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Entangled Photons with no shared Past

  1. Apr 12, 2007 #1


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    OK, here is some pretty wild stuff about entangled photons. In an experiment reported in the Preprint Archives last week, a team of distinguished physicists are continuing to push the bounds of what we can understand as "entanglement". They demonstrate entanglement of 2 photons which have never interacted in the past!

    Here is the abstract:

    Article: Entangling Independent Photons by Time Measurement by Matthaeus Halder, Alexios Beveratos, Nicolas Gisin, Valerio Scarani, Christoph Simon, Hugo Zbinden

    "A quantum system composed of two or more subsystems can be in an entangled state, i.e. a state in which the properties of the global system are well defined but the properties of each subsystem are not. Entanglement is at the heart of quantum physics, both for its conceptual foundations and for applications in information processing and quantum communication. Remarkably, entanglement can be "swapped": if one prepares two independent entangled pairs A1-A2 and B1-B2, a joint measurement on A1 and B1 (called a "Bell-State Measurement", BSM) has the effect of projecting A2 and B2 onto an entangled state, although these two particles have never interacted or shared any common past[1,2]. Experiments using twin photons produced by spontaneous parametric down-conversion (SPDC) have already demonstrated entanglement swapping[3-6], but here we present its first realization using continuous wave (CW) sources, as originally proposed[2]. The challenge was to achieve sufficiently sharp synchronization of the photons in the BSM. Using narrow-band filters, the coherence time of the photons that undergo the BSM is significantly increased, exceeding the temporal resolution of the detectors. Hence pulsed sources can be replaced by CW sources, which do not require any synchronization[6,7], allowing for the first time the use of completely autonomous sources. Our experiment exploits recent progress in the time precision of photon detectors, in the efficiency of photon pair production by SPDC with waveguides in nonlinear crystals[8], and in the stability of narrow-band filters. This approach is independent of the form of entanglement; we employed time-bin entangled photons[9] at telecom wavelengths. Our setup is robust against thermal or mechanical fluctuations in optical fibres thanks to cm-long coherence lengths. "

    In other words: A1 and A2 share a common past, B1 and B2 share a common past, A1 and B1 are made to interact at a later time in such a way as to make them indistinguishable, and the result is that space-like separated A2 and B2 are now entangled (at least partially, but to a level that can be demonstrated).

    It really makes one think about what entanglement means, as if things weren't strange enough already! :smile: And by the way, the results are fully in keeping with Quantum Mechanics.
    Last edited: Apr 12, 2007
  2. jcsd
  3. Apr 12, 2007 #2
    I suppose then that one of the pairs was "teleported" from another universe, or spontaneously generated from nothing, as every particle in this universe shares a common past with any other (at least if big-bang theory is true). :biggrin:

    I wonder why some people feel the need to make false (or unverifiable) claims to make things look stranger than they realy are.
  4. Apr 12, 2007 #3
    I’m not that impressed with these types of experiments.
    Very similar to how they construct 3 way and 4 way entanglements.
    Even if two photon entanglement is correct, these constructions do not imply the same “entanglement” being extended to particles with no history.

    Consider A1 & A2 as being perfectly correlated as being identical to each other or negative images of each other. PDC systems create photo pairs that are “negative images” of each other, but by changing the measuring reference of one the “image” can be observed as reversed allowing the two to seem as identical copies of each other. That is an experimenter’s choice.

    Here we are doing the same for B being split into B1 & B2, independent of the any history from A before or after it was split into A1 & A2.

    Now as an analogy the entanglements of the A’s being caused by a traditional wedding.
    We just don’t know if A1 is a husband or a wife. Likewise with B1. When we bring A1 and B1 together we evaluate only that they are the same sex or opposite sex. (The advantage of photon entanglements is this comparison can be measured as a statistical probability between 0 & 1).

    Once we know they are the same sex or not, why would anyone be surprised that distant comparisons between their spouses A2 & B2 give same sex or not results that correlate perfectly with the results seen by A1 & B1. Even when they are tested at a time and place that makes it impossible for the spouse test to have obtained any information from the original A1B1 test. And no need to imagine some bizarre deterministic history going back to the Big Bang either, I find that comfy security blanket view of reality ridiculous.

    The A1 A2 correlation could be the result of 1)a mystic QM entanglement or 2)a classic Einstein unknown hidden variable (wedding) newly established only at the time the PDC split the photon in two. And I’ll grant that experimentally the Einstein unknown hidden variable has been ruled out.

    But, once you have correlation established between A1 & A2 (by some HV or entanglement) and also between B1 & B2. I see no mystery in discovering A2 and B2 comparisons, correlating very well with comparisons made between A1 & B1.
    It can be made to look amazing, but that is just a mathematical slight of hand, worthy of a magician, and not some new mystery.

  5. Apr 12, 2007 #4


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    (Talk about the pot calling the kettle black regarding unverifiable claims...)

