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Comment by Illniyar | Hacker News Reader

I thought quantum entanglement is the idea that two quantum particles separated by distance will behave the same at the same time regardless od distance.

Why is sending photons over distances a verification of quantum entanglement?

Can someone elaborate on what this is?

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It's more subtle than that. In this case with photons it's that the correlation between a photon going through a polarizer at point A and it's entangled pair going through a polarizer at point B is a function of the angle between the polarizers.

The odd thing is it depends on the relative angle of the polarizers at the time the photons go through even if they are far apart. So if you use the analogy of God rolling a dice to determine if the photon goes through or not then He has to know the angle of the other polarizer in a faster than light way even if us mortals can not know that.

Another factor to consider is that relativity showed that whether two events at different locations are simultaneous depends on the observer's reference frame. See (https://en.wikipedia.org/wiki/Relativity_of_simultaneity). So you can't even say which measurement of the state is the one that collapses the waveform. Observers in different reference frames would disagree.

I too am wondering about this. I thought entanglement couldn't be used for communication because observing both particles simultaneously will reveal that they have opposite spins. It's a conformation after observation, based on what the quantum superposition collapses into once you observe the state. But the actual state is also unknown until you observe it.

I'm not a physicist, but I'm sure there're be one on HN that will tell us what this study really means.

Well, if there is one, you will only know for sure after they reply. It's a conformation after observation too.

Not a physicist either, but as I understand it, entanglement is not useful for communication on its own, but allows for tamper-proof key exchange.

Basically, you generate a pair of entangled particles. Keep one of the pair and send the other away. Now both you and your partner randomly select a basis to measure the particle in.

After a few repetitions, you tell each other (over a classic channel) how you measured and discard measurements made in different bases. The remaining results will be strongly correlated, depending on the quality of the entanglement. By comparing a small part, you can find out exactly how much.

If the correlation is lower than expected, you might have some dust in your measurement apparatus, or an eavesdropper has intercepted a few quanta. If the correlation is good enough, you now both have a value that is secret to anyone who did not measure the particles.

The secret value can then be used as a key to encrypt your communication over the classic channel.

If this description is not satisfying, maybe the Wikipedia article explains it better: https://en.wikipedia.org/wiki/Quantum_key_distribution

Isn't that no better than a one-time pad if you need a classic channel?

Quantum key-exchange solves the distribution problem for one-time pads. And a one-time pad is the best security you could possibly get, so I'm not sure what you mean by "no better", since that's a given.

If you need a classical channel doesn't that just create another distribution problem though?

And yeah, but im considering one time pad as good as the security of its key in transfer

I am a physicist.

I only glossed the article, but from what I gathered, this didn't prove anything about QM, rather it was a technical achievement. One of the biggest problems with entangled pair distribution is actually separating the particles over some distance, because when the entangled particles interact with something ("observed") their state collapses and the entanglement may end.

This is what the achievement was, they managed to not destroy entanglement of a pair over a very long distance.

Probably it is similar to what has been done here:

http://www.cbc.ca/beta/news/canada/calgary/calgary-teleporta...

Information can't travel faster than the speed of light. You have to actually take the photons and move them away from one another, then check one which will modify both.

They are breaking a record, that's all.

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