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The Viterbi Algorithm and Quantum Communications
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.'"
So . . . when is ANYTHING Alice says not garbled?
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Spoken like a true married man.
Beer is proof that God loves us and wants us to be happy.
I wish Alice and Bob would just go ahead and do it already. Everybody knows they have the hots for one another.
Who the ---- is Alice?
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
And for those of you wondering...
Markov was Chekov's evil twin on Star Trek.
Intron: the portion of DNA which expresses nothing useful.
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.
Just one nit to pick.
Generally, we are talking about hidden markov models. linky
So you're suggesting Bob and Alice get entangled? That's spooky... too bad we wouldn't be able to watch.
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.
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.
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 ... and for a basic overview of the underlying concepts, of course the Wikipedia page on the Viterbi algorithm is helpful.
Authors: Mark M. Wilde
Thesis PDF
http://en.wikipedia.org/wiki/Quantum_teleportation 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.
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 sends person B one half of the pair; if they're photons, probably through fiber-optic cables. Totally classical, speed-of-light communication. Now, the weird thing is that if person A observes his particle's spin, he can predict the spin of the other one with greater than random accuracy. That's weird, because both particles' spins are determined randomly (there's no "hidden variable" determined when they were created). But you can't poke one particle and see the other particle instantly wiggle, mainly because observing them destroys the entanglement.
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 that. So while I think it is interesting research,I doubt you will ever see it in real life unless they figure a way to copy everything sent and received without the subjects being spyed upon knowing it. But as always this is my 02c,YMMV
ACs don't waste your time replying, your posts are never seen by me.
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.
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