Researchers quantum teleport particle of light six kilometres
sciencebulletin.org
sciencebulletin.org
"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)
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.
For those thinking that this is a step towards faster-than-light (FTL) communication: As far as I know it's fairly certain that quantum entanglement will not allow for FTL communication. Basic principle is that while measurements between both sides will be correlated, it's not possible to tell how they are correlated until both sides compare measurements.
https://en.wikipedia.org/wiki/Superluminal_communication#Qua...
http://physics.stackexchange.com/a/203893
Given that, it seems like the touted benefit of using quantum entanglement here is in securing communications, since your measurements will no longer correlate if a third party is also measuring? At least, that's what I gathered.
Most scientists are fine with the idea that it doesn't matter if the larger physical system has any sort of "consciousness" or "observation"; isolated particles have quantum behaviors, interacting particles decohere towards more normal-seeming physics. However, there are theories that shouldn't be immediately discounted that do still ascribe a role to the particular observer, indicating everything from reality being relative to consciousness being a fundamental aspect of the universe. I wouldn't take these too seriously, but I wouldn't dismiss them out of hand either.
Of course, that's not addressing the worst thing about quantum mechanics: once you've finally rid yourself of the pseudoscience and poor metaphors that make it sound like magic, it seems for just a second like a normal, intuitive process that's just been obscured through poor explanation--then you learn a little more and realize it's far more bizarre than you'd imagined.
This is a great explanation of the process of learning about quantum mechanics.
A good test once you start to think you have figured things out is to check how your own conceptual model deals with Bell violations or experiments like: https://en.wikipedia.org/wiki/Quantum_eraser_experiment
Since this process converts quantum information to classical information (a digital or analog signal), we know that it must lose information. Only by making many measurements can we infer statistical properties of the entire quantum state. This is exactly what happens with the two-slit experiment.
I personally subscribe to the "shut up and calculate" interpretation of QM (or perhaps the projection-valued operator approach when I'm feeling mathematical). The above intuition has served me well so far.
Wow, I've never heard it explained that way. That's brilliant - did you come up with it? Interference patterns are the result of a huge number of individual photons interfering with each other at an enormous rate, and the result are laid out spatially...really lovely.
Actually (technical nitpick) it's even more lovely than what you describe: people have done the two slit experiment with verifiably single-photon sources... and you still get interference patterns. Really, we know that only a single photon exists between source and detector for the entire lifetime of the photon, and yet you get interference. The only explanation is that it interferes with itself, which is what QM says happens.
The original "Wow! QM!" two slit experiments were done with electrons, which were thought of as competely particle-like, whereas light was thought of as completely wave-light. QM of course says both are both.
I had thought the pattern represented interference between the probability distributions of where the photon might be found (which themselves follow "wavelength" and produce wave-like behavior). With two slits you get two distinct probability distributions that overlap spatially at the screen, which when added together cause denser/less-dense areas of photon detection and looks like an interference pattern.
That would have explained why streams of single photons "self-interfere" (they wouldn't--it's the underlying probability of their path that interferes with itself, so the pattern would eventually appear at any emission rate) but that seems so simplistic as to be unlikely to be the truth of the matter, especially with delayed choice and quantum erasure effects.
Do you happen to know a good layman's source for a solid explanation of what we know about double slit right now? I'd really like to understand it, if only at a high level.
No, they're both something else entirely that if you look at them the right way could look like they share properties with either.
It's stranger than that. A single photon can interfere with itself, see for example: https://en.wikipedia.org/wiki/Double-slit_experiment#Interfe...
Also I bet you could mess with the interference pattern by pulsing the light source. This would be the lesser known form of the Uncertainty principle, where energy and time are unknowable.
This view fails to predict the outcome of the quantum eraser experiment.
