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Interactions that cause entanglement

  1. Apr 6, 2008 #1
    I would like to know what are some examples of what two nuclei would have to experience in order to become entangled. Could being part of the same molecule yield such a state? or would they have to be, say, daughter products of a nuclear reaction, etc?

    big thanks,
  2. jcsd
  3. Apr 7, 2008 #2
    I'm pretty sure that entanglement can only arise as a result of fundamental conservation laws such as pair production in the weak decay of neutral meson.

    It can be proven that Coulomb interactions do not lead to entanglement.
  4. Apr 7, 2008 #3
    Could you elaborate? If two protons collide like billiard balls with the same energy, why are they not entangled afterwards? Conservation of energy applies.
  5. Apr 7, 2008 #4


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    Surely all well understood physical systems obey fundamental conservation laws?
  6. Apr 8, 2008 #5
    To be honest I'm not sure about this myself but it's what I've been told by more knowledgeable people than myself at my University. I do believe this is part of what makes quantum computing so difficult.
  7. Apr 8, 2008 #6
    I meant along the lines of particle creation/annihilation processes. In other words, you need quantum field theory to get entanglement. In nonrelativistic quantum mechanics entanglement is added a priori.
  8. Apr 8, 2008 #7
    Actually, almost any interaction leads to entanglement. The problem with quantum computing is that the system entangles with the environment, which you then can't measure effectively. The system you are interested in is then left in a (classical) statistical state as far as you can measure.

    Entanglement refers to the inability to factor a combined state into a tensor product of states of the separate systems. Very few interactions actually produce states which are factorisable even if you start with factorisable ones.
  9. Apr 8, 2008 #8


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    There are plenty of systems that use electromagnetic interactions (capacitive, inductive, photon mediated etc) to entangle two quantum systems. This is e.g. the only way to entangle two solid-state qubits.
  10. Apr 8, 2008 #9
    Please pardon my extreme ignorance of this subject, but would such interactions also entangle the nuclei of atoms? or does nuclear entanglement require nuclear processes?

  11. Apr 9, 2008 #10
    After seeking clarification it turns out that the statements are not correct. You can entangle two qubits by e.g. driving them in a common laser field until they are in the same quantum state and then allowing them to interact via electrostatic dipole interactions.

    I'm not sure what the situation is like for nuclei but I imagine it would be similar.
  12. Apr 9, 2008 #11
    Very interesting! How might that work for nuclei? What, for example, might be analogous to a laser insofar as a few nuclei are concerned? x-ray laser? neutrons in some kind of coherent wave???
  13. Apr 9, 2008 #12


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    I guess an analog *might* be the decay of a free neutron. Spin would presumably be conserved (in the output particles) and maybe you could figure out a way to come up with a Bell Inequality there too.

    The strong force doesn't seem amenable to similar testing. Perhaps you would try to create a gluon detector, not sure but I assume they could exist in the wild. I don't know enough about the strong force to speculate how that would happen. I think it is generally conceded that free quarks don't exist long enough to do much with them. You end up with pions (quark-antiquark pairs) instead, and I think they decay rather quickly as well. So you end up with a particle soup, nothing simple enough to test really.
  14. Apr 10, 2008 #13


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    Not at all. For example, the EPR experiment is non-relativistic. The entanglement comes from conservation of angular momentum. Given that the spin state's magnetic quantum numbers must add to zero -- zero angular total spin state --, only one measurement is required to determine the entire state, complete set of states and all that standard QM stuff. No a priori assignation of entanglement in non relativistic QM.

    That being said, relativistic QM supports the idea of entanglement. In fact virtually the entire kinematical structure of high energy physics experiments, and also low energy particle experiments are highly dependent upon the phenomena of entanglement. It's kind of like, "Buy a dozen, and get one free."

    That entanglement happens is no big deal, but the dynamics of entanglement presents us with some tricky problems. And, naturally, entanglement occurs in classical systems as well.

    Entanglement by interaction; beyond my pay grade.
    Reilly Atkinson
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