Quantum entanglement

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15 Responses to Quantum entanglement

  1. sherwingooch says:

    Dr. Sherwood,

    I would be interested in hearing: What are your thoughts regarding the independence of the order of observation? More specifically:

    Let us examine the case where two observers, let’s call them Charlene and Darlene, are independent observers located respectively in different inertial reference frames ‘C’ and ‘D’. Their velocities relative to Alice and Bob are such that Charlene sees that Alice observes the spin of her entangled electron before Bob observes the spin of his electron, while Darlene observes that Bob determines the spin of his electron prior to Alice. As “elucidated” by Bram Gaasbeek in “Demystifying the Delayed Choice Experiments”

    ( http://arxiv.org/abs/1007.3977v1 ),

    quantum mechanics dictates that the entanglement is independent of both ‘space’ and ‘time’.

    Because the two electrons seem to “know” their spins independent of which is measured first, it would seem to imply either a hidden rule or a deterministic Universe. How does quantum physics resolve this conundrum?

    Thank you,

    Sherwin Gooch

  2. I’ll emphasize again that the key aspect of entangled states, confirmed in detail by many recent experiments, is that the electrons DO NOT “know” their spins from the beginning. As I said, a “reasonable” notion is that at the time of the creation of the entangled state, Alice’s electron is in a specific state (“up”, say), and Bob’s electron is in a specific state (necessarily “down”). This interpretation has been definitively ruled out. Alice’s electron is simultaneously in both the up and down states, as is Bob’s. There is no hidden rule, and the interpretation involving “hidden variables” has been ruled out. Quantum physics doesn’t “resolve” this “conundrum”. Rather, quantum physics correctly predicts the outcomes of experiments, and it doesn’t care whether this description doesn’t “make sense” to us, or seems to be a “conundrum”. Nature is what it is and has no responsibility to satisfy our notions of what is proper or sensible.

  3. sherwingooch says:

    Bruce,

    Please allow me to restate my question more succinctly:

    Let’s assume ideal conditions, so the probability of observing spin-up and spin-down are both 50%, etc. Charlene in inertial reference frame ‘C’, zooming by, but located near Alice, sees that Alice observes spin-up, and immediately transmits a message to Darlene to that effect. Due to the time required for the message to travel from Charlene to Darlene, Darlene will not receive this message until later.

    Now we shift our attention to Darlene, in inertial reference frame, ‘D’, near Bob, at an instant just prior to Bob making his measurement. In Darlene’s reference frame, Alice has not yet made her measurement. Neither has Bob, although he will in the next instant. At this moment in Darlene’s reference frame, there is a 50% probability that Bob will measure spin-up. As it happens, Bob makes his measurement, and Darlene sees that Bob, indeed, measures spin-up. She records this result.

    Sometime later, Darlene receives the message from Charlene.

    Both electrons being observed to have spin-up would contradict the requirement that each of the entangled electrons, when observed, acquire an opposing spin. To maintain this requirement, either Charlene’s message to Darlene must be garbled, or never arrive. Or Darlene must have recorded Bob’s result incorrectly. Or either Alice or Bob must realize after-the-fact that they have made an erroneous measurement.

    Or perhaps the spin-state of one or the other electrons, along with all ensuing causally-based effects, changes after-the-fact to obtain an internally-consistent universe. But, once it has reached consistency and can be observed, this mechanism would have the same appearance to experimental physicists as the spin-state being maintained — throughout — in a hidden variable.

    What does physics say will happen?

    Thank you for your patience. I would really like to know the answer to this question.

    Sherwin

    • Jeff says:

      I think the answer (based on the premises we are apparently given) has to be that Bob has a 50/50 chance of detecting spin-up, and the result of his measurement must be spin-down. When Alice’s message arrives, it reports that Alice had spin-up. (Or vice versa.) From this we would be hard-pressed not to understand that Bob actually always had a spin-down electron. For this not to be so, we’d have to say that the detection events have some super-luminal relationship, which I gather would constitute a hidden variable. I’ve heard of local hidden variables and global ones. Have both been ruled out?

      • I guess I’ve failed in my attempt to report what is now known from lots of experiments of many kinds. You simply must suspend your natural desire to make common-sense, reasonable interpretations of what “should” happen. Hidden-variable theories are ruled out by the many experiments. Unless Alice or Bob makes a measurement, each of their electrons remains forever in a superposition of up and down states, neither electron is up or down. All attempts at interpreting what happens in terms of Alice’s electron having been born in the up or down condition have run aground, through experiments that show conclusively that her electron CAN NOT have been in the up or the down state at the start of the experiment. I again recommend Zeilinger’s book for details.

        It’s admittedly strange, but it’s the way the world actually works. Nature has no obligation to work in a way that seems reasonable to us. Nature just is, and the task of physicists is to discover how Nature actually behaves. As for superluminal issues, I tried to deal with that: Alice cannot use entanglement to send a message to Bob.