Shortest answer to that question: http://physics.stackexchange.com/a/3162
>Imagine at home you put one glove in your coat without looking (and noticing it's only one of the two). After exiting the train you notice it's cold and you pull out that single glove. At this very instant you know it's either the left or the right glove, and you therefore know which one is left at home. However, no information was transmitted by your "measurement". Of course in quantum mechanics this is more complicated because of the not entirely measurable wave function, but this is the basic idea.
Things get more complicated here because we talk about huge classical objects with a lot of atoms. And here it is practically impossible to obtain such a superposition because the glove interacts a lot with the environment. E. g. by looking at the bulge of your pocket you might be able to determine the glove type without opening the pocket.
The essence of the comment is that the correlation in a quantum case may be stronger than in a classical case such as the case with gloves (also true).
The comment doesn't invalidate the answer unless you think it does imply causation (non-locality). It just says that the answer is not the whole truth: that would be the complete mathematical description of QT with corresponding procedures to connect the math with observations.
The glove analogy obviously has its limits. It doesn't make it wrong. Otherwise, the accepted quantum theory is also wrong because it doesn't describe all known phenomena (consider gravity).
> "Particles behave like wave until observed"
Consider photon: it is neither wave nor particle in a classical sense. The math is such that in some cases it is easier to describe it as wave (interference pattern), in others—as particle (photoelectric effect).
It is surprising that the familiar concepts from a human scale (such as wave, particle) are useful in such a wide range of physical phenomena. Though only because they are more familiar does not make them more real than e.g., the concepts produced by the equations of the quantum theory.
All models are wrong—some are useful. (In a sense that there is no territory under a map, only more complete or different maps)
(A) Measurement and observation is NOT a simplified term for a well-understood complex process. It is an atomic term for a not-well-understood (therefore maybe simple or complex, we don't know) process which is really extremely simple if we take it at its surface meaning and don't poke inside it too far.
Let me take the simplest example, the Stern Gerlach experiment. This is really simple: put two long magnets next to each other with a "gap" between them, and preferably give them very different shapes; this creates an "inhomogeneous magnetic field" between them. It turns out a spinning electric charge, when it comes into such a field, should get deflected based on the direction that it's spinning.
Fire a beam of electrons at this apparatus, and you'll notice something interesting. Let me give some coordinates: if the gap between the two magnets is "horizontal" and the electrons go through it "forwards", they split into two beams, half going "up" and half going "down". We say that these have 'spin up" and "spin down" but that depends on a bunch of little arbitrary choices. If you put another apparatus "horizontal" in front of either beam, you'll notice that those ones going "up" all go "up" through the second set of magnets; likewise for the ones going "down".
So there's a lot to unpack here: first off, if they were normal spinny things, then this separation into two beams is really weird! Because what about an electron that's spinning "forward" or "left"? Why wouldn't nature recognize that mathematically it's spinning just as much clockwise as anticlockwise, vertically, and not deflect it up or down at all? So there should classically be lots of particles deflected between these two beams: it is very strange that this does not happen! In fact we can perform the experiment. We can use a second Stern-Gerlach magnet, this time oriented vertically, so that it deflects electrons into 2 beams going left or right, call these "spin left" and "spin right." Now we take the spin-left electrons and first put them through a vertical Stern-Gerlach magnet, make sure they keep going left, great. We just rotate the magnet and we find half of them go "up" and half of them go "down." Nature doesn't know the difference between "left" and some sort of 50/50 mixture of "up" and "down"; those are the same to Nature, at least where an electron's spin is concerned. And that finding is very robust: "up" is a 50/50 mix of "left" and "right" so if you use another Stern-Gerlach magnet on the electrons that went left and then up, you do NOT see them all go left again! They will all go up if it's horizontal, but they will not all go left if it's vertical. Instead half of them go left and half of them go right!
Now, we have a very folksy understanding of what we mean when we measure these things: we stick a very sensitive electron detector in some place, connect it to a counter, and we watch the counter erratically tick upwards. Of course it is so sensitive that it ticks upwards due to all sorts of other noise sources, even without a signal, but when we turn on the electron gun we start to see that in some places, pointed at the place where the beam hits the gap between the magnets, it starts rising much faster than the noise would provide, and in some places it doesn't rise any faster at all; it's all attributable to noise. That's how we know there are these two beams coming out; we move this detector around and see some peaks in the detection rate.
