Sustained, high-fidelity quantum teleportation
news.fnal.gov
news.fnal.gov
The article states that quantum information is being teleported via entanglement. However, it was my understanding that one cannot transfer information in such a way as the "information" is only revealed when interacting with the particle.
Could someone perhaps clarify what's going on?
https://directory.eoportal.org/web/eoportal/satellite-missio...
https://spectrum.ieee.org/tech-talk/computing/networks/quant...
There are actually other uses, too, about error tolerance, allowing you to quality-control some steps of the computation and repeat them if necessary without risking damaging the results of other steps.
Why do you want to transmit qubits? For various emerging quantum information technologies, each with different potential economic impact.
So perhaps distributed quantum computers spread across a university, city or country.
Also, remember, you still need wired or wireless transmission of bits to do the teleportation protocol.
So, before QM there was already a sense in which you could teleport an object: simply measure its state perfectly and send that information to another location and have them reconstruct that state particle by particle. In principle the new system would be indistinguishable from the original system and you could claim that you've teleported it. Now, with the discovery of quantum mechanics, this process no longer works because there is no way to measure the complete state of a quantum system. For example, you could measure the position of each particle to arbitrary accuracy or you can measure the speed of every particle to arbitrary position, but you can't do both (Heisenberg's uncertainty principle). So, it would seem like one could not construct a perfect replica of a quantum system in a new location by measuring its state in the original position.
The cleverness of quantum teleportation is that you use entanglement to sort of short circuit this limitation. You let entanglement do the heavy lifting to sort of "copy" the state from one location to another and then perform a measurement in the original location to uncover just enough information so that the person in the second location can manipulate its system to reconstruct the original state.
It's sort of like reconstructing the state without actually knowing what that state is.
Now, an interesting side effect of the quantum version is that the first measurement of the original system is necessarily destructive. As such, it's not like you'll end up with two copies of the same thing (which is what would happen in the classical version) so there's no discussion necessary around the distinction between teleportation and cloning. Classically you'd be cloning the system but quantum mechanically you'd really truly be teleporting it (in fact, there's a result in quantum mechanics called "the no cloning theorem" that proves that cloning in QM is impossible).
I've always been entertained by the paradoxes where someone is teleported ala Michael Crichton's Timeline, and is then (due to some glitch) "duplicated", leading to interesting quandaries about "who is who".
If Penrose is right about consciousness [0], this means all these fantastical paradoxes are just fantasy, right?
[0]: https://bigthink.com/paul-ratner/why-a-genius-scientist-thin...
Can there be triplicates of entanglement?
Iirc it essentially comes from the fact that time evolution is linear, and a function that sends stateOfIntetest \otimes constantState to stateOfInterest \otimes stateOfInterest for all values of stateOfInterest would be quadratic?
Riker splits, Tuvix!, the one where the other doctor ages and gets de-aged in the transporter, the one where Scotty is stored in it for decades, etc.
[0] https://arstechnica.com/gaming/2017/09/is-beaming-down-in-st...
i.e. Is there some equivalent of a termination code/header sort of thing that the recipient is looking for in the 'bit stream' or whatever? Or am I not even thinking about this in terms of the right analogy?
edit: Thank btw, this comment was fascinating to me.
So, the two people communicating would e.g. start out together and create a pair of entangled systems A and B. The person in possession of system B would then travel far away. The person in possession of system A then decides that they want to teleport a new system C to the person far away. They do this by placing system C next to system A and then performing a measurement on the combined system A+C causing these two states to become entangled. We now have an implicit entanglement between system C and system B that is far away. The person in possession of system A+C now picks up the phone and calls the other person to tell them what the outcome of their measurement on A+C was. The person far away then uses this information to determine a way to manipulate their state B in a certain way (the particular way in which they need to do this depends on the outcome of the measurement of A+C). Once that manipulation is complete, the system they have in their possession (B) is now in the quantum state that C was originally in. The system C, unfortunately has been destroyed in the process.
