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Quantum entanglement and computing

  1. Sep 17, 2013 #1
    LiveScience Quantum Computing Breakthrough

    QE for short.

    Please read that article. I need someone to set me straight here...

    Is QE a real thing, as in it's no longer an idea but is 100% a real thing.

    If two particles become entangled then whatever happens to one particle instantly effects the other, no matter the distance. (What is our answer for why this is possible.. How can 2 particles instantly communicate?)

    How can we develop a chip that allows for QE if we don't know why or how quantum entanglement works.

    The particles that make up my body come into contact with other particles ALL the time, am I entangled with those? Are particles only entangled once they collide at near c?

    If QE is no longer an idea and a real thing, is there some kind of law that states why this is possible, or does it seem to break known laws? Prehaps we just have not found the law yet?

    I'm so lost...
     
  2. jcsd
  3. Sep 17, 2013 #2

    jfizzix

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    Quantum entanglement is a real thing (I'm studying it in grad school).
    It does not allow you to do instantaneous communication though for a very simple reason.

    Single particles just look like single particles. There is no way of telling that a single particle is half of an entangled pair without information about that other particle.

    Even if a pair of particles were as entangled as can be, there is nothing you can do to one particle that would affect the measurement statistics of the other particle in isolation. Of course if you compare the measurement statistics of one particle to those of another particle entangled with the one, you can see that they are correlated. You just can't see these correlations by looking at only one particle though.

    There are cool things you can do with entanglement. Quantum computing is one of them. Superdense coding, is another. Quantum Teleportation is another, unconditionally secure quantum cryptography is yet another. None of these require that messages travel instantaneously, though.
     
  4. Sep 17, 2013 #3
    M-theory suggests that the universe has 7 extra dimensions for a total of 11 dimensions. It might be the case that the particles continue to communicate in some or all of those 7 extra dimensions where their distance could be possibly much smaller, though the communication seems to be initialized by bringing the particles close enough in the 4 well known dimensions of spacetime.

    It is not necessary for science to fully explain a phenomena in order for technology to take advantage of it. For example Colossal Magnetoresistance heads exist in all hard disks >=1TB, though (according to wikipedia) "a fully quantitative understanding of the CMR effect has been elusive and it is still the subject of current research activities"
     
  5. Sep 18, 2013 #4

    jfizzix

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    Two particles cannot instantly communicate via entanglement. (I just posted on this, but thought I'd give a fuller response to your other questions)

    We know enough about how quantum mechanics works to build theoretical models of quantum computers. The hard part is making these models a reality.

    This difficulty is why all modern quantum computing is still on a very small scale. Building quantum computers out of real world materials takes a lot of extra thought because quantum states can be easily disturbed by any outside influence. You can't have a working computer if your information gets degraded and destroyed before the computation is done.

    Particles can become at least slightly entangled whenever they interact with one another.

    There are interpretations of quantum mechanics which say you are indeed entangled with all the particles you come in contact with, and that since you are aware of only one classical reality, there must be parallel realities in relation to the other quantum states. This is the gist of the many worlds interpretation, an increasingly popular, but still a minority interpretation of quantum mechanics.

    Keep in mind that interpretations of quantum mechanics are just that; Interpretations. Everyone agrees on the math. Everyone agrees on the experimental results. Interpretations are always up for debate until someone comes up with an experiment which can tell one apart form another.

    Quantum entanglement is a part of the theoretical structure of quantum mechanics itself. It is a consequence of the superposition principle of quantum mechanics, i.e., that the quantum state of a system can be in a superposition of different states. Entanglement comes into play when the states we are looking at are of more than one particle.

    If quantum entanglement was nonphysical, a lot of aspects in quantum theory would have to be changed, resulting ultimately in quantum mechanics itself being unphysical.

    Entanglment doesn't break any known laws of physics because again, entanglement cannot be used to send information faster than light. It does fly in the face of classical newtonian physics, but just like relativity, quantum mechanics can reduce to newtonian mechanics in the right limits (i.e. slow moving bodies composed of many particles)
     
    Last edited: Sep 18, 2013
  6. Sep 18, 2013 #5

    jfizzix

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    Indeed so. Another sweet example is high temperature superconductors (materials that superconduct at temperatures above the boiling point of liquid nitrogen). We can make them and use them, but it's considered the great unsolved problem of solid state physics as to how high temperature superconductors work.
     
  7. Sep 18, 2013 #6
    So particles can be entangled with billions of other particles and isn't a 1 to 1 relationship but a 1 to many?

    Let's say I have two particles that are seperated. If I physically do something to one of the particles, what will happen to the other one? Once two particles become entangled, what changes about those particles?

    I can visualise it as two particles coming into contact with each other and giving each other a their telephone number and saying "When something happens to me, i'll phone you and you do the same thing" I know... such a crude and probably inaccurate analogy. You've said that entangled particles don't communicate though, so what changes about the entangled particles?
     
  8. Sep 18, 2013 #7

    jfizzix

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    The quantum state of two groups of particles, however many or few there are in each group can be entangled. There is an additional feature of entanglement called the monogamy of entanglement. The more one system is entangled with another system, the less either system can be entangled with anything else. That being said, a single particle can be at least a little bit entangled with billions of others.

    If you do something (i.e. measure) one particle, the other particle will be unaffected. If they are entangled, you will find that when you do look at the other particle, what you find is correlated to what you saw in the first particle.

    This is a particularly difficult question.
    When two particles interact with one another the state which describes the pair of them can become entangled, which is when the state describing them can't be factored out into the state of one times the state of the other.
    The quantum mechanics that first came out in the 1920s didn't integrate relativity, and treats the effect of measurement as something which propagates instantaneously throughout the universe. A couple decades later, a more powerful version of quantum mechanics was developed which did include special relativity called quantum field theory. Unfortunately, I don't know enough about quantum field theory to say if it solves the problem.

    I do know that there are performed experiments which try to show that entanglement correlations somehow don't have a speed limit. These experiments are based on violating Bell inequalities, which are requirements that your measurement probabilities have to satisfy if the effect of measurement travels slower than light. However, since you need to send classical messages back and forth to compare measurements on entangled particles, you can't use these correlations to send messages faster than light.
     
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