Now, a lot of our explanations of how the electron can do all of these weird interactions with the magnets, depend on saying that the electron does not just take one path at one time! Instead maybe it is "spread out" in space or it "takes all the paths available to it" or something -- these funky interpretations make the rest mathematics used to describe the electron unbelievably simple, you just have these "unitary transforms" and this "linear evolution" and all of that complicated quantum mechanics stuff is super-simple mathematically. But when we measure, we just see this counter jittering upward with highly unpredictable increments but with some very predictable average rate. Most of those clicks we'd like to think are actual electrons which have made up their mind to take this path or that path and have successfully made it to the counter. How this happens, is something of a mystery. If the electron takes all paths, why does it end up here or there? If it's spread out so that it's half on this detector and half on that detector, why do we see the counter increment and not, say, fuzz between the two numbers, having half-incremented and half-not? Why isn't our world more fuzzy, if it's made out of these fuzzy probabilities and amplitudes at its core? And yet why, when we use scanning tunneling microscopes, do these electron clouds of these atoms look like little balls, as if those electrons really aren't spread out over all that space but occupy one single place all of the time?
The mystery comes because there's a lot of really easy ways to explain this funky Stern-Gerlach stuff, but most of them view the world in a way that's alien to our own. The measurement problem is "we know how measurement works pragmatically, and it never showed us this alien world before, so how is this alien world 'collapsing' into the familiar world that we all know and love?"
(B) Entanglement has to do with strange correlations between remote systems which you can only notice when they are brought back together. My favorite example is a game where 3 people compete as a team in several trials where we secretly put them at cross-purposes to each other, call it "Betrayal." We split the team of 3 people into 3 separate rooms and we prohibit communication between team members. Each room has a screen that we display a goal on, and two buttons labeled 1 and 0. Sometimes the displayed goals for an individual and the actual goals for the team will be at odds; the individual never gets any reward in these cases: it's only, "if the team gracefully recovers from our meddling 100 times in a row, we will give them all a big cash prize."
Okay, so how do these work? Once the people are settled in their rooms, 1/4 of the time we will broadcast a "control round" where we tell them all "make the sum of your three button-presses even," and start a countdown timer. They win if they all push exactly one of their buttons once before the time is up, and the sum of their pushes is even. Really simple. Then 3/4 of the time we will choose one of them at random to be a "traitor" to the other two: we tell the traitor, "make the sum of your three button-presses even," but we tell the others "make the sum of your three button-presses odd," and the team wins only if they each push exactly one of their two buttons once, and the sum is odd."
It's easy to prove that you cannot win this game more than 75% of the time classically; each of the 4 situations is represented by some equation among the 6 correlated random variables, but when you add all 4 equations together you find out that they reduce to 0 = 1, an obvious contradiction, so they can't all be simultaneously satisfied no matter how you correlate the random variables. It is also easy to prove that if they all start out with a class of entangled states called a "GHZ state", such as
|+++> + |---> = |000> + |011> + |101> + |110>
|+++> - |---> = |001> + |010> + |100> + |111>,
But, you can't use this spooky collaboration to transfer information faster than light. And that's precisely because you can't discover that the two measurements are correlated until you compare them! We instantly correlate but we aren't instantly aware of our correlation; I can't figure that out until you send me a message saying, "hey, is your set of numbers X?" and I say "yes it is! woah! spooky!"
A measurement is an interaction of the quantum system under test/consideration with another system / component that is classical (non-quantum, e.g. big).
Why? Because otherwise we would have to specify the exact details of every measurement device. Model number wouldn't be enough. What is the full quantum state of photon detector with its enormous number of atoms? Unusable.
