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Macroscopic Quantum Entanglement

Posted by michael on from the done-with-mirrors dept.

meckardt writes: "We laugh at the science fiction of such programs as Star Trek, but it can almost be stated as a truism that what is fiction today may be science tomorrow and engineering next week. Researchers at the University of Aarhus in Denmark report in the science journal Nature that they have been able to cause particles to interact over a distance using lasers. The effect, called quantum entanglement, has been observed before, but never with such large amounts of matter. Don't expect transporters next week, but it is interesting that this report hits the streets the same day that Enterprise debuts."

11 of 216 comments (clear)

  1. Re:Clarification...? by mrseth · · Score: 2, Informative

    I think their goal may be practical to quantum computing. I could explain this here, but I already did it here. Basically, you need to be able to create a quantum copier to create a quantum computer. Building a quantum copier is difficult because, due to the no-cloning theorem, arbitrary quantum states cannot be copied perfectly every time. You can either clone a subset of quantum states perfectly or you can copy arbitrary states with a certain probability of failure.

  2. A Clarification... by HEbGb · · Score: 5, Informative

    Clarification:

    Quantum entanglement involves creating a system in which the state (polarization, spin, etc.) of two or more particles are 'dependent on' each other. Measuring the state of one particle defines the state of the other, 'magically', over some distance.

    HOWEVER make no mistake, nothing in quantum mechanics or entanglement theory allows anything resembling faster-than-light information traveling, nor teleportation as we understand it. This is pure fantasy that many physicists subtly or not-so-subtly use to solicit grants, or at least popular press. (There's plenty of this nonsense in sci-tech magazines.) It certainly worked here.

    Here's another example of macroscopic 'quantum entanglement'. I have a bag with two billiard balls, one black, one white. I close my eyes, pull one out and put it in a second bag. Then, I hand you the first bag, and walk across the room with the second bag, and open it. Once I look at the color of the billiard ball in my bag, the color of the ball in your bag 'magically' changes color and assumes a defined state. These billiard balls are entangled, very much like subatomic particles are.

    Can you ever transport information faster than light using this method? NO. Can matter be teleported? NO. I really wish these pop-sci articles would put an end to these misconceptions once and for all...

    1. Re:A Clarification... by sigwinch · · Score: 3, Informative
      Your billiard ball example is equivalent to Einstein's "hidden variables" attempt to explain away quantum entanglement. Bell's theorem demonstrated that the predictions of quantum mechanics were actually inconsistent with such a theory - and subsequent experiments proved him right.
      Bell's theorem only disproves *local* hidden variable theories, of which the billard balls are an example. (On the other hand, the billiard balls are a good example for the nonspecialist who wants to comprehend the effects without breaking their mind against quantum nonlocality.) Nonlocal hidden variables are perfectly allowable by the Bell inequality (and indeed I think they have considerable merit).
      In fact the reproduction of a quantum state - in all its particulars - is as perfect a teleportation as we might ever expect to achieve...
      In my opinion it doesn't count as teleportation. The systems were too homogenous and contrived. When they can do it for condensed matter containing multiple elements, I'll be willing to consider it teleportation.
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  3. Old news by dickDragon · · Score: 4, Informative

    This was presented at the

    International Conference on Quantum Information
    June 10-13, 2001 at the University of Rochester campus in Rochester, New York.

  4. Re:Clarification...? by a_hofmann · · Score: 2, Informative
    You are right, this quantum teleportation allows the transfer of particle states over distance. This is not about transporting matter. This will not allow us to build teleporters.

    Further information and links at the research group from Austria that ran the first experimental verification of quantum teleportation.

  5. Re:Clarification...? by mocm · · Score: 2, Informative

    You don't change their spin, you measure it and find that it is the same. Usually you measure a quantity that has a fifty fifty chance to give one or the other result. The interesting thing is that it always comes out the same for the entangled particles. No matter how far their separation in time and space.

    You can't convey any information that way since you don't know what the result of the measurement will be. But somehow the particles "know".

    For more information see their paper at http://xxx.lanl.gov/abs/quant-ph/0106057
    and references therein

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  6. Re:Clarification...? by renard · · Score: 5, Informative
    Can anybody shed light on this? How would this experiment lead to a teleporter??

    Well, this will get us into some of the most dangerous neighborhoods of quantum mechanics, but I'll see what I can do.

    The quantum entanglement of two particles means that (just as you say) the behavior of one particle becomes perfectly correlated with the behavior of another. In the classic example case, two photons are generated with opposite polarizations. If you can transmit them a distance apart without any interference, then the photons will remain entangled, and a measurement of the polarization of one photon will have immediate implications for the polarization of the other.

    Although this is very useful for quantum cryptography, please note that it will NOT allow you to transmit information any faster than the speed of light. To take the cryptographic example, it allows you to generate a safe one-time pad, known to both sides and to no one else, but you still have to transmit your actual message separately.

    How can quantum entanglement be used for something like teleportation? Well, let us agree first that if I can produce a perfect quantum replica of a distant system, that is equivalent to teleporting the system. Any given electron (for example) is indistinguishable (in a very deep sense) from every other electron in the universe. So for teleportation all we need to do is reproduce a quantum state. You might say it's more akin to a quantum xerox machine than to most people's classical idea of teleportation.

    Okay, so here's how it works: take your two quantum-entangled photons, and instead of simply measuring the polarization of the one nearby, get it to interact with a "target photon" that you want to teleport. If you set things up properly, and observe the outcome very carefully, then the interaction of the two photons on your end will cause the entangled photon - an arbitrary distance away - to enter a new state which is perfectly correlated with the state that your target photon had. Then, once you tell your distant collaborator about the exact outcome of the photon interaction on your side, your collaborator will be able to apply that knowledge to her entangled photon, and produce an exact quantum replica of your original target photon. Voila! Teleportation.

    Note again that no faster-than-light communication is enabled by this. You still have to communicate a regular light-speed message between collaborators to get this to work. The actual experiment was carried out several years ago and is old hat by now. The current experiment improves upon previous efforts by entangling so many more (trillions!) particles.

    The quantum entanglement of so many particles makes the actual teleportation of a similar number feasible, but one final note - even trillions of particles is many orders of magnitude less than the 10^27 or so particles in your average Starship Captain.

    -Renard

  7. Re:Clarification...? by Coolumbus · · Score: 2, Informative
    Pretty good explanation here:

    http://www.qubit.org/intros/comm/comm.html

    (Centre for quantum computation)

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  8. Xarchives Mirror of the paper [maths here] by -douggy · · Score: 3, Informative

    There is a mirror of the paper here at the arXiv.org e-Print archive. 11 pages of pdf fun can be found:

    HERE

    Have fun!

  9. "told you so" by Nihilanth · · Score: 3, Informative

    I find it gratifying that an earlier comment of mine about quantum entanglement was rudely put down as "impossible outside of science fiction and dilbert cartoons" is now receiving some front-page lovin'

  10. Explanations of entanglement and teleportation by hubie · · Score: 3, Informative
    Here are two pages that explain very well quantum entanglement and quantum teleportation:

    Quantum entanglement

    Teleportation