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).
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
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.
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. :)
"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 , 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.)
People don't get confused because Star Trek is "magic nonsense", they get confused because "teleport" is a longstanding speculative fiction word that quantum scientists decided to co-op to describe something almost, but not quite, entirely unlike teleportation.
It's like if you called something "quantum resurrection" and then making fun of people for asking if it brings people back to life because that's obviously impossible, you buffoon, resurrection is actually when someone's already alive and you check and they're still alive, idiot, stop reading the Bible and start reading more Physical Review.
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.
The use of "Internet" in the title and article is deceptive. They did not send data over the Internet, they sent conventional optical data over a 30 mile spool of fiber and quantum entangled photons on an unused wavelength over the same cable in a laboratory setting.
It's not deceptive. The whole point of the study was to figure out a way to transfer quantum information through active Internet cables, and not empty optical cables.
I find the wording highly deceptive. The main feature making the internet globally scalable is not that it goes through fibers, but that it uses package routing. Nobody knows how to do this with qubits, so quantum communication doesn't scale at all. Instead, the community started doing these publicity stunts with very limited applicability.
Their title is specific in saying "internet cables", not the internet. It seems fair to me that this is one step on the journey, that figuring out going through cables comes before figuring out routing between cables.
It's a press release for a specific research paper, not a product release marketed to consumers.
(Though it's possible the researchers themselves agree with you & iandanforth more than with me & ko27, as the title of the actual research paper itself doesn't use this wording; it's "Quantum teleportation coexisting with classical communications in optical fiber".)
This entire paragraph is just breaking my mind. Need to find an quantum entanglement for dummies book or something
>“By performing a destructive measurement on two photons — one carrying a quantum state and one entangled with another photon — the quantum state is transferred onto the remaining photon, which can be very far away,” said Jordan Thomas, a Ph.D. candidate in Kumar’s laboratory and the paper’s first author. “The photon itself does not have to be sent over long distances, but its state still ends up encoded onto the distant photon. Teleportation allows the exchange of information over great distances without requiring the information itself to travel that distance.”
> (...) quantum teleportation only transfers quantum information. The sender does not have to know the particular quantum state being transferred nor the location of the recipient, but to complete the quantum teleportation, classical information needs to be sent from sender to receiver. Because classical information needs to be sent, quantum teleportation cannot occur faster than the speed of light.
From here it's mind blowing until the part where classical information needs to be sent. Then it only seems useful as a way to transfer quantum state safely and undetected. It's really cool that the classical information can be broadcasted, so you can keep the recipient's location secret.
Maybe quantum cryptographic keys, if that's a thing, can be sent this way, or a channel that just teleports quantum data without a chance of of being overheard can be setup.
The presence of a 1550nm 400Gb/s signal is really just a distraction here and doesn’t add to the experiment in a meaningful way. I don’t think anyone is surprised that you can separate channels by wavelength, even if those channels are quantum.
So this really comes down to observing teleportation given the high loss of the fiber at 1310nm at 30km. From that point of view the experiment is pretty solid but I’m not sure it’s worthy of a press release.
The lasers generating the signal are not perfect and generate a Gaussian-like profile of wavelengths, albeit narrow.
In this experiment[1] they sent single photons alongside the classical data:
Alice’s qubit is then encoded onto the horizontal (H) and vertical (V ) components of the heralded photon’s polarization |ψ〉A = α|H〉 + β|V 〉using polarization waveplates.
This photon is then multiplexed using an O-band/C-band WDM to co-propagate with the 400-Gbps C-band classical signal over 15.2 km of spooled optical fiber (SMF-28(R) ULL) to Charlie.
To prevent the detection of C-band photons due to insufficient filter isolation, we cascade two cascaded O-band/C-band WDMs before the FBGs and a 1290-nm DWDM to achieve >190 dB rejection of C-band light.
The lasers are not Gaussian but Lorenztian. This is a detail that’s not terribly important.
The lasers generate frequencies in C-band at ~194THz. The O-band photon source is ~229THz. Classically these fields do not interact.
For passive beam splitting, there is also no quantum interference. The photo detectors are broadly sensitive to both frequency bands. So if the C-band light hits the photodiode you won’t be able to count photons. The filters are there to reduce the background “noise” of the C-band light hitting the photodiode that’s being used for photon counting. The use of filters for this purpose is very common and really not that interesting.