    The fact is that photons A2 and B2 were created by a laser and PDC crystals, and they showed entanglement. Strangely, random photons - the kind that you seem to think share a past - do not show similar entanglement. So if your explanation is supposed to be a serious one, why aren't all existing photons entangled?
    Last edited: Apr 12, 2007
  6. Apr 12, 2007 #5


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    Is it possible to articulate the specific criteria for when two particles must be modeled as entangled? Would it have something to do with situations where measuring one particle gives you some new information about the probability of getting certain results when you measure the other particle?
  7. Apr 12, 2007 #6


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    I don't think your analogy matches the situation. Try this one instead:

    A1 and A2 are an entangled pair of dice, and their values always add to 7. So 1 & 6, 2 & 5, etc. Ditto for B1 & B2. But there is no connection yet between the A's and the B's. Now we run A1 and B1 through our "indistinguishable" black box as they do in the experiment. What happens?

    Now we notice that A2 and B2 add to 7. I don't believe you have a classical explanation for this turn of events. Logically, A2 and B2 should add to any value in the range 2-12 but instead they show a correlation too strong to be a product of chance alone.
  8. Apr 12, 2007 #7


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    I can't articulate this as a general rule, hopefully someone else can.
  9. Apr 12, 2007 #8
    No A2 and B2 summing to 7 is not true unless they are tested only when A1 & B1 was found to sum to 7. Which is exactly what the experimenters are doing!

    Your analogy is the same as mine only much better. Thanks much, if you don’t mind I’ll borrow it with a few improvements. Notice how by using entangled dice as you’ve described when testing A1 & B1 you can reliably forecast results to be found at A2 & B2 as follows:

    A1-B1 will always equal B2-A2
    A1 + B1 will always equal 14-A2-B2
    Since A1 + B1 + A2 + B2 = 14

    So as you see no mystery in finding correlations between A2 & B2 when synchronized by what ever happens at A1 & B1.

    All these types of experiments are doing is extended the all ready known & established mystery of how a HV or Entanglement causes A1 & A2 to be correlated. Nothing new.

    Last edited: Apr 12, 2007
  10. Apr 12, 2007 #9
    What exactly do you have in mind?

    Entangled particle are created in the same place at the same time. This makes it easy to notice correlations between them. For random particles, we know nothing about their past so the correlations, even if they exist, are hidden.

    Anyway, regardless of the validity of my hypothesis, the statement that those photons do not share a common past is clearly false. A more reasonable claim would be that their common past, which they certainly share, is unlikely to be the cause of the observed experimental results. I wouldn't agree with such a claim either, but at least it is not an absurd one.
  11. Apr 12, 2007 #10


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    That is not what they are doing. A1 & B1 are not being tested the same way that A2 and B2 are tested. So they are not limiting to cases in which A1 & B1 are already known to add to 7 (per our analogy).

    In the actual experiment, A2 & B2 are being tested for relative phase with interferometers:

    "To test for successful entanglement swapping, the relative phase [Alpha-Beta] between the interferometers is changed by keeping [Alpha, A2] fixed and scanning [Beta, B2]. As usual for the analysis of time-bin entanglement, interference is observable in the case where, at the output of the interferometers, both photons are detected at the same time. "
  12. Apr 12, 2007 #11


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    I don't get it. First you say that entangled particles A1 & A2 are created at the same time. Then you say A2 & B2 share a common past (which I say they certainly do not). Which is it?
  13. Apr 14, 2007 #12
    First our analogy does not require that A1 and B1 add to a value of 7 to work.
    Note the results of using the dice formulas I posted in post #8

    The selected quote you used implies that correlation of A2 & B2 is independent of any test at A1 & B1. But, they are very clear in their paper that the entire experiment depends on a synchronized “BSM”! The equivalent in our dice analogy is the testing of A1 & B1.
    Notice even their diagram clearly shows they are using a four way coincident counter. Not just a two way counter between A2 & B2.
    Maintaining some known synchronization at A1 & B1 counts is essential as they explain in their paper:
    They are simply using local realistic applications with two non-local sources to spread an entanglement or “non-local” effect over a greater distance.

    So although their results do not display anything in the way of a new non-local event, I agree with their assessment that this complete system may help design a quantum repeater.
    Using our dice analogy as an example, Assume an entangled dice source can only successfully propagate matched dice 10 miles for a max separation of 20 miles. We create a repeater as follows. Putting two sources A and B at mile markers 10 and 30, we measure A2 at Mile 0 and B2 and Mile 40 for a 40 mile separation. Testing of A1 & B1 at Mile 30 must include a feedback signal to adjust the phase of one of the two sources.
    Now by using stable sources and adjusting the phase difference between them from the BSM at mile 30 we need to synchronize A1 & B1. If we are to produce very high probabilities of A1 + B1 equaling 5, 6, or 7. We can advise the quantum receivers at Miles 0 & 40 they will be receiving there signal dice will be producing results of A2 + B2 being 7, 8 or 9. And no need to check with the folks at Mile 30 as long as the BSM synchronization is maintained.