Yes. This is the key point, I think, and it didn't seem well-communicated in the article.
Also, not to get ahead of ourselves (understanding this is research), but what is the use/benefit of this method? We can already send and receive information over great distances with and without wires at seemingly high speeds. Is this a new level of speed? Are there some previous limitations of distances that are now surmountable? Is the power or cost envelope required somehow reduced in some obvious way (not today, but in some future commercial implementation)?
> The receiver does not know that a message has been sent until the first person contacts them classically. It's a common mistake to think that quantum teleportation is a new way of sending information. It's really a way to use classical communication in order to leverage entanglement to bypass various limitations of quantum mechanics.
That destroy my hopes of having (Close to ) Zero Latency Communication with Quantum Teleportation / Entanglement. We are still bound by the speed of light!
There's a classic analogy to this when we talk about entanglement: imagine taking a pair of gloves and mixing them up. Put one in one box and the other in another box. Send one of the boxes far away. When you look down at the box that you kept, there is no way of knowing if it contains a left handed or a right handed glove; it's a 50/50 shot either way. Similarly you have no idea what the other box contains. You then decide to open your box and find a right handed glove. You then immediately know that the other box contains a left handed glove. In some sense this feels similar to what we see in entanglement but I don't think most people would claim that you opening your box somehow compelled the other glove to pick left/right. They were just always that way, you just didn't know which glove was where.
The claim, however, is that in QM it's not like this. Instead, your act of measuring your system actually does compell the other system to change.
For a long time there were a lot of heated arguments around all of this (most prominently between Einstein and Bohr) trying to figure out if the state of either box was truly undecided until you opened it or if there could have been some type of "hidden variable" that we had yet not discovered that nonetheless dictated what the state was (i.e. could it be more like the glove example or was it truly a new "spooky action at a distance"?)
For a long time physicists believed that this was an unanswerable question and should be relegated to philosophy. It wasn't until Bell discovered his inequality that this was dispelled. He designed an experiment that could be conducted to tell the two stories apart. When it was carried out, it was determined that nature is not like the glove example but rather consistent with the truly quantum story around entanglement. In other words, your measurement of your system actually does compelled the other system to change.
To me, this is a very elegant way of addressing this weird action at a distance.
Is it “to change” or “to define”? I’m a layman QM theorist, but I think this inaccuracy appears often in popsci media (if it is one). There was no info to change in the first place, there was only an undetermined coherence of either outcome. Is that correct?
I have to say I am always at a loss when quantum physicists start talking about "measurement".
In the classical world, measuring means looking at a particular variable x in a system S at time t, S(t) and via some process specific to x (which we want to measure), Mx, obtain the value of Mx(S(t)).
In QM by contrast, it seems that measurement itself has an action upon the system so that measuring in fact means looking at some Mx(Z(S,t)) where you actually never know S but only some kind of end product Z that is believed to reflect S but is itself the result of an unknown operation on S that QM people call "collapse".
So you seek Mx(S) but in fact spend your time looking at Mx(Z(S)) and draw conclusions on S... but I have yet to hear anyone explain to me, physically what is Z, how it works, etc. Lots of statistics, but no real understanding of that "collapse" process.
There are some modified versions of QM that tries to place this on a more rigorous footing, but none of them have convinced everyone that they do. My personal favorite is the many world's approach that in many ways is simpler than traditional QM because it says that there's no such thing as a measurement. Instead, when you think you're measuring something what you're really doing is entangling yourself with the system you're measuring which means that your state is no longer separate from the state of the system. There's a part of you that sees each outcome.
This is actually already how microscopic systems work: if two particles collide and get entangled, the state of each particle sort of splits in two. The only thing that MWI says is that this dynamics also applies to macroscopic objects.
You measure the colour of an object by bouncing light off it and seeing what comes back. The objects state is modified when the light hits it, since it imparts momentum.
The dependency on classical communication would imply that it's not lower latency or higher throughput, and will remain subject to signal loss or degradation.
