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You've compounded their lazy science reporting by making quite a few mistakes yourself in your description:

1. There is no duplication or copying of information, the original particle's state is destroyed. Copying is impossible when dealing with general quantum states[1]. (You may be thinking of measuring an entangled pair to randomly produce a stream of classical bits on each side that is the precise inverse of what is produced on the other side. This is not quantum teleportation.)

2. There is a very real notion of teleportation here, in that you take a particle with a quantum state A, and manipulate it in such a way that it stays where it is while causing a particle of the same type in a different place to be in precisely the state A, without actually moving the first particle to the second location. Yes, it's not likely to ever be used to beam anybody anywhere, but for photons it's very much like teleportation.

3. The two particles are not "literally the same", or even figuratively the same. 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.

[1]: https://en.wikipedia.org/wiki/No-cloning_theorem

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

It's easier to think these as random number generators seeded with same value, taken six kilometers apart from each other, and then fetching values from them. No data is transmitted when fetching values from these.

You're thinking of measuring entangled pairs to produce consistent streams of random classical bits. That is not what quantum teleportation is, although both do use EPR pairs. Their experiment is still bound by the speed of light because they are in fact communicating a full, pre-determined quantum state, not random bits.

Are we witnessing the first steps towards interplanetary turn-based seed driven gaming?!

Maybe!

I swear I shot that guy!

> random number generators seeded with same value

That's basically the https://en.wikipedia.org/wiki/Hidden_variable_theory

Not really. Entanglement isn't anything crazy. It's just a conserved quantity being split among particles during some type of constriction (half-silvered mirror would make the velocities of both deflected particles add up to a straight line for example)

The hidden variable theory makes it more complicated than it is.

If any type of interference or interaction happens to either of the particles after their initial mirror deflection, the correlation is gone. The complimentary nature of the measured values only applies when the particles remain untouched - thats why news articles a couple years ago were praising the ability to "entangle" particles for such a long distance/time. It's not easy to prevent all interaction.

The correlation is really just the undisturbed conservation of some quantity that was divided between the two particles.

In any "path" which splits a wave stream into x%/y% chance of trajectory/spin/polarization, etc, there will be an entanglement. All this means is that for every x% that went one way, y% went another, and the total is 100% ( conserved quantity.)

No, you're thinking about quantum entanglement.

Yes, and as others pointed out below, this is essentially entanglement.

This is an effect that uses entanglement to move the state of one particle to a different particle. You're describing only entanglement, so that's only half the puzzle.

And you're describing entanglement in a misleading way. The most important part of entanglement is that you can't model it as two particles sharing secret data. When you touch one, you affect the output of the other. And this happens faster than light. It still looks random on each end, but once you combine the measurements you can see patterns.

Is their any layman's physics explanation of how this works? Obviously you can't transmit information, but one particle can affect the other at distance faster than the speed of light?

My sense was that quantum physics basically makes sense if you go really small and have an understanding of how the particles interact. But entanglement is weird, because somehow there's also a mechanism for them to interact at distance?

(I know, I could try to look it up, but hopefully other people here have the same silly question)

Particles are constantly randomly changing their state. Entangled particles change in tandem.

As the GP said - to find out whether a particular change of state was random or a signal, one would need to compare readings from both.

Readings cannot be sent faster than the speed of light. Thus preserving causality.

While I don't have an explanation for you, you aren't the only one to think it's weird. Einstein called it "spooky action at a distance".

This is, very confusingly, "teleportation of the quantum state". Not unlike how merging a pull request is "teleporting the software state of your repo", but instantaneously. Essentially this is just entanglement at 6 kilometers, described by a blustery article.

No, "quantum teleportation" is a specific technical term for a type of communication that involves both entanglement and a classical communication channel to together create a secure communication channel.

https://en.wikipedia.org/wiki/Quantum_teleportation

The purpose of quantum teleportation is the teleportation of quantum information. Teleportation is not necessary to the creation of a secure communication channel.

Not sure about "instantaneously" part. It's more like I send you a pull request and then I apply it to my repo and tell you to apply it to your repo and you get your repo in certain state which I knew upfront. Repo state teleportation!

Yeah, it know it's not exactly that either, but it's about as magical as that, AFAIU. Which doesn't say it's not useful - ability to merge pulls and get all the repos in certain state is hugely useful - but calling it "teleportation" implying that we're on the verge of being beamed up and down like StarTrek characters sounds a bit more pompous than warranted.

At the subatomic level, is there any real difference between information and a "thing"? Seems mostly philosophical and arbitrary at that point.

Asking that question puts you in pretty good company[1]. There's a reason that the field of quantum information includes people looking fundamentally at physical systems, not just coming up with algorithms and implementations for quantum computers.

