Entanglement and teleportation

  1. I read up on wikipedia.com about entanglement and teleportation but it left me with a few questions. If you go to This Link. You'll see that they give the analogy "Bob has created two atoms called I and II which are maximally entangled". Now obviously, bob can't create two atoms at will so how do two particles become entangled? Other texts in the article suggest that due to the fact that they're identicle particles in the same instant of time, they're basically one particle so what happens to one will happen to the other. But then it says the transmition of information can not go faster than the speed of light. If this is true then I would assume that the communication between the particles is transmitted through some sort of EM wave. There's a lot of confusion right now, could someone clear this up for me? I just realized that my questions might not be obvious from the text so I will list them.
    1) What defines which identicle particles in the same time period are entangled and which are not? Is there an "entanglement process" or are two identical particles that exist in the same point in time automatically entangled?
    2) Does the transfer of particle state information happen instataneously no matter what the distance?
    3) If not, is the information transmitted through some sort of EM wave?
     
  2. jcsd
  3. An entangeled pair can be created by a technique called parametric down conversion. i suggest you google for that. Besides, two entangeled particles can indeed be seen as one 'bigger' particle with twice as much energy (suppose that each particle has the same amount of energy) or twice as much shorter wavelength. This is how entangled pairs can beat the diffraction limit. i refer to my journal for more info. Just look at the faster then light communication entry and the for qubit-lovers entry

    marlon
    ps this is nice : http://marcus.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf
     
    Last edited: May 8, 2005
  4. Marlon, I read your faster than light article. Just to verify, you said that the communication between the two particles themselves is instataneous but the ability to read these particles has to be enabled through a classical communication platform? Do physicsists know how the particles communicate faster than the speed of light or is there not an explanation behind that?
     
    Last edited: May 8, 2005
  5. The faster then light aspect directly comes from the entaglement. If you have an entangled pair of two atoms (one has spin up along the x-axis and the other has spin down, for example) and you measure the spin of one atom along the x-asis, then you automatically know the spin value of the other atom because it has to be the opposite direction. First of all, communication like that is impossible because of the necessary classical phonecall that is required between the two observers. Secondly, both observers need to measure along the very same axis but who say they will do that ?

    regards
    marlon
     
  6. I must have my concept of entanglement wrong then. I thought entanglement meant that what was done to one entangled partical was automatically done to the other. But this is just a matter of "If this particle is moving this way, the other particle has to be moving that way". No communication is done between the particals themselves. right?
     
  7. Lets say; two observers, separated a light year from eachother, have come to the agreement that spin up refers to 1 and spin down to 0 (assuming they live quite long...). (They have come to this agreement trough the conventional ways of communication, in this example taking multiple light years, discerning the side effects such as signal-loss and such). When they agreed this, they also agreed to prepare two isolated photons (one here and one there) and to bring them into a 'long-lasting' state of entanglement.
    Lets assume they have the possibility (by chance or agreed before) to each measure the photon the exact same time or in a time frame allowing FTL.

    Then, Observer A puts the photon into a forced 'spin up' state, which will, due to entanglement, be instantaneously sent to the other photon, thus forcing that one at observer B into an immediate spin down state. Via this way, observer B will be able to read 0 from his photon.

    Apart from the fact that measuring the spin state requires classical communication, the whole procedure will highly exceed light speed.
     
  8. vanesch

    vanesch 6,236
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    In fact it is neither !
    If it were "what is done to one, is also done to the other", you obviously would have a faster-than-light communication channel. Some fools even thought that you could build a rocket motor that way, by having entangled atoms, one in the rocket, and one on earth, and then accelerate those on earth, so that those in the rocket would also accelerate :-)

    On the other hand, it is not just learning about an unknown parameter of the other atom either. Bell's theorem tells us that that is not the case.

    Let us separate two issues: one is discussions on the *mechanism* that is responsible for entanglement: there are many discussions about it, people have different views on what is actually going on (I have my own view which I don't stop defending over here :-). The other issue is about what is actually observed: here, most people agree (there's still a "local realist" crowd who denies all experimental results and claims it are all tricked, or badly analysed, or oversold results, but they are, by most others, seen as kind of cranky).

