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On entanglement

  1. Oct 6, 2006 #1
    Ok, I have a handful of questions:

    1. Particles A and B are entangled, measuring A colapses the wave function of B, right?

    2. The double-slit experiment produces different outcome based on whether or not a particle has been measured, right?
  2. jcsd
  3. Oct 6, 2006 #2
    I am a student of these things also, but if welcome here would like to explore them along with you.

    1. that is my reading also, altho I am uncertain of the meaning of the collapse of the wavefunction. Yes, yes, I am aware that the measurement causes the eigenvalues to take on a single measured value. In this entanglement, I should like to know, physically, what is going on, as I expect should many other people. Shall we assume that the entangled particles inhabit linked timespace, or is the connection more intimate? Is it even possible that the entanglement is some kind of identity, and our notion of separation is an illusion?

    2.That is also my understanding. I have read that Feynmann irritated his lecturers by presuming that there could be three slits, or four, or an infinite number. In this logic he formed his famous path integral formulation, and the basis of QED. IIRC I read this in a paper of John Baez. I could find the link if anyone is interested.

    Last edited: Oct 6, 2006
  4. Oct 6, 2006 #3
    Your have a few ideas just tossed together there. Look up the detail on a few of the words I capitalize here. Use a thread search here where you can review the debate (and there is an ongoing debate) on these matters. Or use Google, library, WikiP etc.
    You need to understand what these things are and where they apply, before you understand the debates, so take your time in looking them up.
    By the way welcome to the forum, - you will find it works best for you if you take the time to understand an issue or term as clearly as you can, so you can ask an informed more specific question.

    ENTANGLEMENT is part of both A & B together the WAVE FUNCTION COLLAPSE doesn’t just go in one because of the other, it is that they are somehow part of each other.
    That part is what is being affected.
    How can two different things have a common part??
    Try SUPERPOSTION – the idea that each A & B are made up of a SUPERPOSTION and a part of that remains common to both of them as long as they are ENTANGLED.
    That shared part of SUPERPOSTION is what will collapse if either A or B is “touched” in a way that could result in a measurement.

    Has nothing to do with entanglement because it is just one photon at a time going by that still manages to create the pattern. Here again is SUPERPOSTION but not entangled just somehow working within the one particle to act as if it were a wave approaching two slits. BUT IF you disturb it near just one of the two slits, even if doesn’t go though the slit where you place the disturbance (measurement) it will not produce the pattern of a wave. How? SUPERPOSITION! Or some other interpertation (that how you get debates!).

    Are these “Correct” descriptions of the reality of physics??
    Can’t say but you can form an opinion as you learn the details of those terms.
    Last edited: Oct 6, 2006
  5. Oct 6, 2006 #4


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    Welcome to PhyscisForums, RProgrammer!

    1. Yes, this is correct.

    2. The different outcomes are triggered by whether you know - or could know - which of the 2 slits the particle travelled through.

    Both of these experiments exhibit properties of the Heisenberg Uncertainty Principle.
  6. Oct 7, 2006 #5
    Two more questions

    Thanks for your responses, and RandallB: I worded my first question poorly, I know entanglement can apply to many more aspects than location or physical properties.
    And I know my second question didn't have anything to do with entanglement, I just didn't want to post two separate threads.
    I said I had a handful, I was just waiting for some answers to the first two.

    These two are about electron spin.
    C. Can someone entangle two electrons by their spins, (Google didn't yield very promising results so if someone could point me a link..)

    D. If no one measured the results of a Stern-Gerlach experiment, would the input electrons be in a superposition of both deflected paths?

    NB: I also know that other particles besides electrons have quantized spins, I just picked electrons because of their popularity.
  7. Oct 8, 2006 #6


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    I don't think they know how to do this technically yet. When someone discovers how, you will see a ton of expermints based on it, and he current photon experiments will become less key for showing quantum reality. What you need is something functionally equivalent to "Parametric Down-Conversion" but for electrons/spin rather than photons/phase.

    How would you ever know? It's long been observed that automatic recording of data works to make quantum things happen. Suppose no-one looked at the photpaper target of the double slit experiment till the next day. The spots or interference patterns would still be there. Or look at the thread https://www.physicsforums.com/showthread.php?t=135098" on the attempt by the desigers of the delayed choice quantum eraser to remove the decision making from human control. No reason to think it wouldn't work the same way for Stern-Gerlach.

    They are I believe the lightest particles that are
    - Fermions, so quantum spin can be observed
    - Charged, so that they can be manipulated with electromagnetism (the only force we really control)
    - Observable in isolation (isolated quarks, for example, aren't)
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  8. Oct 9, 2006 #7
    You may want to double check me on this but Stern-Gerlach experiments on electron spin were the fist Bell tests. PDC’s just make Bell tests much easier to do, so much so you don’t see much of the Stern-Gerlach spin tests.

    Oddly enough one of the best short layman’s explanations of this can be found in “The Dancing Wu Li Masters”.
    But be warned, this book is full of philosophical non-science interpretations (entanglement comes under Enlightenment – The End of Science) and does little to separate the two. So it’s a bit of a chore to be sure you just pick out just the science in the book.