Entanglement isn't some spooky thing. In fact it's required for us to be able to measure anything. The measuring device's quantum state must become entangled with the quantum system it's measuring. We can't really consider each one separately, but we try anyway, and with great success if we do it right.
I like James Binney's take on this: QM is adult physics. It's an admission that when you measure something, you disturb it. When you measure something really small, you disturb it alot, and can't idealise that away as you can for classical macroscopic systems.
He explains it better than I can: https://www.youtube.com/watch?v=NKPI_wurNlo&t=37m20s
EDIT> I should add that for a measuring device, we want it to be correlated to the system under test. I.e. I could make a thermometer that always read 5C (uncorrelated to environment), but it would be a bad thermometer. Quantum entanglement is how this correlation is set up. But it means that now the future evolution of the quantum system depends on the (unknown to us) exact state of the measuring device.
This is demonstrated by the single-particle double-slit experiment [0] and the eraser variation [1].
[0] https://en.wikipedia.org/wiki/Double-slit_experiment#Interfe...
[1] https://en.wikipedia.org/wiki/Double-slit_experiment#Delayed...
Can someone correct this analogy if I'm wrong?
Would that break the entanglement?
http://blog.rongarret.info/2014/12/quantum-teleportation-dem...
(Also submitted to HN: https://news.ycombinator.com/item?id=12549728)
The premise upon which the idea that it is impossible is based is an axiom:
https://en.wikipedia.org/wiki/Wightman_axioms#W3_.28local_co...
Not a conclusion from data or other, simpler premises. It is an axiom that has nice consequences and is generative of much that describes our observations, but it remains an untestable assumption.
This is entirely incorrect. All of the proofs about broken causality (all of which stem from special relativity, since non-relativistic QM doesn't put the speed of light on a pedestal) assume that the laws of special or general relativity hold everywhere.
What you're saying is the equivalent of claiming in 1914 that the theory of special relativity proves that physics cannot operate without an intertial reference frame.
QM and entanglement are really red herrings with respect to the FTL question. The impossibility of FTL is a direct logical consequence of the fact that there exists a universal speed reference in space-time. Entanglement only appears to violate this because people don't understand what entanglement actually is and how it works. When you make a measurement on one member of an EPR pair, nothing happens to the other particle. It's still in exactly the same state it was in before the measurement. So there is absolutely no reason whatsoever to believe that FTL could be possible, and a lot of reasons to believe that it's not. Granted, this is not "absolutely certain" (nothing in science ever is) but it's as certain as it gets.
This is not true. QM theory in general (without talking about particular Hamiltonians, field or otherwise or not) has no reliance on special relativity; it doesn't even have a reliance on positions in three-dimensional space (physicists who work in quantum information routinely work with QM at this level)! So again, you're already specifying QFT (e.g. by something like the Wightman axioms), which includes special relativity as an assumption.
> The impossibility of FTL is a direct logical consequence of the fact that there exists a universal speed reference in space-time.
This is what special and general relativity say. Not only are you begging the question again, but GR doesn't rule out violations of causality: there's a bunch of known models within GR that violate it[1]. Since GR is in fact more certain than SR (in that SR has strictly less explanatory power than GR since it's a special case of it), you must believe that it is as certain as anything else in science that causality violations are real!
You're right that GR doesn't rule out closed timelike curves, but to actually construct one you need to have matter with negative mass. So you can interpret this in two ways, as a prediction that FTL is possible, or a prediction that negative-mass matter does not exist.
Nonetheless, since you clearly understand physics, I'll restate my claim more precisely: the likelihood of FTL being possible is extremely small, for all reasonable purposes indistinguishable from zero on current scientific knowledge, and the existence of entanglement does not change this one bit. It would take something like the discovery of exotic matter with negative mass to move the needle on FTL.
You only need matter with negative mass if you want to construct one by building a machine. Some models for closed timelike curves that could occur in nature (e.g. the Tipler cylinder with infinite length) do not need negative energy.