    I won't go into the mechanism explanations. I will just try to state what is actually predicted by quantum theory, no matter what interpretational flavor. It is about 2 observers, Alice and Bob, who receive each one of the two entangled particles (photons, atoms, whatever).
    Now, they can do only one measurement on the particle, but they have a choice of WHICH experiment they can do, which is parametrised by a variable, theta-Alice, and theta-Bob. (usually a polarisation angle).
    So Alice makes a choice of theta-Alice, and then gets a result (up or down) for the measurement at hand.
    Bob on his side makes a choice of theta-Bob, and then gets a result (up or down) for the measurement at hand.

    Alice has a certain probability of getting "up", P(a_up, theta_Alice), which is only a function of theta_Alice.
    Bob has a certain probability of getting "up", P(b_up, theta_Bob), which is only a function of theta_Bob.
    So far, so good: this is what people mean by 'there is no information transfer': Bob, with his measurement, cannot learn anything about Alice's choice of theta-Alice and vice versa.

    BUT, but...
    If Alice and Bob COME TOGETHER, AND COMPARE NOTES, then they observe something strange: there is a correlation: the probability
    P(a_up, b_up, {theta_Alice, theta_Bob} ) is such that it does not satisfy a property which is called Bell locality.
    In order to explain this in detail, you should study a bit Bell's theorem. In short, it comes down to the following point. Bell worked out what would be the requirement on the joint probability P(a_up,b_up,{theta_Alice,theta_Bob}) when we assume that the two particles share some common "hidden variables", and then have to generate the probability of "up" or "down" at Bob and Alice, INDEPENDENTLY.
    So Bell assumed that there is a common variable lambda, and that P(a_up, theta_Alice) is in fact given by P(a_up,lambda,theta_Alice), and that at Bob's the probability is given by P(b_up,lambda,theta_Bob) ; and that these probabilities are independently generated, once we know lambda.
    This means then that the joint probability is a product:
    P(a_up,b_up,lambda,{theta_Alice,theta_Bob}) = P(a_up,lambda,theta_Alice) x P(b_up,lambda,theta_Bob).
    But we don't know anything about lambda, is just has an unknown probability distribution, P(lambda), so our observed correlation is then, according to Bell:

    P(a_up,b_up,{theta_Alice,theta_Bob}) =
    Integral P(lambda) P(a_up,b_up,lambda,{theta_Alice,theta_Bob}) d lambda

    Of course, there is still a lot of freedom, because of the choice of P(lambda), and P(a_up,lambda,theta_alice) and so, but Bell succeeded nevertheless in writing down some INEGALITIES which the joint probability needs to satisfy.

    Well, it turns out that the joint probabilities for entangled particles in quantum theory DO NOT ALWAYS SATISFY those Bell inequalities.

    What does this mean, statistically ? Well, it just means that one of Bell's hypotheses are not satisfied.
    And Bell's hypotheses are that the probabilities of Alice and Bob observing "up" for their chosen angles are generated INDEPENDENTLY as a function of a COMMON SET OF (HIDDEN) VARIABLES.
    This is a very reasonable hypothesis when "statistical" things happen and when a correlation is observed. If somehow you arrange that there cannot be any DIRECT influence (because there's a big distance, a concrete wall etc.. between Alice and Bob), then if you observe a correlation, you normally assume a COMMON CAUSE (the hidden variable).
    So this is somehow not true in quantum theory: you can have correlations without having a "common cause".
    But it is also true that Bob cannot learn anything from Alice's CHOICE from his local measurement, nor can Alice learn anything from Bob's choice. So this thing doesn't allow you to send information from Alice to Bob.

    cheers,
    Patrick.
     
  9. Well your definition is correct but the communication part is just the fact that if you measure one spin, you automatically know what the other observer will have as outcome when he measures the other atom of the entangled pair

    regards
    marlon
     
  10. Their behavior wrt some detection scheme or other is
    correlated. For example, different, separated parts of
    the (same) television signal (wave) are entangled.

    "Entanglement processes" produce the entangled phenomena
    observed experimentally. The entangled phenomena have a
    common cause (including, but not necessarily requiring, that
    they've interacted prior to detection). Marlon mentioned PDC.
    There are also other experimental processes that produce
    entanglement.