[1]: https://en.wikipedia.org/wiki/Digital_physics#Wheeler.27s_.2...

I think what you are really asking is: what is the substantial reality of 'the thing', if it is merely 'information'.

All following quotes are from the documentary "Atomic Physics and Reality" [1 - 'emphasis' mine.].

John Wheeler:

    Einstein admired Bohr and Bohr admired Einstein. And you
    recall that Einstein felt that 'reality' exists, in effect,
    'out there', something independent of us. And the position
    of Bohr was rather this: that 'reality' is only a word and
    we have to learn what the right way is to use that word.

John Bell:

    Bohr, I think, quite in early in life had decided that
    ordinary concepts such as space and time simply wouldn't work
    at the atomic scale. And it was always a bit unclear what
    Bohr would replace those concepts by, if anything. But
    Einstein was very attached to the space-time description, and
    thought that one should try very hard to extend it into the
    atomic domain.

    And this was the root of the disagreement between the two men.

David Bohm:

    Bohr had developed a view in which he essentially would say
    that 'reality' was unknown or unknowable, or even had no 
    meaning, 'reality' by itself. Reality, except at the large
    scale level of ordinary experience, atomic reality had no 
    meaning, and all that we have are 'phenomena', produced in
    an 'experiment', and that quantum theory deals with the inter-
    relationships of all these phenomena.

Abner Shimony:

    Bohr was not a professional philosopher. How much of the 
    traditional philosophers he read, one does not know. But
    he was an extremely intelligent man and he must have thought
    through some of the issues that professional philosophers had
    thought through very deeply. And certainly one of the issues
    that he had though through considerably, with some help I 
    think from reading, was the Kantian issue of the status of
    'the thing in itself'. The 'thing' as it really is, quite
    apart from Human 'perception' of it. 

    Bohr, like Kant, was very suspicious that one can say anything
    significant, intelligent, controllable, about 'the thing in
    itself'. [Bohr] once said that physics or science in general
    was not about 'nature' but about nature exposed to our 
    'observation'. And that is very Kantian.

--

[1]: https://youtu.be/BFvJOZ51tmc

Actually, I find it very intuitive that reality is just information. It's much easier than setting it apart as ineffable.

That's not what Bohr said. He said we can't know -- per Bell "have no right to know" -- what is "[objective] reality" [at the atomic scale].

I am personally not comfortable with using 'information'. I think Bohm's formulation "phenomena" is more appropriate. Note that, for example, the manifestation of wave/particle duality is an actual physical phenomena. The 'information' bit is our observation of this phenomena.

And in case the question is still on the table as to are these distinctions merely a matter of philosophical taste or aesthetics?, from same source cited:

Alain Aspect

   At the time of Einstein and Bohr it was just a theoretical
   discussion. You could endorse either Einstein's position or
   Bohr's position, and it was just a matter of taste. 

   And then there was the great discovery by John Bell in '65 that
   it is not only a matter of taste, because according to the 
   position you have at the subatomic point, you have predictions 
   which are not the same. 

   If you follow Einstein you have a certain type of predictions 
   -- that is to say you have results that are in agreement with
   what is called the Bell's inequalities -- and if you follow
   Bohr's position you have another type of prediction which are
   in contradiction, in opposition, with Bell's inequalities.

Aspect at this point had conducted a series of EPR experiments that proved that the Einstein line of reasoning was in contradiction with the obtained experimental results.

Well, I'd put "can't know" squarely in the ineffable.

But yes, I think phenomena are the atomic building blocks of science. The logical positivists at least got that right.

> I think phenomena are the atomic building blocks of science

Lacking context, this could seem somewhat opaque.

Would you mind clarifying which definition of "phenomenon" you are intending to convey? (eg Kantian or otherwise)

Information is a mental concept. That is, it's based on the someone knowing the range of possibilities of a certain state of affairs involving things/entities. For instance the top face of a dice. You know there are 6 possibilities, but at the moment, there's only one face at the top. Same with quantum particles, we attach properties to them, and these properties can have one value out of many. Measuring the specific value yields 'information' because you put that value in the context of the possible values you could find. The particle sent you a message, its properties.

But being related to reality, it's easy to be confused or rightfully associated with being a thing. I'd say that it's superposition of epistemology and ontology. You can't really separated to knower from the known.

Sure, but everything qualitative is information. What remains? Is there any other way to think about reality than information?

Seems kind of silly to pull apart information as somehow separate or lesser than reality. Saying "real teleportation" is somehow different than "just information" is nonsensical. It's like saying with quantum teleportation that the two particles aren't "really" entangled, they just have the same state. Well if you can't distinguish the state, what distinguishes them? They're literally the same until something distinguishes them.

