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.'"

Alice

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

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Spoken like a true married man.

Re:

And for that matter what is the chance Alice will just say it once?

Re:Alice

And for that matter what is the chance Alice will just say it once?

Go ask Alice. I think she'll know.

Re:

Go ask Alice. I think she'll know.

Well, we are talking about QM, where logic and proportion do indeed fall sloppily dead...

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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

Re:Alice

She's the chick that Bob is seeing behind Eve's back, if I understand the situation correctly.

Alice 3 Bob

I wish Alice and Bob would just go ahead and do it already. Everybody knows they have the hots for one another.

Re:Alice 3 Bob

Everybody knows they have the hots for one another.

Kids these days. When will they learn that its not worth the risk. Safe encryption should be practised all the time, not just when you feel like it.

Entangling...

So you're suggesting Bob and Alice get entangled? That's spooky... too bad we wouldn't be able to watch.

Re:Alice 3 Bob

oh crap, now you had to upgrade Schroodinger's cat to a fetus

Alice?

Who the ---- is Alice?

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It's a placeholder name, like a variable named "foo"

See http://en.wikipedia.org/wiki/Alice_and_Bob [wikipedia.org]

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She's friends with Lena and, inexplicably, a teapot.

Re:Alice?

You can get anything you want at her restaurant, I know that much.

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'Cepting Alice, of course.

Ignorant Post

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.

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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.

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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.

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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

This is for deriving information from Markov sets

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

Re:This is for deriving information from Markov se

And for those of you wondering...

Markov was Chekov's evil twin on Star Trek.

Re:This is for deriving information from Markov se

Just one nit to pick.

Generally, we are talking about hidden markov models. linky [wikipedia.org]

This could be huge

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.

Re:This could be huge

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!

Get the full Ph.D. thesis here:

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.