So now you have two copies of the same particle in two different locations.
Now the tricky (and useless part): you DESTROY the first box. Was the cat dead or alive before you destroy it?
If it was dead, you kill the cloned cat. If it was alive, you let the cloned cat live.
So now, congratulations, you have teleported the cat just as it really was when you first cloned it.
Obviously this is a gross approximation, but the central idea is that quantum teleportation let you clone and transport a particle, but you have to find a way to capture it's state and send it encoded in light or whatever method you prefer (fax?).
UPDATE: The cat belonged to Schrödinger, actually.
Teleportation is achieved by doing a specific action on one and then communicating to the other to do a specific action to the other.
In no case can information be transmitted faster than light.
It was always confusing why this replica has to be perfect. Even two “you” separated by a nanosecond in time are not perfect replicas at all. Why not to allow some impreciseness and teleport “quantumly uncertain” bodies instead of perfect copies? You can’t even know what’s perfect or not by the same principle. Moreover, maybe our structure allows very imperfect copying while retaining the same functionality after few minutes of teleache.
Can this be used to 'pause' a system, by delaying the transmission of the classic message? Or do you get the state the original system would have had, had you not measured it?
Is it possible to have three entangled systems? It seems that would allow sending the state information from one to the other two, thus destroying the original but leaving two "copies".
Apologies if the answer to this is obvious, I'm a total layman when it comes to this stuff.
I'm pretty sure (haven't been in the field a while) that superdense coding the densest coding that one can gain using quantum entanglement.
Note that "classical" is just stating that a classical channel is good enough to serve that purpose. A channel that preserves quantum state can of course be used, it's just that that isn't required for this to work.
All classical phenomena are quantum, we just use the word "classical" to describe subsets of quantum phenomena acting qualitatively in ways that are consistent with macroscopic phenomena.
Quantum teleportation lets him do that over the phone.
You look inside your box and see a red ball. In that moment you learn that I have a blue ball in my box. You gained that information in an instant, about a box 5,000 light years away.
(This is an explanation I've heard. But I don't personally know if it's accurate.)
Arguably the information that you "gained" was predetermined when you decided which colored ball to place in which box. You didn't gain any knowledge that you couldn't have known before you were teleported 5,000 ly away.
What's also more difficult is that the balls wouldn't have definitive colors, but exist in a probabilistic state / superposition. But you couldn't send the message "I have a red ball" to the individual 5,000 ly away because collapsing your ball's wave will be random and you can't determine which color you want. You could "mistakenly" collapse your ball to blue, not red.
* There is one theory of QM that removes the possibility of choice from the universe.
In quantum mechanics, entanglement is more about the fact that until a measurement is performed on one of the balls, both balls are simultaneously blue and red and will behave as if it was in a superposition of both colors up until a measurement is performed. By extension, any other property that depends on the color of those balls will also be entangled with the balls and behave in a superposition of whatever properties are entangled.
For example if a washing machine washes clothes with hot water if the blue ball is in box A, and washed with cold water if the blue ball is in box B, then that washing machine will be washing clothes with hot and cold water until a measurement is performed on any of the balls or the washing machine itself. The clothes being washed will simultaneously be expanding (from hot water) and contracting (from cold water).
Only once a measurement is performed on any part of the entangled system will every property of the system collapse into a definite state of red or blue, hot or cold, expanded or contracted.
How can we know anything about quantum entanglement when measuring the entangled particles, or any effects they have, cause the whole thing to collapse?
Then we invented the internet and now you can transmit digital information between digital computers over wires / EM signals.
Quantum teleportation / entanglement does the same but for quantum state rather than just digital state, which allows quantum computers to communicate with each other. Without this technology, quantum computers would not be able to communicate with each other over a network. So you still need to use wires / EM waves in order to transmit the data, but the data you are transmitting is quantum.
Technically you're not actually sending qubits over the wire though, you're sending 2 classical bits per quibit which are able to tell the other system what each qubit in the target system is supposed to look like, but effectively you can pretend you're sending qubits over the wire.