Or, to put it another way, "just" information teleporting isn't any less exciting. "real" teleportation doesn't hold up to much thought anyway; it's very difficult to ground it in physical terms. Is there much point assuming people think about teleportation that way?

This is key and why most people don't understand why the name quantum teleportation really is appropriate.

And when you consider the only difference between two particles is the information state they are in. And that you do teleport the information. The name is not misleading.

What about position, though?

What do you mean by that?

"nothing disappears from one place and then shows up in another."

This is false. The quantum data is completely destroyed at the entry point and it shows up at the end point.

You do understand that this has applications that are quite useful though, right? I understand that you're objecting to the terminology, but the actual process is going to be a necessary part of any large-scale "quantum internet", and needs to be mastered.

Dumb, layman question: would it be possible to "quantum teleport" state information from the future?

I've seen articles that describe "sampling" entangled particles statistically without disturbing the entanglement (I assume these are large sets of particles). I've also seen articles that describe properties being affected by future states of the system as the wave function collapses. (Forgive my lack of citation, and poor recollection / understanding.)

Could properties like this be used in conjunction?

Isn't that how actual sci-fi teleportation would work behind the scenes as well? Basically, if I wanted to teleport myself from one city to another. I'd actually die and disappear, and an exact copy myself would be created instantly in the other city. It would be teleportation for the rest of the world but for me it would be death.

If they relied on quantum teleportation: no, not in the slightest. In order to QT a particle (explicitly not using the verb "teleport" here, because it's a very different thing) you need three additional particles for every particle you're QTing. A pair of entangled particles that act as your fascilitator, and a spare particle on the other end to take on the information state of the one you're QTing.

So unless your sci-fi teleporter comes with a massive amount of pre-entangled particles half of which you have, half of which "they" have, plus an equivalent-mass-worth of particles to accept the teleportation subject, it's still just science fiction.

> Isn't that how actual sci-fi teleportation would work behind the scenes as well?

Depends. Some versions are materialize/dematerialize, some, IIRC, are basically traversable short-lived artificial wormholes.

I found this Veritasium video nice for explaining quantum entanglement at a super basic level.

https://www.youtube.com/watch?v=ZuvK-od647c

right, "quantum teleportation" is a woefully misleading name, however this is still an impressive advancement and quantum teleportation will play a key role in wide-area quantum information networks

Since nobody has mentioned it, maybe I didn't understand the implications, but does this mean there is a chance in the future we won't have "bandwidth"? I could replicate data from a server into my personal computer instantly?

Not really. Not unless you and whomever you were getting data from shared entangled particle pairs. While this is a real thing already, some banks use this setup for quantum encryption, it's probabilistic (particles can stop being entangled, so you have to run checks against a series of them and then decide that p>0.999 is good enough for instance) and the owners of each particle is regulated. There is no QT without first making sure two parties each hold half of an entangled pair, and then make sure they both use the same particle for running the QT algorithm against.

quantum teleportation is not instant, since the information about the particle's quantum state is still sent over a classical channel

This is probably also why SETI-like projects turn up nothing. It's likely that almost no advanced civilizations are using radio waves for communication.

I'm only a layman who's mildly interested in this, but this contradicts my understanding... Someone correct me if I'm wrong here.

You can't communicate with quantum teleportation. It sends random "information" (not information at all), because you can't control how the quantum waveform collapses. So you have to collapse the waveform, create a key that encodes your collapsed form to the information you want to transmit, and then transmit that key slower than light i.e. with radio.

I guess you could argue that since the keys themselves are also indistinguishable from noise, we wouldn't detect slower-than-light key transmission either, though.

So far as I know, that's correct. There's even a handy theorem that states this pretty unequivocally!

> it is not possible for one observer, by making a measurement of a subsystem of the total state, to communicate information to another observer. The theorem is important because, in quantum mechanics, quantum entanglement is an effect by which certain widely separated events can be correlated in ways that suggest the possibility of instantaneous communication. The no-communication theorem gives conditions under which such transfer of information between two observers is impossible.

https://en.wikipedia.org/wiki/No-communication_theorem

Modern radio signals are pretty close to noise anyway. You increase capacity by going to the very limit of what your receiver can distinguish. The only thing you can measure from a distance is that certain frequency ranges have more power than background.

you can't "communicate", but it would be useful for qubits in one quantum computer to be sent to another quantum computer without physically moving the qubit. that was what I meant by wide-area quantum information networks

Oh totally agreed. My response wasn't to your comment, but to a comment implying that aliens are communicating "across" us but never through us via means of quantum teleportation and that's why can't detect them.

I agree with you, this is impressive and probably will be very helpful some day.

Thank you! This is definitely one of those things where my understand was inaccurate and this thread has been immensely helpful to improve that.