3 ms·

I posted this b/c I was hoping someone would dive into these quotes: The Delft researchers were able to entangle two electrons separated by a distance of 1.3 k

• zardo 11y ago
  It doesn't allow (non-random) FTL communication. You can use it to generate the same random number in two locations.
  • fizx 11y ago
    While everything you said is true, I feel its worth pointing out entanglement is stronger than simply being able to generate the same number in two locations, a feat easily accomplished via classical means. Edit: Entanglement is akin to the following... Assume Alice and Bob are distantly separated. Alice has an arbitrary float variable called x. Alice and Bob both have a function rand() generating floats between zero and one, with a shared seed so that they can both generate the same random numbers. Classical correlation is that they both can call rand() with their identical seeds to get the same random numbers. Entanglement is that Bob's rand() is actually ((x + rand()) % 1), where Bob instantly and magically has access to Alice's variable x. No information can be transmitted, because the distribution of random numbers hasn't changed, and Bob doesn't have direct access to x. But something is different, and if Bob is clever, he might be able to do some fun stuff. This isn't a perfect analogy, but perhaps it helps.
    • duaneb 11y ago
      > While everything you said is true, I feel its worth pointing out entanglement is stronger than simply being able to generate the same number in two locations, a feat easily accomplished via classical means. Really? That's not random, that's pseudo-random, so the secret is still just the seed distributed before hand. True randomness would be entirely uninvolved from other state, including location or time. This is really just combining the sharing and the generating into one process so you CAN share true randomness without sharing other state.
• tgflynn 11y ago
  I'm not an expert but quantum entanglement definitely does not allow FTL communication. I believe quantum key sharing requires a classical channel in addition to the quantum channel, hence no information is transmitted faster than light.
• fizx 11y ago
  It's the nyt, so they are at best imprecise and at worst wrong. Entanglement can be thought of as a resource that allows you to do unexpected things (win the CHSH game), but sharing information isn't one of them. A better discussion can be had at http://www.scottaaronson.com/blog/?cat=33 http://www.scottaaronson.com/blog/?cat=33
  • finnh 11y ago
    Good old Aaronson, thanks. My recollection of "Quantum Computing since Democritus" informed my frowny-intuition about the NYT article ... should have gone to his blog directly no doubt.
  • weinzierl 11y ago
    The linked article from Scott Aaronson is quite accessible and very worth reading. My favorite quote is: Perhaps the best way to explain local realism is that it’s the thing you believe in, if you believe all the physicists babbling about “quantum entanglement” just missed something completely obvious. What I still struggle to understand is the realism part. The NYT article seems to imply that, when all loopholes are closed, we will have the final prove to live in a non-local world. Realism isn't mentioned. The finding is another blow to one of the bedrock principles of standard physics known as “locality,” which states that an object is directly influenced only by its immediate surroundings. I always thought that local realism meant one of the following: - no realism - no locality - neither realism nor locality The Wikipedia article Principle of locality (https://en.wikipedia.org/wiki/Principle_of_locality#Local_realism https://en.wikipedia.org/wiki/Principle_of_locality#Local_re...) says Any theory, such as quantum mechanics, that violates Bell's inequalities must abandon either locality or realism; Can the result of Bell's experiment be explained when we just abandon realism but not locality?
    • fizx 11y ago
      > Is there an explanation of Bell's experiment that preserves locality and only gives up realism alone? http://plato.stanford.edu/entries/qm-relational/ http://plato.stanford.edu/entries/qm-relational/ I'm not super knowledgeable about this (and whether it even meets the criteria), but perhaps you will find it interesting.
• snowwrestler 11y ago
  It doesn't say anything about FTL communications. I know that's a subject that has been associated with entanglement many times, but I'm not sure this is one of those times. In other words, let's not blame the Times for something they did not actually say. Instead what I inferred from this is that since entanglement can only be resolved once per pair of particles, a communications system based on entanglement might provide perfect forward secrecy even in an environment in which very large numbers are able to be factored by quantum computers.
  • finnh 11y ago
    Yes, the Times article does say this. (copied from my response to a similar comment above): But since the 1970s, a series of precise experiments by physicists are increasingly erasing doubt — alternative explanations that are referred to as loopholes — that two previously entangled particles, even if separated by the width of the universe, could instantly communicate.
