence counting to see it at all. The rules on this are a little confusing, so don't be surprised if you look at it over and over and still scratch your head.
  15. Nov 4, 2010 #14
    I would be interested in seeing evidence for a double slit experiment with an entangled particle where there is no interference. What is it about having a correlated pair that would alter the particles ability to interfere with itself.
  16. Nov 5, 2010 #15
    Another question: suppose streams of electrons A and B are each sent through double slits. If Alice measures which slit for her electrons, creating a particle pattern, will Bob see a particle pattern as well?
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