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Communications Math

The Viterbi Algorithm and Quantum Communications 91

Posted by ScuttleMonkey
from the sneaky-states dept.
eldavojohn writes "There have been a lot of tests in using quantum mechanics to communicate across large distances. But a student & a professor at USC have proven that the Viterbi algorithm can be applied to quantum communication. In the traditional Alice sends Bob a message scenario, 'Bob can reliably spot errors, and knows which message qubits are bogus before he opens the message — crucial, because opening it destroys it; and if it is garbled, he has nothing.'"
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The Viterbi Algorithm and Quantum Communications

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  • Alice (Score:3, Funny)

    by arizwebfoot (1228544) on Monday August 04, 2008 @01:41PM (#24469653)
    So . . . when is ANYTHING Alice says not garbled?

    -----
    Spoken like a true married man.
    • Re: (Score:2, Funny)

      by Tenrosei (1305283)
      And for that matter what is the chance Alice will just say it once?
    • by Anonymous Coward

      > So . . . when is ANYTHING Alice says not garbled?

      Whenever Eve isn't listening. Of course, given that our government is spying on everyone, you're right. You can simply DoS the connection by spying on them all the time.

      Crap, did I just break quantum encryption?

      • Re: (Score:3, Interesting)

        by hairyfeet (841228)
        I know that you are probably joking,but that is exactly why quantum communications will never take off. There is no major government in the world that will allow communications that they cannot spy on,and since doing a MitM attack on quantum breaks it they will simply never let it get past lab stage. The second anyone tries to roll something out in a big way using quantum the governments will scream "its for teh terrorists and the kiddy pr0nographers! Think of teh childrens!" and that will be the end of tha
        • With quantum communications using entagled photons there are no possibilities for any form of eavesdropping, since the photons are bonded pairs. You can't entagle photons that are separated, they must be, in essence, created as a pair, then distributed as endpoints of communication. This technology will eventually lead to the capabilities of having one centralized quantum computer that any number of users with an entaglement link to it can utilize, the power of all the worlds supercomputers accesssed in r
    • by pragma_x (644215)

      You're all going to die down here.

  • Alice 3 Bob (Score:5, Funny)

    by nategoose (1004564) on Monday August 04, 2008 @01:47PM (#24469745)
    I wish Alice and Bob would just go ahead and do it already. Everybody knows they have the hots for one another.
  • Alice? (Score:4, Funny)

    by RandoX (828285) on Monday August 04, 2008 @01:52PM (#24469849)

    Who the ---- is Alice?

  • So as long as he doesn't open it, it might be garbled, or maybe not. Isn't the fucking cat is dead at that point?

    I assume there has to be some sort of value in this discovery, but neither the summary or article seem to do a great job of expressing it.

    I thought the problem with quantum mechanics was in measurement, not knowing something is bad before you measure it.

    • by zehaeva (1136559)
      The value in this is that with these entangled photon's we can transmit data across any distance instantaneously. From here to anywhere in the universe. The only latency in the system would be in encoding and decoding the information. So you here on earth could talk to your grand children with no time delay even if they happen to be on Mars or some where around Beetleguise 4.
      • Re: (Score:3, Informative)

        by brunascle (994197) *

        The value in this is that with these entangled photon's we can transmit data across any distance instantaneously. From here to anywhere in the universe.

        No, you cant. It would violate relativity and causality.

        These quantum communication systems require a classical communication channel, which is restricted to the speed of light.

        • by zehaeva (1136559)
          It should, but what else do you do with two particle that spin in opposite directions from each other no matter the distance inbetween?
          • by ValuJet (587148)

            What would happen to the spinning of one particle if it was the subject to massive time dilation from either being close to the speed of light or within the event horizon of a black hole?

          • Re: (Score:3, Informative)

            by brunascle (994197) *
            You cant control the direction, it's random, so you cant use it to send information. You have to set up a classical channel, and in that classical channel you tell them how to piece together the information in the quantum channel to get the message.
            • Re: (Score:1, Troll)

              by X0563511 (793323)

              At this point. Who knows what we may figure out in time?

              Look where we were 100 years ago. 200 years ago. 300. Do you get what I'm hinting at?

              • by jfengel (409917)

                Yes. You're hinting that you didn't take any quantum mechanics classes.

                • by X0563511 (793323)

                  This is true. But my point being is that we only know that we do not know everything, we don't know how much we don't know. We've made so many advancements recently, so many that I would not rule out more advancements with quantum theory.

                  Perhaps not having an "inside" knowledge of the theory allows me to see it abstractly - as any other science.

              • by X0563511 (793323)

                What!? Troll!?

                Er.. I hope that was a mistake?

          • by DShard (159067)

            Describe a one time pad. The channel transmits true statistical randomness, which is the exact opposite of information.