> the likelihood of FTL being possible is extremely small, for all reasonable purposes indistinguishable from zero on current scientific knowledge, and the existence of entanglement does not change this one bit
Okay, that I agree with, but this unlikelihood is completely orthogonal to (pre-relativity) QM's compatibility with FTL travel/signaling. That was what I disagreed with.
> You only need matter with negative mass if you want to construct one by building a machine.
As I'm sure you're aware, things do move faster than light and/or backwards in time on the microscopic level. But that's not what most people care about when they talk about this stuff. They want to know if you can build a time machine. And the answer to that (on current knowledge) is a definitive "no".
Well, Lisp is secret alien technology: http://lispers.org/
Is there a rule for describing the entanglement of the resulting constituent particles with the pre-existing particle?
If it's possible to actually break entanglement(so that measurement of a qubit resets to 50%-50% random), it seems like you could transmit information faster than light by preparing a 100%-0% distribution with quantum gates and destroying the particles. Then the first particle would change the distribution of its measurements. And so you could prepare 1000 qubits and measure them every hour, waiting to observe a change.
Imagine you had a machine print out two envelopes with the same random number inside. You and a friend take these envelopes and each go 1 lightyear in separate directions. When you open your envelope, you will instantly know what number was in your friend's envelope. There's no usable FTL communication here. Burning your envelope won't communicate anything FTL to your friend. This just means that when you meet up again, he'll say he believes that you got a 7, and you'll say that he's wrong because you threw away the entanglement first by burning your envelope without looking at it.
There are plenty of interesting quirks about about quantum entanglement that this model doesn't contain, but it should cover the lack of FTL communication potential correctly.
That is a very good idea, but doesn't work. The only way to "destroy an entanglement" is to bring the entangled particles back together.
http://www.flownet.com/ron/QM.pdf
or the movie version:
https://www.youtube.com/watch?v=dEaecUuEqfc
And if you get through those you might enjoy this one too:
http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
Suppose a proton A is entangled with a proton B. If B hits an anti-proton C, it gets converted into pure energy(maybe light and heat?). If it's impossible to destroy entanglement, then the subsystem consisting of the light and heat must be entangled with the original particle.
Observations:
* The dimension of the vector space has increased, since there may be many constituent photons released. So it may be impossible to assign a specific dimension to any quantum state.
* What is the rule for determining the resulting quantum state? Is there a book or article?
* If you convert a particle entirely into an electromagnetic wave or gravitational waves, and then reconstruct it, does that preserve entanglement? What is the rule for describing how the dimension of the vector space changes as it undergoes multiple transformations? If entanglement is allowed to propagate to a continuous field, then it seems like you'd have an uncountable infinite dimensional vector space to describe its state.
Yes. Exactly right. That is how/why decoherence happens. That is the reason the universe appears to be classical, and why the arrow of time seems to move in only one direction.
The rest of your questions are essentially asking me to teach you all of QM, and that's obviously not possible in an HN comment. You'll have to take a class or read a book.
I'm sure they did nothing of the sort. At best they transferred an unknown state of a photon to another photon six kilometers away, then confirmed via measuring both.
There is a good reason to call it quantum teleportation and not quantum facsimile.
When you move matter from one place to another it moves, it's not copied. When move classical information (or state) , you can retain the original or multiply it many times.
Quantum teleport demonstrates interesing aspects of quantum information that makes it work like material thing. When information (quantum state) moves, it can't be copied (no-cloning theorem). You lose the original from the sending end just as you would lose a physical object you send to other place.
Quantum theorems like no-hiding theorem, no-deleting theorem, no-cloning theorem makes quantum information act more like material substance than classical information.
There is no physical of philosophical argument differentiate between "real" teleportation and "just moving state".
In order to effect Quantum Teleportation, we run a process that literally wrecks one of the specially prepared bodies, _kills you_, and then if the second step of the process is run correctly on the other pair of bodies, wrecks the other specially prepared body and _restores you_ as you were before the cache-and-restore process, in the previously inert body.