      • Jeff says:

        Thanks for the response. I was just trying to capture the challenge for conceptualization, according to what I understood from your explanation, which did lead to a question.

  4. Sorry, but I don’t know enough about the subject to answer your question, but a quick glance at the physics article you cited suggests that that article does address questions very similar to yours.

  5. sherwingooch says:

    Yes, but like so many things in QM, it also seems to make no sense at all. The fact that both ‘time’ and ‘location’ cancel out of the mathematical description of entanglement seems to indicate that an entangled pair is entangled for all time — including the time before the entangled pair was even created. This begs the question, “what is time?” which, of course, no one knows!

    Thank you for your attention.

    • Bob Rader says:

      Hi Sherwin (and Bruce). The choice of which experimenter measures “first” is an artifact of human language storytelling: it doesn’t matter. Neither can tell that the other electron has (or has not) been measured. After the fact, when we (or they) get all the results to one location and analyze them, those doing that analysis will see the perfect correlation. The perfect correlation is required by our having specified a correlated (spin 0, in this case) state.

      • Bob Rader says:

        (Continuing…)
        The experiment doesn’t make much progress in addressing “What is time?” because it
        implies the universe-wide common “time” of quantum mechanics: the problem is not
        set up with a relativistic notion of time, for example. At which point one should point out that the order of the two experimenters actions can be anything, from the point of view of different observers.

        We set up a problem with a time-independent global state. The interesting thing is that this way of describing the world really does seem to work. The experiments being done to check this out are amazing.

      • sherwingooch says:

        I notice that I neglected, above, to include in my description the detail that, for this experiment to be performed, Alice and Bob are required to be in different inertial reference frames. Mea culpa! (Considering fringe-effects due to Doppler-derived distortion off-axis from the direction of propagation, I don’t think this is strictly true, but let’s avoid that red herring.)

        – — — — — — — — — — — — — — —

        Thank you, Bob. I’m sure you’re right.

        In fact, in the course of writing what was quickly becoming a long-winded response, I hit upon this resolution to what I perceived as a paradox:

        A way to resolve this conundrum would be to declare that two measurements of a single quantum state (entangled state), by definition, make sense only if each measurement is regarded as taking place in an inertial reference frame in which both measurements occur simultaneously. I.e. it makes sense only to compute details related to such measurements if the decoherence is regarded as occurring in space-time continua in which both events occupy a time-like point on a space-like line.

        A more general statement can be obtained by increasing the number of entangled states while maintaining the requirement that all measurements of the state, if made, must be regarded as being made from corresponding reference frames which provide simultaneity.

        This resolves the conflict, but it makes it more difficult to describe related events taking place in the environment of either entangled electron, as they must be considered in reference to this time-like point. (So be it.)

        What does this say about other quantum measurements taking place in the same, or distant, neighborhoods of space-time? Can they all also be connected in this way without leading to a contradiction? (Yes, I think they can.)

        Sorry about being too ignorant to have perceived this before. I thank you both very much for your patience in aiding me to resolve this misunderstanding. I hope I didn’t consume too much of your time.

        Thanks, again,

        Sherwin

  6. To sherwingooch; Your question isn’t clear. Could you please draw a space-time diagram of events as you describe them. In particular, please pay attention to signals you send and light cones they travel along.
    Just saying that something is before or later does not necessarily make it so. Relativity is still a causal theory and I suspect that in fact there is no issue at all. Your drawing events as you describe them may clarify your question.

    • sherwingooch says:

      Dear Zvonko Hlousek,

      Thank you for your suggestion.

      A vanishingly small magnitude of Lorentz-affected variation in relative time is required to toggle the precedence of two quasi-simultaneous events, so I think we can set aside consideration of light-cones.

      But your suggestion to draw a space-time diagram was spot-on.

      Thank you,

      Sherwin Gooch

  7. David Trowbridge says:

    Hello Bruce,
    First let me thank you for suggesting Anton Zeilinger’s book, which I’ve now renewed four times already from the local library. I’ve read it twice and filled the book with colored sticky tags reminding me of the most striking results.

    I was particularly struck by his discussion of delayed-choice teleportation in the section, “A Ghostly Idea,” pp 227-232. He concludes, “The message to be learned is that individual events in quantum physics are primary: the are more fundamental than the explanations that we later construct based on our physical pictures.” My main takeaway is that we probably need to take a much closer look at what our minds are doing when we try to interpret the world. For example, we like to think that objects have properties– that somehow there are attributes connected to things that are completely independent of our internal mental operations. But is that true? The universe does what it does, and presumably has been doing so long before humans emerged from the muck. Now a species has evolved that thrives by telling itself stories. It likes to believe that certain worldviews do in fact reflect reality, but often neglects to examine where those thought processes come from. Perhaps the time has come to look more closely into just what our minds are doing when we apprehend the world.

    These certainly are exciting times we live in!

    David

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