Information is communicated once the entangled photon has reached the other side of the network, at which point a measurement at one end of the network is made and communicated (classically). Once again though, the purpose of the classical bits is not actually to send information, but rather because any measurement of one part of the entangled system results in its collapse (Copenhagen interpretation), and thus a collapse of the entangled quantum system as a whole. So if you have a quantum computer doing some calculation with qubits and those qubits have been entangled with others at the other side of the network, you can perform measurements on the qubits at one end and this will collapse the qubits at the other end. This would not be possible to do as you'd expect from classic digital stuff, where you can just send the info over the wire.
The distance is limited because of fidelity loss in fiber - you actually need your single photonic qubit (entangled with another qubit at the source) to make it from one end of the network to the other. In order to extend the range you can use quantum repeaters, which are effectively just a sequence of entanglements.
As an aside, higher quality fiber made in microgravity (ZBLAN) could also help bring these transmission distances up.
No-cloning theorem in quantum physics says that it's impossible to do exact copy of unknown quantum state. But it turns out that you can teleport it when the original state is destroyed.
You can't have exact `cp` command for the quantum state, only `mv`. If this makes you think linear logic, you are right.
Ie if you teleport and the original isn’t destroyed. Which one are you?
As for whether the no-cloning theorem informs this, there isn't any reason to believe consciousness is related to quantum mechanics.
I know close to nothing about this domain but these two experiments sounds very similar except for the setup (fiber vs. communication with satellite).
My question is, if we can't observe the particle, how do we know that that particle is entangled with the original one?
Honesty quantum mechanics sounds more like a bug in the universe or some quirk we just don’t understand yet.
If you have access to many copies of the same state, then you can do tomography to learn if the state is entangled or not. But this is a different situation to what you are asking in your question. Actually I'm not entirely sure what your question is asking, but I won't delete this comment just yet.
Qbits are analog so it's no surprise it can be done with them as well.
Flash memory already does that in classical chips. There's nothing preventing you from going further - 8 voltages for 3 bits etc.
Is the article inaccurate?
More specifically, it seems that actually there are 2 shared qubits - one half of an entangled pair ahead of time; and one additional qubit when the message is decided. While 0 information can be transmitted using the entangled pair, entangled qubit + 1 qubit leads to 2 classical bits of information.
What improvements does this have over classical communication?
Wouldn’t it be nice to have a cell phone with perfect reception, anywhere? Someone at the bottom of the ocean talking to some at the top of Everest, on the other side of the planet? On the moon? Mars?
Let’s talk quantum internet. No longer would we have to rely on internet providers. No more Comcast, no more AT&T. Devices can just connect to other devices without someone in the middle. Governments could no longer block sites or suppress ideas.
Entangle some matter, give half of it to the space team and then have inter-stellar walkie talkies?
Edit: or can the communication only happen once and then you need to remeasure over classic communication?
As I said, no. How would you relay your measurement results in real-time? Without those measurement results, the receiver would in essence just hear white noise.
That's the perplexing part of entanglement. Somehow it feels like information is instantaneously transferred from A to B, yet the information content is somehow zero so can't be used for proper communication.
When measuring one particle of an entangled pair and you get say "spin up", you know immediately that if someone measures the other particle (with the same measurement settings) they'll get "spin down", and vice versa.
The chance that you get "spin up" or "spin down" is 50/50 and, as far as we know, cannot be affected or determined in advance.
So, on the receiving end, they measure some random combination of "spin up" and "spin down". Without anything else, this information is for all intents and purposes noise.
What you can do however, is to send a message using regular means with what you measured: "up, down, down, up, down". Ok, at least now they can check that what they got was the exact opposite. However that still doesn't tell them anything.
So instead what you do is that you change the measurement settings, and send via regular means not just your measurement results but also your measurement settings. So you'll send "H left, V down, V up, H left, H right". The recipient will then take the measurement settings (H or V) and measure the entangled particles in the same way you did, and then note down that they get the opposite.
