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Is anyone able to explain to me how this demonstrates "information without direct transmission" when a direct 30km fiber line is needed to transmit the data, an

Is anyone able to explain to me how this demonstrates "information without direct transmission" when a direct 30km fiber line is needed to transmit the data, and how you can call it teleportation when it's "limited by the speed of light"? Not doubting they did something special but I couldn't tell from the article what exactly it was that they did which is unique from standard fiber transmission (other than they received a single photon using an atypical frequency).

ko27 2y ago
This is basic QM "spooky action at a distance" stuff. Quantum information is teleported instantaneously, however to make use of that information, you actually need to transfer classical information over regular channels. That doesn't mean it's not useful, see for example the way QKD works: https://en.wikipedia.org/wiki/Quantum_key_distribution https://en.wikipedia.org/wiki/Quantum_key_distribution
eppp 2y ago
I know almost nothing about this but lay understand is this: the state of a photon can possibly be encoded with a vast amount of information, instead of transmitting that information encoded into light, they transmitted the state of entangled photons and thusly recreated the information on the third photon.
jerf 2y ago
Being generous, they say that "direct transmission" is not necessary because they are not "directly" transmitting a "1" or a "0", but instead an entanglement. I agree that, as is fairly usual for this field, this is poorly phrased, probably poorly understood by the writer, and prone to misinterpretation. There absolutely certainly is a "direct transmission" of some photons to achieve the effect, irreducibly so. You can't move entanglement around without some sort of carrier, photons in this case (as they are the most convenient for this task for much the same reasons they're quite convenient for conventional transmissions as well). Quantum teleportation is not related to speed. Quantum teleportation is when you entangle two particles, A and B, and then you take the entanglement on B and send the entanglement itself to some distant particle C. A and C are then entangled in exactly the same way that A and B were (this is the "teleportation"), and B no longer is. This is an intrinsically destructive operation on the original A/B entanglement. The requirement to be "quantum teleportation" is merely than the entanglement is transferred, regardless of speed; if you could send an entangled particle via the slowest shipping Fedex has and transfer the entanglement days later, that would still be quantum teleportation. However, it's all very delicate and that's not practical or possible, so photons are one of the better ways to do this since they go fast and can actually travel some distance while still entangled. This is an intrinsically light-speed limited operation because it requires actual particles to be transmitted, and then of course the resulting entanglement can not be used for "instantaneous" communication either, so quantum teleportation has nothing whatsoever to do with faster communication. It's about securing it, not speeding it up. I would guess that the researchers themselves do not particularly love this summary of their work, but I'd also guess that they've thrown themselves against the brick wall of obstinate misunderstanding about everything "quantum" enough times that they may well have given up trying to get University PR to write about it accurately. There must be some training camp somewhere that they send all the aspiring science communicators to where they say the word "quantum" over and over and if the aspiring writers do not wiggle their fingers ominously and go "wooooo wooooo" in a scary Halloween voice in response every single time they hear it they get their hands thwacked by a ruler.
- inkcapmushroom 2y ago
  Thanks for the explanation of what "teleportation" means in this context. I guess that word is maybe not the best choice as "quantum teleportation" conjures up all sorts of images in an average person's head, but that does seem to be the case for a lot of quantum stuff. I guess where I still get hung up with entanglement is the idea that "classical bits" are necessary at all in the operation. I get that reading the state of the particle on the receiving side is destructive, so you can only do it once. But how is it that just reading it doesn't give you any information without those "classical bits"? Is it something like "there's 10 different ways to read it, and if I don't use the right way I get out some junk, but if I pick the right way I get a definite signal"? So then the only information you're sending to the receiver side using classical bits is the instruction manual on how to read this particular qubit, without which they can't be certain if they really received something at all? I also get that we might be getting past the point where it's easily explainable without a lot of math jargon. :)
  - jerf 2y ago
    "I guess that word is maybe not the best choice as "quantum teleportation" conjures up all sorts of images in an average person's head, but that does seem to be the case for a lot of quantum stuff." The word teleportation is IMHO a reasonable term for what is occurring, because of the fact that particles are otherwise indistinguishable from each other. If you have some state on an electron, and you perfectly convey that state, entanglement and all, to some other electron, there's a fairly realistic sense that you have "teleported" that electron, because its quantum state and that it is an electron is all there really is to a particle. Particles don't have their own identity. It isn't really realistic to trace "the" path "an" electron takes through the universe. (The recent PBS Space Time about electrons has an illustration that will, in passing, demonstrate this: https://www.youtube.com/watch?v=KXPhaAsnrfs https://www.youtube.com/watch?v=KXPhaAsnrfs , around 13:05.) You can't teleport large systems, but it's not a math restriction, it's just a brute physical restriction; you need a non-trivial machine to transmit something and there's just never going to be an engineering solution to transmitting macroscopic quantities of quantum information somehow. People get really confused about this because Star Trek transporters are magic, straight-up lunatic nonsense magic, but it defines the term for people, and those actually move matter around. Quantum teleportation does not; if you want to teleport matter (protons, electrons, etc) it requires the target already be present at the other end. So to "quantum teleport" a kilogram of water you'd have to have a kilogram of water already at the other end, and you'd still have a kilogram of water at the original source. It's only the quantum state that would move. So it has nothing to do with teleportation in the Star Trek sense. The other key thing to understand about quantum communication is that you basically don't "transmit" any information at all. The payload that comes out of these systems is basically a stream of zeros and ones, where you know that the other side has the same stream, but neither side controls the stream, so the stream is quantum-level random. (Some engineering may need to be done for this, e.g., one side may be inverting all their bits, but this is the end result.) From here you can use this to construct keys that you know are not intercepted, and you do indeed send other conventional communication that is obscured by the quantum bit stream. (You can think one-time pad for this as at least a model, whether or not that's what they do.)

