Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How 2 particles become entangled?

  1. Jul 17, 2015 #1
    I don't have clear how two particles initially "independent" in the sense of "not entangled" become then entangled because of their mutual interaction (and in this last case, when and how I can say they "interact"?). How do I know how should they approach or how strong their interaction should be and for how long, so that they then become physically entangled?
    I'm not looking for the mathematical description of an entangled state but a physical description of the process, if that is possible.

  2. jcsd
  3. Jul 17, 2015 #2


    User Avatar
    Science Advisor

    Think of entanglement as a mathematical description of correlation, and ask yourself how two initially uncorrelated particles may become correlated by interaction? Even in classical physics, it should be clear and intuitive that two phenomena can be correlated only by mutual (either direct or indirect via a third party) interaction. Does it help?
    Last edited: Jul 17, 2015
  4. Jul 17, 2015 #3


    User Avatar
    Science Advisor
    Gold Member

    As a "rule", it must be impossible in principle to know or predict the values of the entangled properties. That may not help you much, because your next question is likely to be "how do you do that?"

    Usually there is some conserved quantity, such as spin or momentum. And the individual particles have interacted in such a way as to make any of a multitude of indistinguishable outcomes possible. In a classical interaction, the outcomes are deterministic and there is no entanglement. An unknown but predetermined outcome is not entangled either.

    If you took any interaction of quantum particles a la a Feynman diagram, there is a good chance there is entanglement between one or more properties of the output particles related to conservation.
  5. Jul 17, 2015 #4
    Lightarrow: You are already further along than Einstein:

    "Einstein and others considered such {entangled} behavior to be impossible, as it violated the local realist view of causality (Einstein referred to it as "spooky action at a distance")"

    It's a short 'wait'.....in fact, no' time at all:


    "...According to the formalism of quantum theory, the effect of measurement happens instantly.....The seeming paradox here is that a measurement made on either of the particles apparently collapses the state of the entire entangled system—and does so instantaneously, before any information about the measurement could have reached the other particle...."

    "..Experiments have been performed involving measuring the polarization or spin of entangled particles in different directions, which—by producing violations of Bell's inequality—demonstrate statistically that the local realist view cannot be correct. This has been shown to occur even when the measurements are performed more quickly than light could travel between the sites of measurement: there is no lightspeed or slower influence that can pass between the entangled particles.[6] Recent experiments have measured entangled particles within less than one one-hundredth of a percent of the light travel time between them.[7]
  6. Jul 17, 2015 #5
    Actually Einstein was correct. It makes no sense at all for particles to be joined at the hip when miles apart.
  7. Jul 18, 2015 #6
    Not sure to have totally understood. Does mutual interaction always entangle two particles? About interaction: how close they have to came to say they have interacted?

  8. Jul 18, 2015 #7
    You wrote something, but you are very cryptic :smile:
    I deduce the subject it's not very easy.

  9. Jul 18, 2015 #8
    Sorry but in your reply I don't grasp the answer to my question.

  10. Jul 18, 2015 #9


    User Avatar
    Science Advisor

    Ask yourself analogous questions about classical objects. Does mutual interaction always correlate them? How close they have to come to say they have interacted?
    Quantum physics is not so much different from classical physics.
  11. Jul 18, 2015 #10
    In classical physics I can just take two mass points and say their total momentum is conserved if no force is applied on this system. If I take two any electrons, their total momentum must be conserved as well, in the absence of forces of them. Maybe in this case it's difficult to establish that no forces acted on them? Maybe in QM what si difficult is to know if two electrons or two photons can be considered as part of the same system during a certain interval of time during which measures on them can be performed?

  12. Jul 18, 2015 #11
    It is an insight only...The Wikipedia article I linked to already gives a decent overview. If you only read the brief excerpts above, try the article in total.

    Here is another from Brian Cox [physicst]:
    ".... everything IS connected to everything else."

    Interpreting quantum mechanics continues to be a subject of discussions in these forums.