Note now that you suddenly got a way to communicate some actual information. By agreeing in advance that a Vertical measurement means 0 and a Horizontal measurement means 1, you can send information to the recipient.
However also note that you had to make a measurement of your particles and then send the results using regular means, limited by the speed of light. So why bother with this complicated setup? Why not just send the data without all this entangled stuff?
And indeed, for just sending plain messages it makes no sense to use entangled pairs.
However as I noted in my other post, the inability to clone entangled states means an eavesdropper can be detected using the entangled setup.
As I've understood it, to "listen in" the eavesdropper has to destroy the entangled state by measuring it, and there's no way to perfectly clone the entangled state before doing that.
The recipient can compare the entangled data with the measurement results (sent via classical means) and detect statistical inconsistencies if there is an eavesdropper.
edit: I see their page[1] mentions quantum metrology, which I found reference to in a page[2] describing work to improve GPS and similar detection using quantum entanglement. Not sure if it's directly related but seems like there should be room for some interesting work using this quantum network in this area.
[2]: https://news.engineering.arizona.edu/news/quantum-entangleme...
Why is that? This keeps happening. Quantum entanglement is not a new concept. It's almost certainly older than 99.9% of the people on this site. Yet the myth that it could enable FTL communcation continues to persist.
Imagine that you have 2 coins and give one to someone on earth to flip and one to someone on the moon to flip. The outcomes in either location is random 50/50, but interestingly they are perfectly correlated (H<->H, T<->T). When you flip a coin you don't have the ability to pick it's outcome. The only thing you can pick is whether or not to flip it at all. So, imagine you want to communicate one bit of information from earth to the moon and decide that a 1 will be encoded as "flip the coin" and 0 as "don't flip the coin". When you're standing on the moon and want to reveal the information you flip the coin and see e.g. H. There are two ways this could have happened: either the earth coin had already been flipped and showed H or the earth coin had not yet been flipped and you just randomly got a H. In other words, the outcome by the flip is useless by itself.
"2 generals command 2 armies hundreds of km apart. They want to attack a common enemy, there are 2 options, A) And all-front attack. B) From the flanks. The generals want the plans to remain uncertain until the last second before the attack. So from an intermediate point, they send two "entangled coins" , one to each general, the coins will arrive at both sites at the exact start of the battle.Both will show the same face when "measured". The generals have agreed previously that if they turn out "heads" they will both attack from the flank, in the other case, they will do a front-attack.
Of course you dont need a quantum system for this, you could have agreed on other stuff (like it if it is raining that day at certain place or sending a framed coin by regular mail) but I think the quantum solution is the more elegant, assuming no 3rd party snooping.
So really it just front-loads that portion (I.e. removes it from consideration as part of the proposed solution) instead of allowing it to slow down solving the overall problem. It requires precomputation/agreement beforehand, rather than during the time allotted to the problem.
The speed of that precomputation would still be unchanged, and still be the slowest portion.
However, as he said, in this situation the two generals could have agreed on something else, like if it was raining or not.
In fact, something they could have done is to flip a coin, and split with a copy of the result, which they only look at when they launch the attack. It would have the same effect.
With a random variable (the coin flip), that is hidden until revealed, we achieve the same results as quantum mechanics.
Scientist call them "hidden values" and Einstein hopped we could explain the "spooky action at a distance" with such hidden variables. But we can't, Bell proposed an experiment with entangle state which measurement could not be explained by such hidden variables, and Aspect did the experiment and obtained the predicted results.
So there exist situation where we can use entanglement to achieve better results, for example in "non-local games", where players sharing entangled state can win with probability 1, when "classical player" with probability < 1
Do you have an example for this? This is really interesting
Yes but the difference is that in this case, the plan already exists from the moment the coin is flipped until the result is revealed to the generals. In the QM case no one (not even God) knows what the result will be until it is actually measured,but once that happens the result is known "instantly" at both places (assuming both coins arrive at the same time), so in that sense is a FTL communication (of course it is not real communication)
Like, keep parsing the incoming information until you see a period (for example).
