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.
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?
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!)
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.
I'm not prone to study too deeply because I've learned (apparently) very bad analogies or schemes for entanglement.
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).
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:
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.
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.
> 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!
That's basically the https://en.wikipedia.org/wiki/Hidden_variable_theory
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.)
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.
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)
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.
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.
[1]: https://en.wikipedia.org/wiki/Digital_physics#Wheeler.27s_.2...
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.
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.
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.
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.
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.
But yes, I think phenomena are the atomic building blocks of science. The logical positivists at least got that right.
Lacking context, this could seem somewhat opaque.
Would you mind clarifying which definition of "phenomenon" you are intending to convey? (eg Kantian or otherwise)
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.
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?
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.
This is false. The quantum data is completely destroyed at the entry point and it shows up at the end point.
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?
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.
Depends. Some versions are materialize/dematerialize, some, IIRC, are basically traversable short-lived artificial wormholes.
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.
> 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.
I agree with you, this is impressive and probably will be very helpful some day.
"Quantum teleportation" is a process of information duplication using particles that already exist, and have been positioned such that there is enough distance between them that we can rule out direct interaction between them (that we know of given the current state of physics). Quantum teleportation is the process by which we then manipulate only the particles on one side of the distance divide, such that particles on the other side "end up" reflecting the same state that the ones we manipulated were.
Although "ending up" is probably the wrong term, because we use particles in special states of which we already know they're entangled, then split them up (which does not cancel entanglement) and then we make use of their entanglement property: running an algorithm involving particles on one side should yield the exact same result as running the same algorithm on the other side, so a much more interesting algorithm is one that you run on one side in one way, and on the other in a different way, to effect a "data copy" without ever actually copying data (and very much without any kind of teleportation. The fact that you run your process with "the same particle" is the special part. Being able to even have two particles that are literally the same is a pretty bizarre bit of physics)