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How are atoms entangled and can it be done remotely?

  1. Jul 31, 2011 #1
    I'm a bit confused about quantum entanglement. First of all, how exactly do you entangle two atoms? I've heard about how they entangled atoms in quantum teleportation experiments, but I don't get how they did it. I've heard that the atoms must "interact", but what exactly does that mean? Do they have to touch? Do they have to be within a certain distance - like a foot or an inch - from each other?

    Also, can atoms be entangled if they're really far away? Like if one was in New York and the other was in LA, could you entangle them? Or would you have to, for example, entangle them both in LA and then take one to New York?
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  3. Jul 31, 2011 #2


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    It could be difficult to keep them undisturbed for several hours needed for flight from LA to NY.
    But the quantum teleportation experiments had been performed between distant locations. Actually, the distances are still much shorter tyhan LA-NY - they are limited by light attenuation in optical fibers and furthest distances are in order of 50km. But multiple such experiments had been performed between Bernex and Bellevue (11km apart) - where Univ.of Geneva labs are located and between Vienna and it's remote location (20km away, I don't remember town name...) by Vienna Univ group.
  4. Jul 31, 2011 #3
    Right, but my question is regarding how entanglement is established. I understand that keeping atoms entangled is difficult, but how do they get entangled in the first place? I know that sometimes entangled photons can be created by an excited atom. But how do atoms themselves get entangled?
  5. Jul 31, 2011 #4
  6. Jul 31, 2011 #5


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    What can be achieved is not 'entangled atoms' (what does that mean???) but entangled states of some their property: e.g. excitment if we consider only two energetic levels, or their spin, or... any single simple property.
  7. Jul 31, 2011 #6
    Well, you can entangle photons, right? So why not atoms?

    In any case, I suppose what I meant to say was entangled states. How do the states of, say, two hydrogen atoms become entangled? Do they have to come into physical contact? Can it be done remotely?
  8. Jul 31, 2011 #7
  9. Jul 31, 2011 #8


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    Because of complexity. Photon is fully described by its momentum vector and spin. That's all. Nothing more. For entanglement experiments we usually ignore momentum vector and play only with spin (or polarisation, if you prefer) states.

    But what is a full description of atom??? Spins of electrons? Occupation of orbitals? Should we go so deep as to states of gluons keeping quarks in its nucleus together? Quark spins?...
  10. Jul 31, 2011 #9
    If atoms are near they interact electromagnetically even though they are neutral.
  11. Jul 31, 2011 #10
    I've heard that the atoms can also interact long-distance, by exchanging photons. Can the photons just travel through space or air? Or do they have to go through some channel?
  12. Jul 31, 2011 #11


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    Open your eyes wider to see some photons travelling though air. Go outside to see some photons, which, after reflecting from the Moon travelled through space.
    (Ooops - there is new moon tonight, you must wait till morning to see some photons travelling through space from Sun...)
    Last edited: Jul 31, 2011
  13. Aug 2, 2011 #12
    Theoretically, For making an entangled state, at first we have to make an "interaction" between the particles such as electrons, etc., i.e. we have to add a potential (between our particles) in our Hamiltonian, otherwise the state of our system is a multiple of state of single particles and so no correlation during every kind of measurement, which equals to no entangled state. It can be understood clearly when we look at Schrodinger's equation.
  14. Aug 2, 2011 #13
    To entangle two particles put them in the same state with the exception of spin which must be opposite. Now we know the spin of the particles must remain opposite. So if you measure the spin of one particle the other must be opposite of what you measured. One particle gives you information about the other, they are entangled.

    If you separate the particles over a distance without disturbing them you will find that they remain opposite in spin upon measurement.
  15. Aug 2, 2011 #14


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    They do not need to be opposite in spin, and their states do not need to be identical. You can entangle any number of particles, for example. The 3 quarks in a proton are entangled. And in fact the particles do not even need to be the same type, although usually they are.

    Usually there are 2 required factors: i) the total of some observable is conserved; ii) which particle has which value is unknown.
  16. Aug 2, 2011 #15
    That is a better way of putting it. That explains how something like a photon can be entangled since they can be in the same state with the same spin.