    Also, RProgrammer, when you do learn how to separate Science from the philosophical you will see your question “D” doesn’t belong here but in one of the Philosophy Forums.
  9. Oct 9, 2006 #8
    By my understanding, quantum entanglement requires two events that may be spatially separate, but connected in a way that allows them to be described as one event.

    I would submit that the twin slit experiment does provide evidence of entanglement.

    Consider: a particle has been fired and the resulting interference pattern surrounds both slits. The moment you take a reading at one slit, the interference pattern will vanish at both simultaneously - regardless of the distance between them.

    Now although we are only talking about one particle here, the collapse of it's wave function in respect to it's possible location does seem to make this a demonstration of both the uncertaintly principle and quantum entanglement.

    If this is the case, then I beleive the twin-slit experiment may offer possiblities for faster than light communication via entanglment - more so than the example of measuring two particles with different quantised spins.

  10. Oct 9, 2006 #9


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    A couple of points:

    1. By definition, entanglement is where multiple particles are sharing a single wave function. Entanglement has been demonstrated with 2, 3, 4 and I think even more particles. But a single particle cannot be entangled, even though it is in a mixed state.

    2. There is no known mechanism for transmitting information FTL, either with a double slit setup or by entangled particles. All the schemes you can develop have some catch that ultimately leaves you with nothing. But trying never hurts... :smile:
  11. Oct 9, 2006 #10
    I think you need to consider more carefully exactly what you’ve said here and the impossible things you have assumed within the statement.

    1 - How does the interference pattern get established by just one particle hitting your observation screen just one time – it cannot.

    2 - A interference pattern of many hits does not “vanish” at a “moment” you start taking readings at one slit – a future pattern fails to build as long as you are disturbing the area of one of the slits enough to be able to take a reading, but you do not have to actually take the readings.

    The idea that resolves the paradox of how an individual particle can participate in building up a pattern without knowledge of or help from other particles is called SUPERPOSITION.

    No other experiment outside of Bell implies Entanglement.
    But, it is not hard to argue that Entanglement and Superposition must somehow be related, but they are not the same thing.
  12. Oct 9, 2006 #11
    Last question.. (Hopefully)

    One (hopefully) final question:

    E. In the double slit experiment, would an interference pattern develop if a third party knew, or could know which slit the particles went through?
  13. Oct 9, 2006 #12


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    If the wave function was reduced, whether in the retina of a human being or in some abiotic interaction, then the ambiguity would be resolved and the interference wouldn't happen. This covers all machines, transmissions, third parties, presumably teleportation of photons etc. Although I'll bet experimenters would love to try that last one, just to show they could, and rule out the possibility that there was non-QM physics there.
  14. Oct 9, 2006 #13
    Dr Chinese and RandalB:

    Yes I'm aware that the cited examples of entanglement involve two or more particles quantised in spin-up and spin-down positions.

    Nevertheless, Wikipedia defines entanglement as a phenomenon
    If this description is accurate, it may open the door to interpreting twin slit superposition as an example of such entanglement. I completely recognise that this is debateable and the two are not usually classed together.

    What I've been arguing is that, through the occurence of an interference pattern, the two slits now qualify as spacially separate objects whose quantum states have to be described with reference to each other.

    RandallB wrote:

    I may be misunderstanding you, but it sounds as though you are rejecting the very quantum dillemma that lies at the heart of twin slit superposition.

    We're agreed that the twin slit experiment is performed by projecting a beam of particles, usually photons. However, the interference pattern is the result of the superposition of each individual particle and its potential journey through one slit or the other - not by one particle interfering with another. Indeed, the projecting beam can be dimmed so the photons enter the slits one at a time. The interference pattern still occurs.

    Agreed, the wave function doesn't have to be collapsed by a measurement. Just blocking off one slit will do it. But as I understand it, a photon detector that isn't powered can be placed close to one of the slits and still not eliminate the interference. Once activated, however, the pattern dissappears - regardless of whether it detected a particle there or not. It's potential to detect a particle was enough to collapse the wave function and force the particle to 'decide' exactly which slit it entered - even if it went through the other one. Thus, a photon detector at Slit A, by not getting a reading, establishes by default that the photon entered Slit B. This eliminates the interference pattern.

    RProgrammer: this probably answers your question too. Knowledge and detection seem inseparable. If an observer knows that a particle entered one slit, it can only be because it was detected.

    What might be interesting is to imagine a microbe sized observer by one of the slits, who does have such knowledge. From his perspective, there is no interference pattern. From the human observer's viewpoint there is. This ties in with a slightly Einsteinian idea that wave functions and their collapse may be observer dependent.

    As regards faster-than-light transmissions, I think the jury is still out on that one.
    Last edited: Oct 9, 2006
  15. Oct 9, 2006 #14


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    Then could you please write down the entangled wavefunction of the double-slit experiment?

