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It's still the consensus that we can't (even theoretically) send a bit of information this way, right? Like, no quantum telegraph?

So we measure the particles and find that they are in inverse states. I assume there's a reason for this, but why are we sure that we're not just reading off a random number generator that's set from the same seed?

Third time I'm writing this in this thread, but it causes so much confusion: in order to quantum teleport one qubit, you transmit two classical bits of information, say by ethernet. So there's no way to take this to work as a sneaky means of sending a classical bit.

It's my sacred duty to keep repeating this fact until the whole internet knows. (It's even in the first sentence of the wikipedia page on quantum teleportation!)

Quantum teleportation is actually kind of the opposite. You transmit a quantum state (which if your particle is as simple as a photon, can be used to create a copy of the particle) by using an already-separated entangled pair and sending two classical bits any boring old way (fiber, wire, smoke signals).

If you don't get the classical information on the receiving end, any attempts to read the quantum state will garble the quantum transmission such that it'll be indistinguishable from random noise. Since those classical bits are limited by the speed of light, so is the overall transmission of the state.

So why does the reciver need the two bits?

I'm not prone to study too deeply because I've learned (apparently) very bad analogies or schemes for entanglement.

The teleportation process involves using a measurement to entangle the receiver's side of the pair with the original state in a useful way. Since measurements are probabilistic (and the measurement is along two qubits) there are four outcomes of this measurement with four different kinds of "garbling" of the transmission (one of which is no garbling).

The receiver needs the two bits to choose which of four kinds of garbling occurred, so it can correct them with the appropriate post-processing (if necessary).

So it's kind of like a traditional error-correcting code, just sent along a separate channel?

Sort of. The only issue with that interpretation is that either piece of data in isolation conveys none of the original information, even probabilistically. A better analogy would be sending two classical bits by different channels, with the desired bit being the XOR of the two, and one of the bits being chosen by a random number generator.

> why are we sure that we're not just reading off a random number generator that's set from the same seed?

We aren't. In fact, according to the decoherence / many-world interpretation (and possibly others) that's exactly what happens.

When you "measure" (aka. entangle yourself with) one of the two photons, you end up (find yourself) in one of the infinitely many possible "worlds", in which both photons have a single, definite state (always the opposite of each other, in this setup.) It's as simple as that.

I keep recommending this sequence of blog posts as the best introduction to the topic:

http://lesswrong.com/lw/r5/the_quantum_physics_sequence/

"Duplication" was the wrong term, I should have said "destructive-read/write", which I did in another comment.

2. There is no teleportation in the common man, sci-fi interpretation. In order to effect the information transferral we need three times as many particles as we're transferring the information of. Per particle whose state is to be tranferred, we need two entangled particles and a target particle. To translate that to the "teleporting a thing" case, if you were to teleport a cake, you'd need a cake, two (entangled) cake-equivalent mass collections, and a target "generic particle collection" to be turned into "your cake" after running the QT algorithm. It's only teleportation if you pretend that triplicate wasted-once-used mass isn't there, which we can't do: it's there, and this is QT, not 'regular' teleportation (ignoring the fact that there is no such thing outside of works of fiction =)

3. fair enough, I simplified that the wrong way.

https://en.wikipedia.org/wiki/Quantum_entanglement and https://en.wikipedia.org/wiki/Quantum_entanglement#Applicati... might work better as highlighting links here, although something like "Quantum Computation and Quantum Information" by Nielsen and Chuang would be a much better reference.

What common sci-fi man?

> The particles when measured will result in the other particle ending up in the opposite of the measured state, and this inversion persists even if we do operations that preserve the quantum state.

This is the part that I don't get - what's the physics mechanism behind this? It sounds very Schrodinger's cat.

It also sounds like a great way to tell if your information has been unknowingly accessed when it's travelling between two supposedly secure locations.

Outside of the realm of speculative theories about how measurement works (none of which have managed to really shed any light on the matter yet), there's not much known here other than "that's what happens when you use the prescribed rules for measuring a quantum system on a particular quantum state we call entangled." I promise if we ever get a concrete answer to that question it'll get posted to HN shortly thereafter XD.

> It also sounds like a great way to tell if your information has been unknowingly accessed when it's travelling between two supposedly secure locations.

That's actually one of the currently possible applications of quantum mechanics to cryptography[1]. There are already multiple commercial implementations!

[1]: https://en.wikipedia.org/wiki/Quantum_key_distribution