Slashdot Mirror
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.'"
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
It's a placeholder name, like a variable named "foo"
See http://en.wikipedia.org/wiki/Alice_and_Bob
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
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.
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.
Ignorance is Bliss -- And the Opposite is True -- Genius is Madness
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.