    Do you see the point?
  14. Apr 14, 2007 #13


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    Yes, they are doing 4 way coincidence counting. But A1 & B1 do not go through interferometers. The reason to check A1 & B1 is to verify that they are indistinguishable. This is necessary to cause A2 and B2 to become entangled. If one arrives early then they can be distinguished (in principle).

    Therefore, the only subset that is considered is a subset in which the A1 & B1 photons hit detectors in the BSM, but we don't know which hit which detector. So this is not equivalent to reducing to our dice subset in which A1 & B1 add to 7. If it were, then you would be correct.
  15. Apr 14, 2007 #14
    I don’t understand your point. You agree that they “check A1 & B1 is to verify that they are indistinguishable”.
    In our dice analogy that would mean A1 and B1 would both be 1 or 2 or 3 etc. at the same time; that is equal to each other.
    They don’t need to sum to 7.
    If you ensure A1 and B1 are that indistinguishable you are simply selecting only very synchronized events with local realistic testing.
    If giving the reality of entangled sources at A & B, any local realist view must under these conditions, and following the rules of in post #8, conclude that the dice A2 and B2 will be equal to each other with out need to see the values produces by A1 and B1. Just knowing that they are the same (A1=B1) is all that is needed.

    (Edit in additional example)
    Thus if everyone knows that the conditions are that source A produces entangled particles such that A1 & A2 are always identical or inverse images of each other. (In our dice case we are use inverse images or opposites sides of a die to set the value for 1 & 2)
    AND the same applies for source B.
    AND we restrict testing at A2 and B2 to only those events that are coordinated with 1) that A1 & B1 can be compared and 2) that comparison of A1 & B1 will always meet some BSM rule such as summing A1 + B1 to some number, or in this case only when A1=B1.

    Now a Local Realist, even one accepts two body entanglement results as true but believes cause is some unknown HV. He only needs “local real” explanations to explain this experiment with out any further paradox.
    If he sees a 5 at B2 he needs no addition information to immediately know:
    1) due to source entanglement; B1 is a 2
    2) due to A1 = B1 BSM rule; A1 is also 2
    3) due to entanglement of other system; A2 = 5
    4) Therefore A2 = B2 or 5.

    The relationship between A2 & B2 to dependent on the BSM rules applied.
    No new weird action at a distance beyond what already exists in the two independent entangled systems is show here.
    Last edited: Apr 14, 2007
  16. Apr 14, 2007 #15


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    "Indistinguishable" in this context means that their source cannot be determined. I.e. you do not know whether which was the A1 photon and which was the B1 photon. This is required in order to entangle A2 & B2.

    "Indistinguishable" in this context does NOT mean that they are identical. They aren't, and they are not post-selected as identical.
  17. Apr 15, 2007 #16

    Like any other two particles in the universe. In this case, every electron/quark in the source A interacted with every electron/quark in the source B through EM and gravitational fields.

    What evidence do you have to support this bold assertion?
  18. Apr 15, 2007 #17
    No they don't state they are simply constructing a condition where A1 and B1 are just "Indistinguishable" from each other. That could be done by not looking at them. They are clear in there paper their paper that A1 and B1 must actually be observed in a method that results in them being "Indistinguishable". They are further clear that their experiments only attain the characteristic they need with "synchronization" of A1 & B1.
    The only photons in the BSM are A1 & B1.

    They are allowing there desire to find something important to allow their own experiment to fool them with what is little more than a magician’s misdirection. You’ve always done so well in explaining entanglement and the OQM position, I’m a little surprised you’re not picking up on this flaw in this paper.

    As to the ueit "common past" argument; it doesn’t really apply here;
    It is part of not accepting the single source entanglement in the first place.
    It is a debate not worth entering into.

    In the single point entanglement case they claim all three separated parts of the entanglement system share a common past: The PDC and both distant measuring devices for particles 1 & 2 including whatever setting they have no matter how or when the setting is made. They share a “common past” though the interaction of every particle they have ever encountered in their respective pasts, even if you have to go all the way back to the Big Bang if needed.

    It’s an absurd argument not worth debating in the single entanglement case, so likewise in a two entanglement system problem.
  19. Apr 15, 2007 #18
    So, do you agree that the particles used in the experiment discussed in this thread share a common past or not?

    P.S. All particles in the universe interact continuously, at infinite range, through EM force. So, it is misleading to speak of "every particle they have ever encountered". All particles (at least the charged ones) "encountered" every other particle.
    Last edited: Apr 15, 2007
  20. Apr 15, 2007 #19


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    Not so bold, I simply base this on what you have previously agreed.

    If two photons were created a minute ago, how do they share a past with photons that have yet to be created? If you say all photons share a past, it is you that is making a bold assertion. There is not one iota of evidence that photons bear any element of an earlier history.
  21. Apr 16, 2007 #20


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    These authors are highly respected and well-published. I don't think they have let anything about their "desire" lead them to improper conclusions. And I might point out, these results are fully in keeping with Quantum Mechanics anyway. It is not something they are likely go to far astray on.
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