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Quantum Telecommunication for dummies (me)

  1. Aug 12, 2012 #1
    In another thread in another forum I started side tracking the thread. I'm trying to understand why communication is not possible so I am reposting as suggested some of the info over here in a more appropriate forum to continue the discussion.

    I'll start with my favorite post that helped the dummy (me) get a starters grip on entanglement.

    which was followed by me:

    @DennisN - That was awesome! Thanks. Let me see if I have this straight then. So for them to know, they measure say 100 photons and get 70% head 30% tails. Then go measure the other sample from lab B and it should be something statistically close to 30% heads and 70% tails. Is that correct? I now see how this cannot be used for communication.

    The last part I'm not understanding is wouldn't you have to measure both samples at exactly the same time to know they are entangled? I mean when I go back to Lab B to make the measurement, who's to say that the results don't come up exactly as with Lab A since the coins are rotating and would only represent the mirror at the exact moment the measurement is made...

    Followed by

    and a similar response by DennisN

    next is my follow up question...
  2. jcsd
  3. Aug 12, 2012 #2
    Ok so I'm lost on time-dependency. I'm not understanding how you can make an observation in Lab A on a group of photons and be able to make an observation 10 minutes later in Lab B to get a result that would substantiate a claim that entanglement has occurred. ???

    I would think after making the observation the photons are now spinning again so you'd get a random sample in Lab B.... Or do photons spin or whatever they do at exactly the same rate? So the ratio would always be the same no matter when you took the sample? Even that wouldn't make sense because if they spun at the same speed, the ratio of a particular sample would always be identical even without entanglement on one set of photons..... I'm lost
  4. Aug 13, 2012 #3


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    Hi sday, great you moved it here! I just want to stress again that my coin analogy was only meant to give you a basic taste of it. Entanglement is quite strange, and there's really no good comparison in the "normal" macroscopic world.
    Don't assume you are a dummy :cool:. I think you can get to understand entanglement (but don't expect to understand it completely - I think it's safe to say nobody really does. :smile:)
    Photons are not spinning (that's one of the problems with my analogy, sorry :redface:; the spinning (rotation) was meant to represent the indefinite state of polarization). In reality, it's the polarization of the photons that is central. Before the measurement, the polarizations are indeterminate. After the measurements, the polarizations are determinate, and the photons are subsequently "destroyed" in the photon detectors.

    An entanglement testing scheme goes something like this:
    1. Entangled photon pairs (A & B) are created with a laser and a crystal.
    2. Photon A goes through one polarizer and then to a photon detector which counts the photons (Alice).
    3. Photon B goes through a different (but similar) polarizer and then to a photon detector which counts the photons (Bob).
    Steps 1,2,3 are repeated for many number of pairs, and with different polarizer angles. The result is then plotted in a graph (y-axis=correlation, x-axis=angle between polarizers). This graph will show a correlation that is stronger than what one would expect with a so called Local Hidden Variable theory.

    (I started writing a longer and better reply, but I found out that I was too tired; my thinking/writing got bad :bugeye:, so I have to wait until tomorrow)

    I'm also hoping Dr Chinese et.al. will discover this thread; I know there are others here who are better at explaining it than me.
  5. Aug 13, 2012 #4
    Thanks DennisN. Although incomplete, I thought the coin example helped me make one step.

    1. So once they are created, their polarization is unknown until measured. Once measured the photon disappears. I assume you mean only the one measured disappears, and the mirrored version is stil viable until it is measured which statistically will have a complete mirror copy of the polarization state.... or will always have a mirror image if it was entangled, and the entanglement doesn't always occur. Once a measurement is made on group B, that photon is now also destroyed.

    2. On a side note, how does a photon just sit there? I always thought a photon was light travelling at C, not some object that could be brought to a stop for your examination.

  6. Aug 14, 2012 #5


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    Hi again, sday! I will try to answer your questions, and also provide some good links IMO.
    As I said, time has AFAIK no influence on the entanglement, what matters is if any of the two photons A or B gets disturbed by the environment before any of them are measured; in that case, the entanglement can be broken.

    It's easy to delay one of the measurements: simply let one of the photons travel a longer distance before it's measured; e.g. put Bob's polarizer and detector at a greater distance from the laser & crystal than Alice's (or vice versa). This won't break the entanglement.

