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?