    "Particle state information" is something that *we*
    generate via theory and observation. Are the separated,
    entangled physical phenomena *causing* each other
    (instantaneously or superluminally)? There's no direct
    evidence of that. But some interpretations have it that
    that's what's happening. My personal opinion is that that
    sort of *causation-at-a-distance* probably isn't what's
    happening.

    The correlations are a function of analyzing (even
    via spacelike separated events) motional properties that the
    entangled phenomena have in common due to their having
    interacted in the past, or being created at the same time
    and place (eg., a wave moving omnidirectionally away from
    its source and rotating parallel to some plane-- separate,
    individual points on the wave are entangled wrt the
    rotation). Separated objects in any *system* of objects
    moving together as a group are entangled wrt the movement
    of the system as a whole.

    Information, in the sense of something being communicated
    from one place to another, is transmitted electromagnetically.
    There might be other waves in nature moving faster than EM
    waves, but nobody has detected that yet. So, as far as
    anybody knows, the speed of electromagnetic radiation in
    a vacuum is the upper limit.

    Nothing *needs* to be being transferred instantaneously or
    superluminally to understand why the correlations of entangled
    phenomena are what they are. For example, in the case of photons
    entangled in polarization, light waves emitted (presumably by
    the same atom) during the same interval are analyzed by
    crossed linear polarizers. No nonlocal causation needs to be
    happening -- the polarizers are simply, in effect, analyzing
    the same light at the same time, and a cos^2 theta correlation
    for coincidental detection emerges (which is what would be
    expected if the same light is being analyzed by crossed linear
    polarizers).

    Now, I'm aware of analyses of this that conclude
    that the light incident on the polarizers can't have been
    made the same by the emission process, that it must happen
    at the instant the detection that initiates a coincidence
    interval occurs. But, these analyses are flawed, imho.
    One way to approach it is by considering where the qm
    projection along the plane of transmission (by the polarizer
    at the initially detecting end) comes from. There's, imo,
    a sound physical basis for it. Anyway, what results is
    a probability of 1 for the initiating detection, and
    a cos^2 theta probability at the other end for the same
    interval. So, the joint probability of detection
    (the probability of coincidental detection)
    wrt any interval is 1(cos^2 theta). And, experiments
    support this prediction.

    The assumption of the causal independence of spacelike
    separated individual results holds as long as one is
    careful to modify the probabilistic picture following
    the initiating detection. Maybe current 'pictures'
    of spin and polarization are inadequate to describe
    exactly what is happening. But, the plane of polarization,
    and the intensity, of the light transmitted by the first
    polarizer (associated with the start of the coincidence
    interval) is a subset of the emitted light incident
    on each polarizer for the common interval. This
    light produced a photon, which represents maximal
    intensity for that coincidence interval, at the first
    detector. So, it follows from standard optics that
    the probability that it will produce a photon at the
    second detector (via analyzing the light from the
    same emission, or set of emissions) is cos^2 theta,
    where theta is the angular difference between the
    settings of the two polarizers.
     
    Last edited: May 8, 2005
  11. This pretty much sums up my conception of the process. There just can't be any impossible or mysterious factors involved. We just haven't identified all the properties and restrictions on their motion. Unless I miss the point, communication at a distance is merely speculation, right?
    :shy:
     
  12. Well, there can't be any *impossible* factors involved. :)

    But, there are mysterious factors involved, and they have,
    imo, as much (maybe more in the case of entanglement) to do
    with the way competing formulations are analysed as with
    the entangled phenomena themselves.

    The (speculative) inference of instantaneous or
    superluminal *causal* relationships between the separated
    phenomena is allowed, logically, given certain assumptions
    (or, more strictly, the experimental negation of certain
    *interpretations* of certain assumptions via the formulation
    of probability statements regarding joint detection, and
    the restriction of alternatives).

    I've outlined the reasons why I don't think that experimental
    violations of Bell inequalities are telling us what some people
    seem to think they're telling us. Was Bell wrong? No, he
    said his formulation regarding probability of joint detection is
    incompatible with qm. It is. It's also incompatible with
    experimental results, which support the qm formulation.
    The problem is that the usual lhv formulation, following Bell,
    doesn't take into account that the probabilities for individual
    detection have changed once a detection is registered and
    a coincidence interval is initiated. If you give the qm
    projection operator the correct, imo, physical interpretation
    in these experiments, then the qm formulation can be
    seen as a sort of lhv theory itself.