          • Re: (Score:2, Informative)

            by closetpsycho (1175221)
            http://en.wikipedia.org/wiki/Quantum_teleportation [wikipedia.org] I'm not a physicist, but from what I could gather from the wiki page, when you make the change, Bob's qubit could become 1 of 4 different states. Alice needs to send (through a classical channel) information to Bob saying what changes need to be made to his in order to get the desired outcome. If somebody is eavesdropping, all that they get are the changes, not the starting state or the final state. Thus ends today's lecture on quantum communications.
          • Re: (Score:3, Informative)

            by BungaDunga (801391)
            I despise- despise- pseudo-scientific mystical mumbo-jumbo about quantum entanglement. Yes, Einstein thought it was "spooky". And it is, and the behavior of entangled photons is indeed counterintuitive and violates classical notions of probability. But it does not, as far as anyone knows, violate either the speed of light or causality. Google "tachyon pistols thought experiment" to see what would happen if it could.
            The basics of entanglement are thus: Person A produces an entangled pair of particles. He se
        • Re: (Score:2, Informative)

          No, you cant. It would violate relativity and causality. These quantum communication systems require a classical communication channel, which is restricted to the speed of light.

          Actually, you are wrong, since the communication occurs along the entaglement linkage, which is in a higher order dimension than space-time, which dimension it is all depends on which version of M-theory you currently ascibe to.

        • Re: (Score:1, Informative)

          by Anonymous Coward

          No, it would violate your perception of relativity and causality.

          It is quite possible that the information could follow a pathway that actually does go all the way across the universe, but if it's at the correct angle it really doesn't take much, if any, time relative to the position on either end.

          Stupid humans, still thinking that time is somehow different from distance.

    • by aoeu (532208)
      I think that the problem here, is how bad.
    • by vegiVamp (518171)

      The cat may or may not be dead, but if you wait long enough to open the box, it'll starve anyways.

  • by Sir_Real (179104) on Monday August 04, 2008 @01:53PM (#24469861)

    For those wondering what use this has.

    Say you had.... a buttload of code, and wanted to find the context free grammar for the language. You could use a Viterbi algorithm to pull out a statistically likely parse tree (the Viterbi Parse). The thing you're pulling from is often called a Markov process which is a model for the evolution of a state changing, memoryless system. So, over time, you can retrieve a grammar from a running process.

    How this applies to QM is left as an exercise to the reader (5 stars, unless you're Don K His-self, in which case it's 2).

    ianaqp

  • This could be huge (Score:5, Interesting)

    by xZgf6xHx2uhoAj9D (1160707) on Monday August 04, 2008 @02:08PM (#24470113)

    I'm not smart enough to figure out the details of what they've done, but it sounds like really promising work. "Communication" is perhaps too narrow a term for the applications, though.

    A big part of the problem with building quantum computers right now is keeping the qubits stable. The real world is constantly trying to "observe" (or interfere with) the qubits. When that happens, your quantum states break down and you lose your computation. This is a bit reason why we've only been able to build small (5-qubit) machines: it's very hard to keep things isolated and stable.

    If you have a practical error correction code scheme (using a Viterbi decoder, like in this article), then things might be a bit easier. Maybe instead of 5 very stable qubits, you could have 20 sort-of-stable qubits, where you expect that half of them will be lost to noise. It would still be a net win.

    • by hansraj (458504) * on Monday August 04, 2008 @02:15PM (#24470209)

      A big part of the problem with building quantum computers right now is keeping the qubits stable. The real world is constantly trying to "observe" (or interfere with) the qubits. When that happens, your quantum states break down and you lose your computation. This is a bit reason why we've only been able to build small (5-qubit) machines: it's very hard to keep things isolated and stable.

      [Emphasis added]

      I think the qubits' behavior is very suspicious. Surely if the qubits have nothing to hide, they shouldn't have any problems!

      • by Qubit (100461)

        Stop looking at me!

        Also, there's nothing under this huge tarp. Nope. Nothing at all...

  • by Falkkin (97268) on Monday August 04, 2008 @02:26PM (#24470369) Homepage

    TFA is a bit short on details, as expected for a general-audience press release. In particular, they throw the word "Viterbi" out there without ever explaining what the heck it means; probably an artifact of USC containing the *Viterbi* School of Engineering. The juicy technical bits can be found in his thesis here:

    Title: Quantum Coding with Entanglement
    Authors: Mark M. Wilde
    Thesis PDF [arxiv.org] ... and for a basic overview of the underlying concepts, of course the Wikipedia page on the Viterbi algorithm [wikipedia.org] is helpful.

  • I just wanna know when I can have my cake and eat it too.
    • by mcpkaaos (449561)
      If you don't mind a little bulimia you could always eat your cake and then have it.
    • by mikael (484)

      It's in the cake-box along with the cat. As soon as we know the state of the cat, we'll know the state of the cake.

  • Frank Shoemaker would call this noise.

  • Is it just me, or is the basic idea really rather obvious? (At least for someone with a basic understanding of both quantum computing and coding theory)

    That is, it's fairly obvious you can use convolutional codes with qubits, and if you did you could use the viterbi algorithm to decode it without destroying/measuring the original qubits.

    With that said, reliable communication of qubits may be particularly important for scale in quantum computing -- we can build a large quantum computer out of a network of sm

  • Ok, I've never understood this. I'm sure someone smart out there has the answer.

    Say you have a pair of entangled atoms, photons, whatever - and you move one of them across the universe (assuming you could keep the entangled state) and you flip the bit on the one back home. According to what I understand about entanglement, the particle on the opposite side of the universe immediately flips as well. Doesn't this violate the basic rule that the speed of light is the speed limit for the universe?

    Als

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