(I know it's not using actual characters, just using the idea to illustrate the point).
There's no parsing; it's all or nothing.
Yes, information can travel with infinite speed.
No, cause-effect relationship cannot travel faster than light.
Could a QM answer be otherwise?
On the one hand, we have the simplest of radio waves -- which moves a little bit of information (depending on how it is encoded, how the radio wave is modulated) over space, and on the other, we have quantum teleportation, which also moves information over space.
On the one hand, the simple radio wave dissipates according to the square of the distance -- the "inverse square law" (https://en.wikipedia.org/wiki/Inverse-square_law), whereas presumably (I do not know), the quantum teleportation does not.
Which leads to a side question -- is there a physical limit to the distance that information can be transported via quantum teleportation?
If not, then there's yet another example of light speed violation, and if so, then maybe the quantum teleportation isn't really quantum teleportation -- but rather some form of radio/wavelike communication at a distance, albeit, one physics has yet to describe...
My point is simply this -- when you have a new fangled, not well understood phenomena (in this case Quantum Teleportation), and you have an old well understood phenomena (for example, radio waves -- but it could be any wave or member of the EM spectrum that conveys information over distance, for example, light), well, when you have two apparently disparate phenomena like that in Physics, you want to understand WHAT AND HOW ARE THEY UNIFIED -- rather than the endless set of attributes that are different between them...
You see, an advanced Physics would be able to express/quantify/explain both radio waves on the low-end of understanding, and every single possible form of quantum teleportation on the high end... IF IT WERE CORRECT...
Observation: What science at this point in time in earth's history needs to do is the following:
1) Construct FTL waves out of Slower-Than-Light waves...
2) Construct Wormholes (Wormhole = Black Hole = Quantum Telportation = Portal, use whatever language you wish!) out of the FTL waves.
Of course, we'll leave #2 for the future.
Goal #1 should be to construct a single, solitary, FTL wave (and be able to detect it over a distance, otherwise what's the point?) from Slower-Than-Light waves.
By default, if you're moving Faster-Than-Light, then the path/space you are moving in has similar characteristics to #2, which, over time, should yield a greater understanding of that phenomenon... (is the space compressed, or the wave expanded, or both?).
In fact, maybe it would make sense to go back to Zeno's paradox of Achilles and the Tortoise: https://en.wikipedia.org/wiki/Zeno%27s_paradoxes#Achilles_an...
That is, if you think about each, each EM wave that exists in space -- must first cross half of that space, but before that, it must cross half of that space, etc., ad infinitum.
So, what happens if an EM wave is made of nothing but space, but space that is twisting, space that is vibrating?
If (and it's a big IF!) that's how EM waves work -- then you already have part of all of the above -- BECAUSE AT THE SMALLEST SCALE IN AN EM WAVE -- SPACE ITSELF IS BEING COMPRESSED AND EXPANDED...
You see, conventional Physics uses words like "Electric Field" and "Magnetic Field" -- rather than such words as "Twist Vector Of Space", "Compression Vector Of Space", "Expansion Vector Of Space", and "Oscillation/Vibration Vector Of Space" (and of course, those are relative to size/wavelength/frequency, etc.... "relative to scale", as I like to say...)
But, I think todays Physics -- would get so much more out of itself -- if we stopped using words like "Electricity" and "Magnetism" -- and instead replaced those with "Something is happening to SPACE here!" (where the something was a more accurate description of what was actually going on!)
But anyway, we need to know/understand/grok the root of all phenomena -- in the simplest of terms -- otherwise Physics will keep inventing a plethora of words and phrases to describe what can be in essence, described by a few simple fundamental understandings in the tersest of ways...
Anyway, feel free to call me a crackpot <g>... I don't claim to be right -- I only claim that researchers may wish to investigate these subjects further!
Phrased another way "Here be dragons!" -- in programmer parlance! <g>