    It seems like a good bet that all the properties of light, electricity,
    etc. haven't been identified yet -- at least not precisely enough
    to give a clear picture of the physical details of what's happening
    in the entanglement experiments.
     
  13. Oh, I see. So it's more saying that because the photons were produced at the same exact time, any reading of the particles will be probably the same depending on the point in time the photon was "read". (seeing the same light at the same time). I was thinking like the idiots that were going to use it for rocket propulsion. Haha. Thank you for clearing that up for me. I thought that the actions of Alice would produce an effect to Bob's photon.
     
  14. But entanglement isn't one of those causual relationships, right?
     
    Last edited: May 8, 2005
  15. I don't think so, but some pretty smart people do.

    The problems arise because of the way some lhv formulas
    are done. If you describe joint detection in terms of
    the product of the *initial* (prior to detection) individual
    probabilities, then you get some predictions that don't agree
    with qm (or experiment). But, the probability of individual
    detection changes upon a detection being registered at one
    end or the other. When that's taken into account, then
    the idea that the filters are analyzing a common property
    (or properties) imparted at emission is ok.
     
  16. More like, because the photons were produced at the same exact
    time *and place* (like from the same atomic 'burp'), subsequent
    analysis of the photons by the same sort of device will produce
    results that are correlated.

    There's a lot of great stuff written about this sort of thing.
    If you're really interested, then you should read all of Bell's
    work on this (and check out all of the citations, including the
    EPR paper, the Aspect papers, etc). That should set you back
    at least a few months, but it will give you a much better
    understanding of the difficulties involved -- and the
    considerations that led to the belief by some that there
    are superluminal 'influences' (or whatever you want to
    call the nonlocal stuff) in nature.
     
  17. Ok. I'll check out those articles. Thank you for helpin me out even through my confusion. I must go now.
     
  18. vanesch

    vanesch 6,236
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    Science Advisor
    Gold Member

    I read, and re-read this several times, and I can't make up what you mean. I am reasonably well acquainted (or so I think) with Bell's reasoning.
    What do you mean by "the probabilities for individual detection have changed once a detection is registered" ??

    cheers,
    Patrick.

    EDIT: btw, this has probably already been cited, but I just found a very very thorough reference on all things Bell:

    http://plato.stanford.edu/entries/bell-theorem/
     
    Last edited: May 9, 2005
  19. Prior to detection the probability of individual detection at each
    end is .5. A detection at one end or the other starts the
    coincidence circuitry. A 'coincidence interval' is electronically
    defined and, for this interval, the probability of detection at
    the detecting end is no longer .5. It's 1. The probability of
    detection at the other end for this interval is no longer .5, but
    cos^2 theta (where theta is the angular difference of the
    polarizer settings). So, the probability of joint detection is
    1(cos^2 theta).

    The transmission axis of the polarizer at the initially detecting
    end is taken or projected as the global emission parameter,
    because:
    (1) the intensity of the detected light is a subset of the
    emitted light.
    (2) the transmission axis of the polarizer at the initially
    detecting end represents the or 'a' plane of maximal
    transmission by the polarizer(s) wrt the light emitted
    during the interval (a photon *was* produced out of
    light that was filtered from the emitted light).
    (3) PMT response is directly proportional to the intensity
    of the light transmitted by the polarizer.
    (4) the intensities of the light between the polarizers and
    their respective PMT's are related by cos^2 theta, which
    therefore represents the probability of joint detection
    for any coincidence interval.
     
  20. DrChinese

    DrChinese 5,658
    Science Advisor
    Gold Member

    As far as anyone can tell, that is EXACTLY what happens. Of course it is just as likely that it is Bob's actions that affect Alice's results. These scenarios end up being indistinguishable, which is of course a bit puzzling.
     
  21. DrChinese

    DrChinese 5,658
    Science Advisor
    Gold Member

    That is certainly an unconventional description of the situation. Since the results change upon the "first" detection, as you also point out in other posts, and that "causes" the results at the other detector to immediately change, you are saying that the results ARE dependent on space-like separated observer settings. That is the opposite of a LHV interpretation.
     
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