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Comment by themgt | Hacker News Reader

Well, from my read, it'd be theoretically possible to transmit at near-infinite bandwidth using quantum entanglement (since you just keep adding more entangled particles). There'd still be the latency of at least the speed of light though

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Entanglement alone can't be used to transmit information. If this is the first time long-range entanglement has been possible then the communications advances they're talking about would include anything that requires parties to share entanglement over a distance. So quantum encryption, key exchange, superdense coding (is that even likely to be useful?) etc. Someone who knows what they're talking about want to weigh in here?

Entangled particles communicate instantaneously through space. That is to say, communication faster than the speed of light. This was actually one of the reasons that Einstein did not want to accept quantum theory. In fact, he proposed quantum entanglement as an argument against the validity of quantum theory. According to quantum theory, transfer of information faster than the speed of light should be possible (via quantum entanglement). Therefore, quantum theory was incorrect/incomplete- that was Einstein's reasoning.

There would still have to be an entanglement step. The photons need to be coupled. This is currently accomplished by a laser across long distances. I suppose that in this sense there is a latency at the speed of light to add new particles. But once the photons have been entangled they can communicate instantaneously.

No. Sorry to shatter the dream of FTL communication. You cannot achieve it with entangled particles. While it seems like there is some transfer of information going on its much better to think of two entangled photons as a pair of dice which will roll the same number wherever they are. Spooky, yes. Useful for cryptography, certainly. But sadly no FTL communication. At best its FTL transmission of complete noise.

Edit: See http://en.m.wikipedia.org/wiki/Quantum_teleportation for more.

Edit 2: If you say that the particles themselve are 'communicating' ftl, then you are perfeclty correct, yes. Unfortunatly one cannot use this behaviour to transmit arbitrary data.

I don't understand. If they can measure and compare the states of two entangled photons to determine that the states are identical, then couldn't they encode information in the intervals between state changes by measuring the photons at a previously agreed upon rate? Or is it simply impossible to impute a state transition on one of the entangled photons?

Comparison of the states of each entangled particle, to find the signal, requires communication (light-speed); so while there was information sent, you can't figure out what it is until you receive a signal from the source of the transmission, thus no causal violations.

This is also why it's interesting cryptographically, unless you have information on the movements of both particles you can't pull a signal from the movements.

Why would you need to compare the states of the particles if you know a priori that they are entangled? Doesn't the theory say that the states will always match? I still don't understand why one side couldn't just measure the interval between state changes and use that channel to read information.

So you couldn't get a predictive behaviour from each particle, without external communication? Am I understanding you correctly?

Correct. But in theory a small classical signal could be sent to essentially say "measure/extract data from the entangled particles in this way", and then the bandwidth from this communication would be based on the amount of data extractable from the entangled particles, while the latency will still be capped at however long it takes to send the classical signal

I think this was on HN recently. http://arstechnica.com/science/2012/10/quantum-entanglement-...

Ah, that's too bad. I was about to imagine future hackers who would tap into a secret FTL communication with their own particles to steal entangled data :)

Actually, it is an open question. Delayed choice entanglement seems to be able to send messages back in time, but the experiment was unable to tell whether it was communication into the past or simply an after-the-fact relabelling of events.

"In Quantum entanglement, part of the transfer happens instantaneously. Repeated experiments have verified that this works even when the measurements are performed more quickly than light could travel between the sites of measurement" http://en.wikipedia.org/wiki/Quantum_entanglement

Yeah it occurs instantaneously but because you can't influence either particle without destroying the entanglement you can't use it to communicate faster than the speed of light. http://curious.astro.cornell.edu/question.php?number=612

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