    This sort of takes away the mystery of entanglement though. Since you would expect if the observables are conserved that when measuring one you would know the other, due to the conservation.
  17. Aug 2, 2011 #16
    But my theoretical question about entangling states of two atoms, one on the moon and one on Earth, remains. Could those two atoms, if they were originally not entangled and in no way correlated, exchange photons and then have entangled properties? How hard would that be? Would you have to worry about aiming the emitted photons, interference from air, radiation, etc.?
  18. Aug 3, 2011 #17


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    Yes, that is called Quantum Teleportation. Wiki on it and follow references.
    In order to make any 1-bit property of those atoms entangled, you must have a pair of entangled photons (one in each location) and additionally exchange 2 bits of "classical" information.
  19. Aug 3, 2011 #18
    Most probably I know less about this subject than you. But I'll still try.

    I guess an H2 molecule split into it's individual atoms should be 'pretty' entangled for you in momentum, spin and what not. After any event resulting in 2 (or more) particles (ahem!) and by observing one particle, if you can 'precisely' predict the nature of second particle, you have an entangled pair.

    Any interaction between 2 particles make them 'entangled'. After a cue-ball hits a snooker-ball, they are entangled by momentum. By observing the cue-ball momentum, you can precisely predict the snooker-ball's momentum.

    For a un-entangled snooker-ball (a snooker ball of unknown mass pushed by some 'mysterious force' now moving at an unknown velocity), you cannot measure or predict its momentum without 'inadvertently-manipulating' its momentum. But for an entangled snooker-ball, you can predict the momentum, by measuring the momentum of the cue-ball. You did not change anything by doing it, you just know it now. It doesn't make the snooker-ball a 'special' snooker ball. You removed the possible uncertainty of momentum (by collapsing the probability function). I know I am missing the Heisenberg here, probably because the example I chose doesn't have anything quantum about it!

    At a quantum level the effects are more prominent, primarily because of the complementarity breakdown. That's what I want to believe now.

    Pardon me if I am being really stupid here by trying to make some sense into everything.

    For me things should make sense. This is physics, not theology. I didn't opt out of 'hidden god' theories to study 'hidden variable' theories. Sorry! :)

  20. Aug 3, 2011 #19


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    I guess an H2 molecule split into it's individual atoms should be 'pretty' entangled for you in momentum, spin and what not.
    Nope. If you split H2 (e.g. hitting it with X-ray) you'll get just two separated atoms, not entangled. That's not so easy to produce entangled pairs of atoms.

    Any interaction between 2 particles make them 'entangled'.
    Nope. Only those interactions which lead to random states of particles with constraint that sum of the values is precisely known. That is a case of creating entangled pairs of photons using parametric down conversion: two photons are created in this process, while we know that sum of their spins is 0, sum of their momenta is equal to momentum of the initiating photon (well defined, as coming from laser).

    a cue-ball hits a snooker-ball... Pardon me if I am being really stupid here by trying to make some sense into everything. For me things should make sense.
    Never, never, never more use snooker-ball picture while thinking about quantum phenomena!
    It leads you on swamps - usually to paradoxes or results contradicting experiments. Snooker ball picture fails even with simplest double-slit experiment.
    As till now no one (although Einstein spent several years on it) developed any self-consistent and consistent with experiment mechanistic model of quantum phenomena.
    Sure, we all (well... most of us...) try to reach some understanding going deeper than just getting precise results of calculations. And we need some intuitions and ordered view. But realistic mechanistical models are definitely not a right approach - although some wise men (Einstein...) never abandoned them.
  21. Aug 3, 2011 #20
    Where I have a disagreement with you is sticking on to randomness. Randomness is a generalized behavior. 'Orderliness' is a specialized subset of Randomness. Naturally any theory capable of explaining random behavior should be explaining orderly behavior too. Isn't it?

    Now snooker-balls will produce interference patterns if someone makes a DS apparatus with slit widths in the order of 10-34, then manage to pass the snooker ball through that and then measure the fringe gap in the order of 10-34.

    All these factors combined make this DS experiment invalid. But for electrons, other sub-atomic and atomic particles, the experiment seems to be valid and you do see interference patterns (though we don't know how and why)

    QT holds good every time. It's just some experiment are invalid or the results are unobservable/incomprehensible.

    Going by this there could be macro-level entanglements. It's just either the experiments might be invalid or the results might be unobservable/incomprehensible.


    ps: can a stream of point-like singularities create interference fringes? yeah, why not :)
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