And that's only if the second half of the process gets run correctly. It's quantum operations, so any quantum interference may disturb the system and you stay dead forever. Or, in QT terms: entanglement is lost and the information obtained is no longer correlated.
This is so far removed from what non-science considers teleportation that it is a misnomer at best, and a populist term when used outside of science at worst =)
[1] http://www.tuhep.phys.tohoku.ac.jp/~hotta/extended-version-q...
[2] http://journals.aps.org/pre/abstract/10.1103/PhysRevE.93.066...
Matter is not being moved here.
"There is no physical of philosophical argument differentiate between "real" teleportation and "just moving state"."
Sure there is.
Arguably - that nothing was copied or transferred in the first place and that the particle was always in a given state until it was observed.
something that we don't know is happening
That said, this is a chain of comments that skirts dangerously close to not-contributing IMO, since I'm not sure making cryptic comments that lead readers that don't happen to reach the right context sensitive interpretation to produce more comments asking about it aren't actually worse than a pure non sequitur, from a discussion point of view. Having a clever counter-example (as the GP does) is good. Leaving it unexplained and ambiguous is not.
https://en.wikipedia.org/wiki/Identical_particles
Since there is always some degree of uncertainty w/r/t location and velocity, there's no way to tell which particle is which over time.
> “The challenge was to keep the photons’ arrival time synchronized to within 10 pico-seconds,”
> “Since these detectors only work at temperatures less than one degree above absolute zero the equipment also included a compact cryostat,” said Tittel.
The dark fiber seems like it was important for synchronizing the clocks. And while they claim this could be used for encryption keys, that is really a roundabout way of saying that very little information was actually transmitted/received, although the article doesn't say exactly how little.
If this technology was refined, you'd just use this system to send secure messages without the need for an encryption key.
As far as I know, I don't think that meshes with what actually happens. This has nothing to do with the message being secure; anyone with all the pieces could read the message.
The important part is, since observation effects quantum state, the receiver can determine whether a third party observed the key. If the key matches, then there's a guarantee that it was not observed, and therefore is safe to use.
But, there are quotes in this article to the contrary. e.g.:
“Such a network will enable secure communication without having to worry about eavesdropping, and allow distant quantum computers to connect,” says Tittel.
Was my understanding mistaken?
https://en.wikipedia.org/wiki/No-communication_theorem
But together with a conventional communications channel (e.g. radio signals), you can use quantum entanglement to create a secure communications channel. This combination is called quantum teleportation (as opposed to just quantum entanglement):
My understanding is this:
1. send entangled bits to two separate locations A and B
2. determine their states (which will always be exactly opposite) on both sides
3. send data using classical means from A to B, xor'd with the quantum measurements
4. decrypt the data at B, with: data ^ quantum_measurements ^ 111111 (the last step being "invert all bits and ^ representing xor).
If that's it, how is this much better than sending block of identical entropy to A and B, and using a OTP? Theoretial tamper-proofness?
If you have a secure way to distribute your OTP I'm not sure there'd be a benefit to using quantum teleportation at all.
No, because you do not choose the information to be verified on either side. You're not choosing a key and "encoding" it into "quantum bits", you're measuring the quantum state of two entangled particles.
If the quantum state matches, you have provably secure transmission. If the transmission were intercepted, the act of intercepting it would cause the measurement verification to fail due to the observer effect. The parties would be able to see that the key was intercepted because the particles were prematurely disentangled.
This is quantum key distribution, which is a subset of (and often misleadingly conflated with) quantum cryptography. It is not possible to use quantum cryptography to send provably secure arbitrary data of your choosing, it's only possible to generate and distribute provably secure keys and use those for classical cryptographic communication.
The reason why it's important to call it quantum key distribution is because it makes it clearer that this only solves one problem in a greater cryptosystem. Key distribution is a significant problem in cryptography, and provably secure key distribution is a great leap forward, but it's not the only problem.