    • tgflynn 11y ago
      The problem is with the usage of the word "communicate". Normally communication would mean "transmit information". Quantum entanglement does not allow this. With entangled particles a measurement on one instantly changes the statistical behavior of the other. However there's no way to see this effect until you compare the statistics between the two particles (which probably requires an ensemble of particle pairs to be measured, not just one), so you can't actually use the effect to transmit information. I don't think there's a good English word for the concept of an influence that isn't causal and doesn't transmit information. That's why it's often called "spooky action at a distance" and why attempts to describe it with simple English sentences are likely to lead to confusion.
    • duaneb 11y ago
      Yea, communicate is the wrong word. Perhaps "could continue to share state" would be better, but perhaps that would be confusing to the non-quantum-inclined audience.
    • snowwrestler 11y ago
• duaneb 11y ago
  This does NOT imply FTL communication. The actual key is arbitrary/random, so no information is transferred. However, both can read the same particle state. So it's more like they sent the particles to each other (or from the center), "caching" whatever value is there in two separate locations. However, you don't know when the entanglement is broken, so you STILL can't communicate FTL.
• lisper 11y ago
  No, entanglement does not produce FTL. Entanglement is nothing more than (the first step of) a measurement. http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc
  • dchichkov 11y ago
    Can you give a link that clearly describes "measurement-as-a-cascade-of-entanglements process" in some reliable source? Best, some QM textbook. Basically exactly the same as this http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf (5.3), but in a well-known textbook or some review paper in a journal with good reputation. I run from time-to-time into a discussion which just needs such pointer...
    • lisper 11y ago
      My paper is based on this one: http://arxiv.org/pdf/quant-ph/9605002v2.pdf http://arxiv.org/pdf/quant-ph/9605002v2.pdf I don't know if that exact paper was ever published, but an earlier version was formally published here: C. H. Adami and N. J. Cerf, “Information Theory of Quantum Entanglement and Measurement,” Physica D 120 (1998) 62-81. But this paper turns out to be a rehash of an older result (Zurek, 1991) http://arxiv.org/abs/quant-ph/0306072 http://arxiv.org/abs/quant-ph/0306072 which in turn is more or less a rehash of an even older result (1955) by von Neuman: http://arxiv.org/pdf/1311.7649v1.pdf http://arxiv.org/pdf/1311.7649v1.pdf (See the references for the original source.) And there is some evidence that even Bohr and Heisenberg actually understood this. Basically, the physics community bifurcates among those who deny this is true (e.g. Lubos Motl) and those among whom it is considered common knowledge. Indeed, when I submitted my paper to Physics Today back in 2000 it was rejected on the grounds that it was nothing new. The hard part of QM is not understanding it. The hard part is accepting that it is true. Because people really want to believe that the reason that we can make reproducible measurements is that these measurements are a faithful reflection of a underlying metaphysical reality that is actually "out there" in some sense. But that turns out not to be the case. As David Mermin so eloquently puts it (http://arxiv.org/abs/quant-ph/9609013 http://arxiv.org/abs/quant-ph/9609013) we are in fact made of "correlations without correlata."
• jameshart 11y ago
  Why do you think that these quotes imply FTL? They talk about a network allowing communication over a large distance, but they don't specify a communication speed. Why would you assume that means that it would be instantaneous/faster than light? A more parsimonious reading of the article would assume that the author knows, and assumes you know, that FTL communication is impossible, and that the interesting aspect of the communication possible through an entangled quantum network is, as stated, its security and resilience to eavesdropping.
  • finnh 11y ago
    The interaction between the particles is instantaneous. There is no mention of a non-FTL communication betwixt the particles. And then there's this sentence in the same article: But since the 1970s, a series of precise experiments by physicists are increasingly erasing doubt — alternative explanations that are referred to as loopholes — that two previously entangled particles, even if separated by the width of the universe, could instantly communicate. So I don't think my reading was insufficiently parsimonious.
    • semi-extrinsic 11y ago
      http://xkcd.com/1591/ http://xkcd.com/1591/
• badcircle 11y ago
  this may not elucidate a clear answer, but I've heard it described as such and thought it might help: entanglement works sort of like creating a matching pair of gloves. You can put each glove in separate boxes and ship one to New York, one to San Francisco. You open the box in New York, finding the right-hand glove and know instantly that the left-handed mate is in San Francisco. So too bits can be entangled, and measuring one, gives you a "cheater's peek" at what the other's value is -- they're integrally linked. This seems immensely useful regarding encryption, because in my mind, this lets you mimic the public-, private-key model on the small scale.