  16. Oct 9, 2006 #15


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    The analysis of the double-slit experiment does not even consider the state of the slits, you only consider the state of the photon, the slits represent restrictions on the allowable paths for the photon.
    Interference is not observer-relative in this way; if at any point the photon interacts with another system (a detector, a microbe, whatever) such that an examination of that system could in principle tell you what path the photon took, then all observers will see the interference destroyed. For example, in the version of the double-slit experiment which uses electrons instead of photons, the experiment must be done in a vacuum or else the electrons' interactions with the air destroy the interfarence, in spite of the fact that in practice no human experimenter could deduce the path the electron took by examining all the air molecules immediately after the electron passed through them.
    I believe it's actually been proven that according to the known laws of QM, entanglement can never be used for FTL transmissions.
  17. Oct 9, 2006 #16
    Your still dealing with some kind of philosophy answer, like maybe the moon isn’t there if you don’t take the time to look at it. Measurement is not the point, it is whatever is used to effect that measurement that matters. A rain shower or a beam of light in front of just one slit, that will cast a trail of water or detectable shadow on the floor but still allow the particle to go through the slit to mark a spot on the wall. It still doesn’t matter if you, a 3rd, 100 or zero that look at the shadow on the floor to take a measurement. The fact the something was crossing the opening that can be affected by the particele as it pass is all it takes to stop the pattern from building up. Not that any actual measurement of any effect is made let alone known by anybody.

    As to Simon 6 thinking that slits need to be entangled (???), not that a particle that has Superposition, is so far off the mark – all I can suggest is go back and keep reading.

    And both of you need to remember, you are trying to understand an interpretation of what is happening here – superposition & entanglement have not been proven to be facts.
  18. Oct 9, 2006 #17
    My point

    So, what all this has been leading up to is this:

    I have heard numerous places that Faster Than Lishgt information transfer cannot be achieved.
    So, I would like to further my understandings of quantum physics from someone explaining to me what prevents this scenario from succeeding:

    Given that you can entangle two electrons by their spins, you do.

    The two Electrons (Signal and Recipient) are sent their separate ways, to the transmitter (signal) and to the reciever (recipient).

    On the transmitter's side, he either measures or does not measure the spin of the signal, based on the value of the binary bit he wishes to transfer.

    On the recievers side, there is an apparatus similar to the Stern-Gerlach experiment, which changes the course of an electron based on its spin.
    There is also a double slit setup, which the previous device is connected to.
    They are set up in such a way that based on the spin of the electron, it will travel into a specific slit.

    My hypothesis is that if the transmitter does not measure the spin (and thus know the recipient's spin) then an interference pattern will develop from the electron being in a superposition of both deflections.
    And corrsepondingly the superposition would collapse if the transmitter measured his signal electron and thus cause a particle pattern, which would be observable by the reciever.

    So, that being said, I would like to know why and how this setup cannot effect Faster Than Light transfer.
  19. Oct 9, 2006 #18


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    Very clever, I think you've basically reinvented the delayed choice quantum eraser, unless you'd already heard of this sort of experiment before. The answer is somewhat subtle--basically, you'll never see any interference in the total pattern of electrons that go through the double-slit, but if you do a coincidence count between the subset of electrons on the screen whose entangled double was measured in such a way that it becomes impossible to tell which slit the first electron went through, you will see an interference pattern in this subset. But since you don't know in advance which electrons' doubles will be measured in this way and which won't (you could prearrange it, but that would defeat the purpose of FTL communication), you can't see this interference pattern in the coincidence count until the experimenter at the other end has sent you an ordinary message at light speed or slower telling you which electrons he measured in which way.

    I remember some old threads from way back which had longer discussions of the same sort of idea:

    Last edited: Oct 9, 2006
  20. Oct 9, 2006 #19


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    As JesseM points out, this gets a little tricky. But I had once proposed the exact same setup (using photons) previously, only to realize that you don't get interference from entangled particles! You can see this in this enlightening article by Anton Zeilinger, p. 290, Figure 2.

    Experiment and the foundations of quantum physics
  21. Oct 9, 2006 #20
    So you’ve been baiting us all along – just to give us THIS!! (Lame)

    Come on RP you can’t just ignore everything you were suppose to be learning from your “Lead up” questions just so you can reveal the great inspiration you’ve been holding back since the OP anyway. When what you should have learned was more than enough to tell you that this is just non-sense.

    Take a good look at the links JesseM offered and you will see yours will not come close to matching a “delayed choice quantum eraser". You need to address losing the pattern by messing with just one side of the slit. Here you ask us to force feed the double slit by messing with both sides and only allowing selective photons to one slit or the other based on a measurement! It’s been well demonstrated that intervention of much small amounts is more than enough to void any possiblity of producing a pattern and you doing it on both ends of the “quasi entanglement test”.

    The “delayed choice quantum eraser" test do not force such selections – there they duplicate a double slit using a half-mirror allowing the photon to go though or deflect. The experamantor does not select which way photons go though.

    At least try to use or think about the information you’ve already been given - if you had you’d have figured this on your own easily enough.
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