How would you tell the difference between a set of qbits entangled with the right other set of qbits, and with a different set of qbits? If an attacker were to intercept the whole stream/set of entangled particles, wouldn't he be able to perform a MITM attack by sending you particles entangled with his particles instead?
The way I mentally model "sending entangled particles and measuring them" is "sending two streams or blocks of exactly inverted random data to two locations, in a format that is impossible to recreate/forge as setting the right spins of particles is not possible". Is it so that this measurement can only be performed once, or can it be measured again later, giving the same results? Does the security of quantum entanglement come from an inability to create entangled particles with specific values, and if so, how does that solve wholesale MITM'ing, were the hole stream is intercepted on the way to one party, and then replaced by a wholy different stream with an attacker-controlled "twin" set/stream?
What I mean is, if one or both streams/sets of entangled particles were intercepted as a whole before arriving at A or B (or replaced before measurement and after arrival), wouldn't that allow for an effective MITM attack? I just find it hard to see how this technology solves the physical access problem.
A third party also viewing the entangled qubit would necessarily modify the measurements also, causing correlation failure. A man-in-the-middle attack would only be effective if they also intercepted the measurement information and replaced it with their own. In which case, yes naturally someone who can intercept all your communication and replace it arbitrarily can arbitrarily control your communication.
As is used today, signing the measurement information would be an effective mechanism for preventing this. The attacker should be unable to correctly sign their own measurement information, and so you will be able to detect that the key transfer is unreliable.
Once the key is reliably transferred, then it's theoretically a secure one-time pad. That is, you know that it was transferred to the party at the other end of the entangled qubit, and that it was not intercepted along the way.
In short, quantum entanglement is an effect that causes two quantum particles to share state instantaneously over arbitrary distances. It can not be used to transmit information faster than the speed of light, essentially because while it is possible to manipulate the particle at one end, it is not possible to arbitrarily set it to a chosen state (and as described fully in the no communication theorem)[1].
Quantum teleportation is a way to transmit quantum information, ie the quantum state of a 'qubit', using both quantum entanglement and a classical communication channel. Because classical communication is required, no faster than light communication is possible. However, quantum teleportation is necessary if you want to transmit quantum information.
To very briefly sum up how it works, you start with a qubit whose state you want to transmit, along with two entangled particles, and a 'receiving' qubit that will receive the state of the sending qubit. Through an interaction between the sending qubit and the entangled particle on the sending side, the quantum state of the entangled particles is set to one of four possibilities. Which of the four possibilities resulted is sent via the classical communication channel from sending to receiving end. The receiving end then uses that information, along with the receiving-end entangled particle, to manipulate the receiving qubit into the identical state as the sending qubit, thereby 'teleporting' that state from sending to receiving end. The Wikipedia article has a more thorough layman's description, as well as the underlying math[2].
Caveat: I'm an engineer, not a physicist, so I may have made a mistake here as well, but the main take-away is that quantum teleportation is not the same thing as quantum entanglement, and its purpose is not FTL communication, but rather communication of quantum states.
[1] https://en.wikipedia.org/wiki/No-communication_theorem [2] https://en.wikipedia.org/wiki/Quantum_teleportation
Isn't dark fibre either unlit capacity or leased fibres?
The thing is, although the quantum state of the photom collapses instantaneously on both entangled photons, we can't manipulate at which state the photon will collapse, and so, we can't really send information using quantum entanglement.
We can't also confirm the entanglement until we can compare both measurement results or else we could send information from the future as in the quantum eraser experiment https://www.youtube.com/watch?v=2Uzytrooz44
Edit: formatting and typos
So, i stopped reading :)
These ubiquitous subscribe forms provide a vector for inflating the size of mailing lists. Mailing lists with more addresses sell for real money, and sell well.
It doesn't matter if the data is accurate. Only the perception of the data matters. The more addresses the better, the more datapoints per address, the higher the quality of data. The age and activity against the data matters.
