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Quantum Entanglement

  1. May 26, 2008 #1
    Quantum Entanglement!!

    How "Quantum Entanglement" happens?!
    As far as i know, it's one sub-atomic particle (i.e. electron) sendig data to another particle no matter the distance between them ... How does it work? and why?
    And could it work between big particles, molecules, and living beings? or is it just specified for quantum scale?
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  3. May 30, 2008 #2
    Entanglement is one of the most impressive properties of the quantum mechanics. It occurs just for very small particles, since that in the scale of Planck (quantum scale) is possible, for a quantum state, to exist in superposition with another. This fundamental propertie, named superposition of quatum states, is crucial to quantum entanglement existence.
    Entanglement occurs because of special correlations that can exist between these quantum states and, at least in theory, it is possible to transfer one state configuration to another state no matter the distance between them. In fact, it is just possible because of the entanglement, but it is important to emphasize some aspects:
    1. The information is transferred from a place A to a place B, but when this occurs, the information in A is destroyed and it just exist, before the teleportation, in B. (no cloning theorem)
    2. Despite the information to "travel" from A to B, it is impossible for somebody in B to read this information without talk about the state with somebody in A. In fact, noting can travel faster than the light velocity, neither information.
  4. May 30, 2008 #3
    No, that's not -- at least as far as anyone can tell -- what's happening. There's a less exotic explanation. If two particles have interacted via collision, or common origin, or interaction with a common torque variable while spatially separated, then they will be entangled with respect to this commonality. For example, two opposite-moving photons emitted simultaneously from the same atom will be entangled with respect to the angular momentum imparted from the atom via the emission. If streams of these sorts of photon pairs are analyzed by two spatially separated polarization filters, then a predictable joint detection curve emerges. The functional relationship between the angular difference of the polarizer settings and the rate of joint detection is cos^2 Theta, where Theta is the angular difference of the spatially separated settings at A and B for any given photon pair.

    The observation of entanglement depends, of course, on one's observational perspective. If you just look at A or B by itself, or at (A,B) without reference to the angular dependency then no entanglement will be evident -- just randomness.

    I think so. You could think of certain aspects of the motions of, say, Earth and Mars as being entangled with respect to the motion of the solar system.

    This is the sort of entanglement that becomes evident when you look at two spatially separated parts of a larger body or system of bodies.

    I suppose I would say that the essence of entanglement, quantum or otherwise, is that spatially separated analyzers are analyzing some common motional property.

    It might be important to note here that, at least so far, it's impossible to say, wrt quantum phenomena, what the common motions actually are independent of detection attributes. However, the assumption that filter-incident disturbances (whatever they are) associated with paired detection attributes are the same is all that's needed to make a certain sort of sense of the experimental results.
    Last edited: May 30, 2008
  5. May 30, 2008 #4


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    In fact, it is possible for something to go faster than light, as long as no information is carried. For example, the group velocity in a wave can be > c, but that's ok, because the energy of the wave is carried by the wave velocity (of the "envelope" wave, so to say) which is always < c.

    Now consider e.g. the EPR paradox. A spinless particle decays into two electrons, which must then have opposite spins. But until we measure one of these spins, the spin is in a superposition of the two (call them up and down). Now we can let these electrons fly (in principle) arbitrarily far. But then if we measure the spin of one, we instantly know what someone measuring the other electron's spin would get. So apparently there is some "instant" communication between the electrons, because they are in an entangled state (and definitely knowing one of the spins will fix the other one). The point is that no real information is carried (when you measure the spin, you cannot deduce whether someone else has already measured the spin of the other electron, let along deduce what the result would have been. Only if you compare it to such a measurement, you will find that they are opposite, but such a comparison requires information to be sent - with a finite speed. It is not possible to make up some binary system where "A measures spin" is 0 and "A does not measure spin" is 1, because B would still find up half the time and down half the time, without knowing whether A actually did the measurement.
  6. May 30, 2008 #5
    I Agree with CompuChip about the group velocity to takes values > c. But, how he well exposed, the information is carried with a finite speed. About ThomasT comment, it is important to observe that the angular momentum of the photon, and the atom energy spectrum are both quantum states and they can be entangled. In fact, you can use a large classes of quantum states to create entanglement, and, since that a quantum system have a lot of different observable, some of these can be entangled or not with other systems, or even with itself, creating entanglement between different observables of the same particle.
    About the entanglement of Large systems I disagree completely. The phenomenon denomine decoherence prevent,for large systems, any quantum states superposition, and the entanglement do not exist without it. It is because of the decoherence that we do not observe superpositions of classical states.
  7. Jun 3, 2008 #6
    I agree that we don't observe quantum superpositions and quantum entanglement in any classical sense. However, the phenomena of superposition and entanglement originated in the world of our sensory experience. We observe superpositions and entanglements every day. What we want to understand is how and why quantum superpositions and quantum entanglements are the same as and different from everyday superpositions and entanglements.