ko27 2y ago

This is basic QM "spooky action at a distance" stuff. Quantum information is teleported instantaneously, however to make use of that information, you actually need to transfer classical information over regular channels. That doesn't mean it's not useful, see for example the way QKD works: https://en.wikipedia.org/wiki/Quantum_key_distribution https://en.wikipedia.org/wiki/Quantum_key_distribution

eppp 2y ago

I know almost nothing about this but lay understand is this: the state of a photon can possibly be encoded with a vast amount of information, instead of transmitting that information encoded into light, they transmitted the state of entangled photons and thusly recreated the information on the third photon.

jerf 2y ago

Being generous, they say that "direct transmission" is not necessary because they are not "directly" transmitting a "1" or a "0", but instead an entanglement. I agree that, as is fairly usual for this field, this is poorly phrased, probably poorly understood by the writer, and prone to misinterpretation. There absolutely certainly is a "direct transmission" of some photons to achieve the effect, irreducibly so. You can't move entanglement around without some sort of carrier, photons in this case (as they are the most convenient for this task for much the same reasons they're quite convenient for conventional transmissions as well). Quantum teleportation is not related to speed. Quantum teleportation is when you entangle two particles, A and B, and then you take the entanglement on B and send the entanglement itself to some distant particle C. A and C are then entangled in exactly the same way that A and B were (this is the "teleportation"), and B no longer is. This is an intrinsically destructive operation on the original A/B entanglement. The requirement to be "quantum teleportation" is merely than the entanglement is transferred, regardless of speed; if you could send an entangled particle via the slowest shipping Fedex has and transfer the entanglement days later, that would still be quantum teleportation. However, it's all very delicate and that's not practical or possible, so photons are one of the better ways to do this since they go fast and can actually travel some distance while still entangled. This is an intrinsically light-speed limited operation because it requires actual particles to be transmitted, and then of course the resulting entanglement can not be used for "instantaneous" communication either, so quantum teleportation has nothing whatsoever to do with faster communication. It's about securing it, not speeding it up. I would guess that the researchers themselves do not particularly love this summary of their work, but I'd also guess that they've thrown themselves against the brick wall of obstinate misunderstanding about everything "quantum" enough times that they may well have given up trying to get University PR to write about it accurately. There must be some training camp somewhere that they send all the aspiring science communicators to where they say the word "quantum" over and over and if the aspiring writers do not wiggle their fingers ominously and go "wooooo wooooo" in a scary Halloween voice in response every single time they hear it they get their hands thwacked by a ruler.

inkcapmushroom 2y ago
Thanks for the explanation of what "teleportation" means in this context. I guess that word is maybe not the best choice as "quantum teleportation" conjures up all sorts of images in an average person's head, but that does seem to be the case for a lot of quantum stuff. I guess where I still get hung up with entanglement is the idea that "classical bits" are necessary at all in the operation. I get that reading the state of the particle on the receiving side is destructive, so you can only do it once. But how is it that just reading it doesn't give you any information without those "classical bits"? Is it something like "there's 10 different ways to read it, and if I don't use the right way I get out some junk, but if I pick the right way I get a definite signal"? So then the only information you're sending to the receiver side using classical bits is the instruction manual on how to read this particular qubit, without which they can't be certain if they really received something at all? I also get that we might be getting past the point where it's easily explainable without a lot of math jargon. :)
- jerf 2y ago
  "I guess that word is maybe not the best choice as "quantum teleportation" conjures up all sorts of images in an average person's head, but that does seem to be the case for a lot of quantum stuff." The word teleportation is IMHO a reasonable term for what is occurring, because of the fact that particles are otherwise indistinguishable from each other. If you have some state on an electron, and you perfectly convey that state, entanglement and all, to some other electron, there's a fairly realistic sense that you have "teleported" that electron, because its quantum state and that it is an electron is all there really is to a particle. Particles don't have their own identity. It isn't really realistic to trace "the" path "an" electron takes through the universe. (The recent PBS Space Time about electrons has an illustration that will, in passing, demonstrate this: https://www.youtube.com/watch?v=KXPhaAsnrfs https://www.youtube.com/watch?v=KXPhaAsnrfs , around 13:05.) You can't teleport large systems, but it's not a math restriction, it's just a brute physical restriction; you need a non-trivial machine to transmit something and there's just never going to be an engineering solution to transmitting macroscopic quantities of quantum information somehow. People get really confused about this because Star Trek transporters are magic, straight-up lunatic nonsense magic, but it defines the term for people, and those actually move matter around. Quantum teleportation does not; if you want to teleport matter (protons, electrons, etc) it requires the target already be present at the other end. So to "quantum teleport" a kilogram of water you'd have to have a kilogram of water already at the other end, and you'd still have a kilogram of water at the original source. It's only the quantum state that would move. So it has nothing to do with teleportation in the Star Trek sense. The other key thing to understand about quantum communication is that you basically don't "transmit" any information at all. The payload that comes out of these systems is basically a stream of zeros and ones, where you know that the other side has the same stream, but neither side controls the stream, so the stream is quantum-level random. (Some engineering may need to be done for this, e.g., one side may be inverting all their bits, but this is the end result.) From here you can use this to construct keys that you know are not intercepted, and you do indeed send other conventional communication that is obscured by the quantum bit stream. (You can think one-time pad for this as at least a model, whether or not that's what they do.)