There are reputable companies that trade on these details and pay the salaries of expensive developers.
> https://en.wikipedia.org/wiki/No-communication_theorem
If you have superluminal communication, signals can travel into your subjective past, which means causality pretty much breaks down entirely. There's no theoretical reason why causality has to hold, but it would make life extremely "interesting"
I imagine that what you're describing about superluminal communication could be used to make a gadget like this.
I'm also wondering if anybody knows which webcomic I'm referring to. It'd be embarrassing if this turned out to be an XKCD number...
http://www.nature.com/nature/journal/v436/n7047/full/436150a...
It sounds more like an SMBC comic than an XKCD one, societal collapse for unexpected reasons is an ongoing theme.
"or one millionth of one millionth of a second"
The article itself says (or at least is written to imply) that ten picoseconds is one millionth of one millionth of a second.
Since ten is not one, one of those two statements (and definitely not ten of those two statements) must be incorrect.
>“The challenge was to keep the photons’ arrival time synchronized to within 10 pico-seconds,” says Tittel. “That is one trillionth, or one millionth of one millionth of a second.”
The media plays fast and loose with terminology either out of plain ignorance or the desire to sell a story.
Maybe I'm remembering my QED special topics class incorrectly, but I thought the state teleportation is believed to be instant. The problem is to extract the state information, the transformation operators still need to be communicated. So you still have to get on the phone to tell your colleague across the planet what to do to see the entangled result. It's been years, so maybe I'm a little off on that?
Causality also isn't a hard-and-fast law. We've just never seen it violated before. It's one of those physics problems that is really hard to find evidence to support or discredit it.
I understand the temptation to think that every "law" is just waiting to be broken, but I can easily read the same data and come to the opposite conclusion about the strength of our current "laws".
Why can't it happen simultaneously? Why is distance relevant in this case?
See my top-level post for more information [1].
This is the part where you are potentially wrong. Entanglement is about correlation. In order to derive the original message, you need the local entangled state, along with data from the remote actor. You could then correlate this information to derive the original remote entangled state -- ie, some message. Of course, this is now useless to you, as they could have simply sent the original message and saved you the correlation step.
However, I don't know of any reason that the local quantum state could not be measured prior to the arrival of the data from the remote actor. The math says that the measurable effect should happen instantaneously. But that data alone is not sufficient to derive a message.
The communication does not really happen between the state that is 'teleported' - you still have to manually send the photons 6km or whatever. But - very crudely - you can pass information from one entangled particle to it's pair 6km away and observe the results.
So, you still have to have a fiber or whatever.
Also - because it's all probabilistic and you don't know for sure if it worked in any given instance, you still need another classical channel to communicate information about what was passed in the first place.
The receiver does some operation on their entangled photon based on the results of the senders measurements. After these operations their entangled photon will be in the exact same state as the sent photon.
So to answer your question, information is transmitted classically but that information would not be enough without the use of entanglement.
> So teleport in this case doesn't really mean teleport
I would disagree with this statement. For all intents and purposes, the photon receiving the "transfer" is now indistinguishable from the photon that was transferred, before the event took place. That is to say, it has a completely identical quantum state following the transfer, and the "original" photon no longer has that "original" quantum state. They are only separate in space, and the transfer event took place at a defined "instant in time". It is is indeed ok to call this teleportation as (although this might seem odd in the practical sense) it is not possible to prove that the original photon was not teleported through space-time.
Cue endless discussions such as: What does teleport REALLY mean?
Changing the title to "Researchers quantum teleport particle..." would make it clearer.
On the other hand, as far as I understand quantum mechanics, there is nothing about the "original" photon that was not teleported/transferred to the new location. It would be impossible to create an experiment that could conclude that the "target" photon was not in fact the original photon. When looking at it this way, it is not so unreasonable to call the process teleportation :)
Isn't that how teleportation works in, for example, Star Trek?