    An interesting entanglement discussion in these forums revolves around a tv show from physicst Brian Cox...[becox]

    He actually participates in the discussion! Some don't like his model and interpretation, yet he says clearly that's what they teach at Manchester. He posts an excerpt from his book which describes his view of entanglement: [my layman synopsis: everything is entangled, always has been, always will be. Not only that, effects happen in 'no time at all'...are instantaneous. ]

    "It seems that we must conclude that the pair of identical electrons in two distant hydrogen atoms cannot have the same energy but we have also said that we expect the electrons to be in the lowest energy level corresponding to an idealised, perfectly isolated hydrogen atom. Both those things cannot be true and a little thought indicates that the way out of the problem is for there to be not one but two energy levels for each level in an idealised, isolated hydrogen atom. That way we can accommodate the two electrons without violating the Exclusion Principle. The difference in the two energies must be very small indeed for atoms that are far apart, so that we can pretend the atoms are oblivious to each other. But really, they are not oblivious because of the tendril-like reaches of the Pauli principle: if one of the two electrons is in one energy state then the other must be in the second, different energy state and this intimate link between the two atoms persists regardless of how far apart they are...There is an intimacy between the particles that make up our Universe that extends across the entire Universe. ....This is one of the weirdest-sounding conclusions we’ve been led to so far in the book. "

    Also see Fredriks post #29 for a critique of the becox 'theory'.....Note that whether indistinguishable particles constitute entanglement is also an issue for discussion. In addition, one has to keep in mind that environmental decoherence, interactions with everything else, subdues entanglement.

    Please note I am pointing you to the entanglement discussion as a whole, not becox's particular interpretation.

    Right you are. As is often the case, the math definitions are not readily subject to a single interpretation. What do they mean?? When asked which interpretation of QM he favored, the famous and humorous physicist Richard Feynman said: "Shut up and calculate."
  13. Jul 18, 2015 #12


    Staff: Mentor

    Two free electrons in proximity with unaligned spins, can emit a photon to fall into lower energy triplet or singlet entangled states. The photon carries off the energy difference.

    A free electron becoming part of an atom becomes entangled with the other electrons in the atom. I also speculate that electrons in an atom are more entangled with other electrons in the same shell, than with those in other shells. (physicists, please correct my speculation if it is wrong.). Therefore absorbing or emitting photons that change energy states in an atom changes entanglement.

    Those could be some processes leading to entanglement, there may be others. Remember, entanglement is not just binary, there are degrees of entanglement from 0% to 100%.

    Photons can be emitted in entangled pairs. In that case, they are entangled from their beginning.
  14. Jul 18, 2015 #13
    Does anyone know if the online Leonard Susskind video lectures on entanglement address this question? I have found his insights on other issues very interesting, sometime uniquely. I have some doubts the advanced math in the lecture series will address this particular issue. I hope I am wrong.
    Last edited: Jul 18, 2015
  15. Jul 18, 2015 #14


    User Avatar
    Science Advisor
    Gold Member

    Sorry 'bout that... but sadly: the more you understand entanglement, the easier it is to understand. I KNOW that is a bit cryptic. :)

    If you have a helium atom with 2 electrons in the ground state: those electrons are spin entangled. 2 particles, described by *one* state rather than separate 2 states. While you intuitively know that one has spin +1/2 and the other has spin -1/2, that hardly tells the story. If you measured both of them on ANY identical basis, you would get the same result, that being: one has spin +1/2 and the other has spin -1/2. And that result is independent of the separation between them. Considering there are an infinite number of bases that you could choose to measure electron spin, that is quite a feat!
  16. Jul 18, 2015 #15
    Here is a 2004 discussion on the same subject with what I believe are some insights which may help...


    Seems like the staff member is saying that since all quantum interactions take place in increments of h, an interaction which provides that level of energy entangles and one which falls short, doesn't.