    If the deep reality underlying quantum experimental phenomena is essentially wave interactions wrt a hierarchy of media, some particulate and some not, then I see no reason to believe that quantum scale wave behavior is essentially different from the wave behavior that we can observe every day with our ordinary sensory faculties. If that is the case, then FTL or instantaneous propagations aren't necessary to understand quantum entanglement experimental phenomena -- and analogies with large scale classical phenonomena can be used to facilitate such an understanding.

    Of course things get even more conceptually difficult when particle phenomena are included. Hence the need for a complementary particle-wave picture, and a uniquely quantum superposition principle and approach to modelling experimental quantum entanglements.
    Last edited: Jun 3, 2008
  8. Jun 3, 2008 #7
    Quantum entanglement and quantum superpositions are very different from everyday superpositions and entanglement. Firstly because for classical superpositions we do not observe the parallelism inherent of the quantum superpositions. Secondly I do not agree that we observe entanglements everyday. Where do you observe this? Furthermo, some quantum entanglement states do not satisfy the Bell Inequality which do not occurs for any classical state. States that do not satisfy the Bell Inequality certainly are quantum states.
  9. Jun 5, 2008 #8
    An example of a common observation of entanglement (according to the way I described entanglement in my first post) would be if you and and your neighbor tune to the same channel on your radios or television sets and get the same program. Another would be the motions of Los Angeles and New York City with respect to the motion(s) of the Earth.

    There's no way to know if what I suggested as a possible basis for understanding quantum entanglement is correct or not. This is due to the existence of that pesky little fundamental quantum of action.

    Anyway, the original poster wanted to know what how quantum entanglement happens. The correct answer is that nobody knows. But we can speculate a little as to the physical nature of quantum entanglement, eh? I would be very surprised if it were essentially different from entanglement as I described it.

    As for quantum superposition of states. It can be communicated to the layman as being an expression of the various mutually exclusive outcome possibilities in any trial of any run of any quantum experimental preparation. It's not a statement of some actual physical state that exists independent of observation. Its basis is the principle of linear superposition (we're combining amplitudes) -- which is part of any wave theory.
  10. Jun 10, 2008 #9
    it is said that particles, even entire atoms go through a state of non-existence for a short period of time, as protons jump from energy levels. its said that they do not exist between the levels, just in them themselves. so it can be said that they have jumped without travel...telleportation if you will. who's to say that atoms, even entire structures couldn't blink out of existence, then blink back again nearby. if that is possible, then why cant it reappear farther away. if a beings atoms all cease to exist at the same time, the recreate and realign them selves a distance away, that would be perceived as telleportation. the only problem would be if they didn't realign in the same pattern and structure, that would happen if the information was mixed up with itself and bonded differently
  11. Jun 10, 2008 #10


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    You need to be VERY careful about applying various exotic principles, especially when you are misinterpreting them. For example, the "quantum teleportation" that has been demonstrated is NOT a teleportation that you see in Star Trek movies. It isn't a teleportation of whole objects, but rather the information about the property of entangled objects.

    Furthermore, there's nothing in QM that says about "non-existence" of objects. Superposition, where a number of orthorgonal physical properties coexist, yes. But whole atoms disappearing? No.

    Our PF Guidelines, if you haven't read it, can be found here.

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