    (10 minutes is a very long time for photons, though, but there are other ways of entanglement; the (quantum) spin (NOT rotation!) of two electrons can also be entangled, but I think it's a good idea to focus on photons and polarization instead.)
    (As we talked about, photons are not spinning, the spinning was representing the indeterminate state of polarization)

    I think a good way to formulate it would be that e.g. photon A is indeterminate before it passes Alice's polarizer and determinate after the polarizer (thus, in the coin analogy, the rotation of the coin definitely stops, but as mfb said, we really can't see the coin rotation stopping, what happens it's actually a matter of interpretation. The "stopping of the coin" is representing what is called a wave function collapse in quantum mechanics).
    Yes, the photons polarizations are unknown, but entangled (they share the same state).
    Yes, when the photon hit the photon detector, it is absorbed (and counted). The photon detector is placed after the polarizer.
    Yes, exactly.
    Yes, ideally. The polarizers and detectors in the experiment are positioned to catch as many entangled pairs as possible (actually there are two types of polarization entanglement, one in which the photons share identical polarization, and another in which the photons share orthogonal polarization).
    You are correct. As I said above, the photons are absorbed and "destroyed" by the photon detectors, so you could say the detectors stop the photons.

    One of the interesting things with entanglement is what actually happens when/after Alice measures her photon and before Bob measures his (assuming Alice measures before Bob). It seems that when photon A gets determinate, so does photon B. But that seems contrary to common sense, and further, there is no known mechanism for this :confused::smile:. What really happens is the subject of many debates, and it sooner or later seems to boil down to different interpretations.
    (personally, I'm pretty agnostic/minimal concerning interpretations)

    Here are some links concerning entanglement which might be enlightening/interesting;

    (I hope I got my description right, and that other PF members correct any possible mistake of mine)
    Last edited: Aug 15, 2012
  7. Aug 15, 2012 #6


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    A lot of great points you have explained to sday, quite nicely I might add!

    I wanted to add a comment about the time difference between the Alice and Bob observations for sday:

    - The ordering does not change the results in any observable fashion. Alice first, Bob first, same time - no difference.
    - We have no idea what actually happens with entanglement as to the physical mechanism (or collapse of the wave function of any particle being observed. So we have no way to really probe if there is some underlying difference due to one being measured first. All we know is that the math works out the same regardless of ordering, and that is verified experimentally.
    - Looking at it another way: there is no test to perform on a single photon which will tell you definitively whether it is entangled with another or not. That can only be accomplished by checking the other photons too.
    - The photons travel at c, so holding them around for a while is usually not practical. However, it is technically possible to swap the entanglement to a place it can be stored. It can later be retrieved. Again, not practical.
  8. Aug 15, 2012 #7
    yes, BRAVO.....

    The same entangelment is obvserved when the observations are made simultaneously.

    Even when a pair pf photons are totally unpolarized and they may be arbitrarily far away from each other, if their preparation is an entangled state, the single-photon polarizations are 100% correlated.

    From Dr Chinese in another discussion:

    Here are a couple of great papers that show these concepts:

    1. Violation of Bell's inequality under strict Einstein locality conditions
    Gregor Weihs, Thomas Jennewein, Christoph Simon, Harald Weinfurter, Anton Zeilinger

    This demonstrates what is often called Quantum Non-locality by executing a Bell test with observer separation of 1+ km. The correlations are non-local (if you assume a causal setup). The setting of one observer appears to influence the results for the other, with propagation speed over 10^4 c. This influence does not, however, include the ability to send a signal.

    2. Experimental Nonlocality Proof of Quantum Teleportation and Entanglement Swapping
    Thomas Jennewein, Gregor Weihs, Jian-Wei Pan, Anton Zeilinger

    Quantum teleportation is used to demonstrate that effects can precede their causes, leading to an acausal interpretation of the setup. In this case, the decision to entangle 2 particles is made AFTER the particles have been detected and observed to have Bell type correlations. Thus, a future influence changes the past. This influence does not, as before, include the ability to send any type of signal.
    Last edited: Aug 15, 2012
  9. Aug 15, 2012 #8
    I think the posts above may underplay that light can actually be slowed...it can:


    In the last few years, I know I saw a research abstract ...maybe Harvard or IBM research where light was slowed incredibly....and 'stored'.....in connection with quantum computing and likely a reference in a discussion in these forums. Maybe somebody can post a reference??

    Slow light is also a big deal in optical communications [fiber optics] where silicon chip fabrication techniques can be used for switches that combine electron and photon circuitry greatly reducing optical switch costs.
  10. Aug 15, 2012 #9
    Thanks DennisN for breaking apart each of my questions. It helps me significantly in my pursuit of knowledge. Many times answers gloss over the details of the questions which doesn't help the "seeker of knowledge" :)

    In fairness to all those helping me understand this, I'm going to spend a day or so going over those links to help me better understand. One parting question before I "study up".

    When the photon is split into pairs, are they just rotating, or polarizing at the exact same speed without anything really interacting which is why they "appear" to be a mirror or entangled? If I flicked two coins at the exact same speed at the exact same moment in space... they could be said to be "entangled", because without any other influence, their exact match on speed would guarantee knowledge of the "spin"/polarization of the other coin. ????

    thanks again, this will be great reading material on our little family vacation as the rest are busy counting sheep. :)

  11. Aug 15, 2012 #10
    If we want to observe entanglement between the photons, we need to make sure we can distinguish the particular pares that are entangled to each other. In an experimental setting where the photons are created by parametric down-conversion and they are measured at about the same distance from the source, and due to the fact that there are many photons hitting both detectors, the pairs are identified by what is called coincidence-counting, which means that only those pairs of photons that hit both detectors at about the same time are considered. So the time of arrival does matter, but for a different reason than that originally suggested.
  12. Aug 15, 2012 #11
    When we say that two particles are entangled, we actually mean that one or more of their properties are entangled. What enforces this entanglement in the case of two particles being created together is usually some conservation law. For example it is common to create a pair of photons whose total sidewise (linear) momentum is zero. In that case, once we measure the momentum of one of the photons, we know that the other one will have a momentum of the same magnitude but opposite direction. If it is the spin that is entangled, the most common situation is that the sum of both spins needs to be zero. So if one spin is up the other one is down (measured in the same direction on both sides).
  13. Aug 16, 2012 #12
  14. Aug 16, 2012 #13
    Just read that last night. I'm still digesting what I read. I feel like I'm trying to figure out how Houdini pulled the Rabbit out. ...how to peel the magic away so the concept makes sense to me. :)
  15. Aug 17, 2012 #14
    That is logical.

    That may not be logical unless you use 'quantum logic'.

    Niels Bohr said:
    "If quantum mechanics hasn't profoundly shocked you, you haven't understood it yet."

    Physicists deal with evidence. And that quantum [experimental, observational] evidence has some decidely 'non classical' components. It doesn't always 'make sense' but must necessarily be self consistent and consistent with observations.
  16. Aug 20, 2012 #15


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    Thank you very much, Dr Chinese and Naty! :smile:
    That is a very good question, and it is probably a more difficult question than you might think. AFAIK, I believe there is no certain good answer to it (today), but I will try to describe the issue in more detail;

    The two rotating coins analogy represents the photons' indeterminate (but entangled) state of polarization. In reality I don't think we can really say what's exactly happening to the polarizations of the photons (see note 1 below) from the time they are created in the crystal to the time they pass their polarizers and subsequently are detected in the detectors. But the models (physics/math) describe that the photons' polarizations are in an entangled state before any of them (A or B) passes its polarizer, and when one of the photons polarization is determinate, so is the other (and they are now disentangled). Experiments performed match what the models predict.

    Note 1:
    If Alice only checks her own measurements, i.e. the stream of photons A, then the photons seem to be unpolarized, i.e. the polarizations seem to be random (and the same goes for Bob, of course). Technically speaking, 50% of photons A pass polarizer A (according to Malus' law), regardless of at which angle polarizer A is aligned. (a note on terminology: I'm actually not sure whether an individual photon from stream A can/should be called unpolarized or not before it passes the polarizer; I would be very happy if someone else shines some light upon how one should refer to the photon... :smile:). Further, AFAIK, experiments are usually set up with linear polarizers (but it seems circular and elliptical polarization states can also be achieved).

    (again I hope others fill in and/or correct regarding what I wrote above)
    Sday, it is quite natural that it takes time to connect the dots concerning entanglement. It took me quite a fair amount of time to start to get a grip of it (to be honest, I'd say it's one of the most intellectually challenging topics I've ever encountered). I've read many articles/papers/threads about it on this forum, and this has helped me considerably. Personally there are still things about entanglement I have not quite sorted out yet, and I'll probably start my own thread about them later (when my thoughts get sufficiently coherent :smile:).

    Some extra notes: you'll probably encounter these abbreviations if you haven't already:
    EPR = Einstein, Podolsky, Rosen; an early critique of quantum mechanics.
    SPDC = Spontaneous parametric down-conversion; a method for creating entangled photons.

    I also forgot mentioning this page before;
    QuantumLab (University Erlangen-Nuremberg)
    On this page there is an online interactive entanglement experiment simulator (Flash).
    Last edited: Aug 20, 2012
  17. Aug 21, 2012 #16


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    I just want to correct/clarify something I wrote in post 3, since it was badly formulated and could easily be misunderstood. I wrote
    "angle between polarizers" should be "angle difference of the two polarizer settings" (it's weird that something which is quite easy to visualize sometimes can be hard to formulate with words :smile:).
  18. Aug 21, 2012 #17
    Thanks for the follow up information. I've read through most of the articles posted.

    One thing I'm fuzzy on is what is exactly happening with the polarization. I understand that it is the magnetic orientation of sample. The detector detects 120 degree increments in angle of polarization. Once a measurement is made, the sample (not photon) becomes oriented parallel to the detector right? So when a sample passes through a detector as in the Bell experiment, the sample may be oriented at say 90 degrees and in the experiment would get counted by the detector that reads the 120 degree polarized samples... after it passes through the detector it would be oriented at 120 degrees.... does this mean that the other entangled particle also changes orientation after the one is detected... (Seems like you could test for that with detectors on one side oriented slightly different than the other)

    Also, do you have any idea how a particles magnetic orientation is detected? How is it detected? I'm trying to fill in as many gaps as possible. Reading the Bell proof is difficult to follow until I know for details... since that is where the devil is. :) I understand the 5/9 concept, but need to understand how the particles are getting grouped before I can buy the locality premise.

    ...falling back on my coin analogy, as Dennis said, they are entangled before they go through the emitter... I think the coin analogy can still handle that with just one coin. Lets say that the Heads side is entangled with the Tails side. The emitter splits the coin in half and is sent to each detector. Lets say a laser cuts the rotating coin in half so you have a tails coin and a heads coin. Also, during the cut the two halfs rotation is modified equally and at a random amount, and you cannot "see" their state until they come into contact with the detector. You could still say they are still entangled with an unknown "polarization". If you know the orientation of one, you know the orientation of the other, because their rotational direction and speed was identical... which by itself doesn't mean entangled, just that you can infer the properties of one by knowing their deltas are identical.

    When I read about the detector, it sounds like they aren't able to measure "exact" states or properties which would seem to me that such generalizations in accuracy make it hard to know if you are measuring entanglement, or something like my coin analogy. I'm obviously missing something, because people much smarter than me have this figured out... I'm just not getting it.
  19. Aug 22, 2012 #18
    sday: If you read [edit: you need to STUDY them] DennisN's posts and the Wikpedia article I posted, you can answer your own questions to the extent answers are understood.

    Dennis already gave as good an answer as exists:

    So there is no '90 degrees' before measurement!! [yes, it's weird!!]

    [Note that 'spin' is only one form of entanglement.]

    the 'change' is from indeterminate to determinate [to 120 degrees].

    Dennis posted the same:
    Wikipedia sez:
    You may be heading down a dead end here....the method of detection can be anything you want; the fact is we do not understand the precise mechanisms at work. The predictions of quantum mechanics ARE counterintuitive...even Einstein was baffled:


    again Wikipedia:
    There is no precise answer:

  20. Aug 22, 2012 #19
    @Naty1 - thanks for the breakdown. I am studying the various papers. Some of my questions came from reading this one http://www4.ncsu.edu/unity/lockers/users/f/felder/public/kenny/papers/bell.html

    So if the state is indeterminate before measurement.... is that because we have no way to measure it without influencing it... so it is determined, we just have no way of knowing what it is until measured. Like the coin analogy. If you are not looking at the coin, the current spin is in an indeterminate state until you look at it. (although it does have a state while not looking at it, we just don't know what it is) (I recognize spin is only one attribute, but I'm picking on it to maintain some semblance of focus)

    By measuring one particle, are we saying that the other also went from an indeterminate state to a determinate state? And is it only a determinate state once we measure the second particle?

    ...I'm the caveman asking if the strange formation of rocks laid in a circle started the fire... uggg ...and sorry

    One of my questions as you pointed out is asking essentially what is asked in the last wiki quote... and there isn't enough known to answer the questions in between. The subject is so obscure that I fear my lack of base physics knowledge is simply too great for me to make much progress. It sure is intriguing though.

    thanks again for trying to expand my understanding. sincerely
  21. Aug 23, 2012 #20


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    :biggrin: No problems. As I said before, I think entanglement is one of the toughest topics there are. And it is quite provocative to our common sense.
    Perhaps, but I think you have made some progress. You understood why it can't be used for instant communication. And judging from your questions and reasoning it seems to me that you have been thinking quite hard on the subject.

    I will reply to your posts #17 and #19 in more detail later when I have time; I must make time to think it through properly first.
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