    See what you think of post # 26 from staff.
    "I don't know what a force strong enough to affect the probability of a spin but too weak to change the spin would be in QM. Spin (around a given axis) only comes in a discrete set of states, so you either leave it in the state it is, that is, you don't interact with it, or less you influence it and then you are operating on the spin state with some operator and the state goes into some set of discrete eigenvalues. It looks to me to be all or nothing."

    If that's true, I don't immediately see how different degrees of entanglement are achieved.

    Also maybe the following.........
    #16,21,26...and more advanced, #29, a correction in #33, is #51 correct? then the thread goes off on a tangent.
  17. Jul 18, 2015 #16


    User Avatar
    Science Advisor
    Gold Member

    Entanglement can be made to be anywhere from 0 to 100% in the ideal condition. This is done by varying the amount of knowledge you gain about one (or both) of the particles. Collapse is definitely not all or nothing. And in fact collapse can occur on one basis (say spin) without any collapse occurring on some other basis (say momentum).

    It is not currently known if collapse is a distinct physical process. This is somewhat interpretation dependent.
  18. Jul 18, 2015 #17


    Staff: Mentor

    What I wrote in #12 about emitting a photon was paraphrasing Susskind. He definitely said that the singlet pair has lower energy than a triplet pair which is lower energy than an unentangled pair of electrons. That makes sense if entangling is spontaneous, but disentangling requires an external interaction
  19. Jul 19, 2015 #18

    If studying a particle tells you that there was no interaction with another particle, then the other particle must somehow become informed about that result, so it can agree that no interaction happened.

    When the universe has statistical information about a bunch of particles, but no information about the individual particles, then those particles are entangled.

    When new information about individual particles in a bunch of entangled particles appears into the universe, the new information must agree with the statistical information.
  20. Jul 19, 2015 #19
    Leading on from this is something I've been wondering about recently. Classical gravity, introduces correlation and entanglement between particles all across the universe. At large distances we know that this is not strong enough to induce decoherence, however at shorter scales the magnitude of the force is unbounded.

    It seems to me that the role of gravity must be somehow significant in determining how classicality emerges.
  21. Jul 20, 2015 #20


    User Avatar
    Science Advisor
    Gold Member

    This is incorrect. Relativistic gravity does not lead to entanglement and there cannot be entanglement swapping via the gravitational force either.

    On the other hand, presumably quantum gravity - if there is such a thing - would participate in some kinds entanglement.
  22. Jul 21, 2015 #21


    User Avatar
    Science Advisor

    Classical gravity cannot cause entanglement, but can cause correlation. Suppose there is a strong local source of gravitational field which attracts quantum particles. Suppose also that you do not know where that source is. Then, by finding one quantum particle particle at some position, there is a larger probability that other quantum particles will also be found nearby. This, of course, is a purely classical effect, despite the fact that particles are quantum.
  23. Jul 21, 2015 #22
    You are, of course correct. Reading my post back, my wording was clumsy.

    I was trying to illustrate a case where correlation and entanglement are not treated equivelantly, using existing models.
  24. Jul 22, 2015 #23
    I would like to know if someone, somewhere, has done an experimental work like this:
    1. Produce two quantum entangled entities (particles or what else)
    2. send one to the antipode local of earth
    3. Change the state of the first of them and write down the Universal Time of the occurrence
    4. At the second place monitor in a continuous way the quantum state of the second
    5. When, at the second place, will happens a change in the quantum state, write down the Universal Time of the occurrence

    If done, will both figures of Universal times equals? are there some experimental evidence of that?

    I think that If QM equations are not local and if they describe both entities at once (because they are entangled) then the UTs will equals.
  25. Jul 22, 2015 #24


    User Avatar
    Science Advisor
    Gold Member

    Here is a major problem with your idea: Continuous monitoring of such a quantum state will NEVER show any connection to Particle 1's similar state. In fact, the first such check will cause entanglement to end (on that basis). Same is true vice versa.
  26. Jul 22, 2015 #25
    Thank you by your reply, DrChinese.
    Yes, I have forgotten that.
    Do you know if there are some experimental way to verify that the UTs of quantum state changes will equals?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook