A rudimentary quantum network link between Dutch cities
tudelft.nl
tudelft.nl
Besides hard-working PhD students, another key ingredient that our research institute QuTech facilitated, was the collaboration with expert hardware and software engineers, allowing us to quickly transform new ideas into (deployable) products. A great show of what's possible when academia mixes with professional engineering. But of course there was enough hacking and tinkering going on that it warrants to be on HN ;)
You can reply here if you have any questions, I'll be checking throughout the day. Thanks!
Personally, I always wonder why point-to-point connections are called "networks". The information is not quantum at any node, even if there are multiple nodes in a system.
Then there's "quantum internet", which makes no sense at all. What are we going to do, run direct fiber from every computer to every other computer directly? You can't hop safely or anything. Don't get me started on the total bullshit that is the "quantum repeater", now we need "quantum switch" too?
We call serial port connections things like "link", "connection", etc. We typically don't call them networks until we start linking them all together with simple routing logic that doesn't inherently require access to all the unencrypted information the packets contain and such.
To me these are all just signs that the whole scheme is/was and will forever be mostly crankery.
Quantum networking is an oxymoron. It doesn't allow end-to-end encryption and in exchange gives back extremely fragile single link security properties.
As far as your routing/switching qualms go I think they are mostly addressed by entanglement swapping? Person A and person B can each make an entangled pair and send me half, and I can (locally) do stuff which leads to the halves they keep at home becoming entangled. Then they can use teleportation or whatever to do whatever they want between themselves without me knowing anything about it.
Good luck with all of that.
Its been well understood since at least 1993
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.71...
No, you don't have to do that. A quantum network would be a web of point-to-point quantum links, with paths formed by routers choosing links. Same as a classical network.
To be a bit more concrete what an operating quantum network would look like is a bunch of routers using links to build up entanglement with their neighbors. When an endpoint wants to send a message across the network, a path from source to destination would be determined and entanglement across the links of that path would be consumed to move the message across the network [1][2]. The reason it's done this way, instead of directly sending the message, is that entanglement can be cross-checked before using it [3] and quantum networks really don't like dropping packets due to the no-cloning theorem.
> We typically don't call them networks until we start linking them all together with simple routing logic
Yeah I agree that it would be more accurate for this press release to say they made a quantum link.
> To me these are all just signs that the whole scheme is/was and will forever be mostly crankery.
Don't confuse difficulty with crankery. It'll be awhile before anyone reports an experimental realization of a true quantum network, because it'll be awhile because anyone can make a quantum router. The issue is that a quantum router is for all intents and purposes a fault tolerant quantum computer, and that is its own hard challenge being worked on separately. In particular, a quantum router needs to be able to store qubits reliably for non-trivial amounts of time, and to perform reliable operations on those qubits in order to cross-check stored entanglement.
[1]: https://en.wikipedia.org/wiki/Quantum_teleportation
[2]: https://en.wikipedia.org/wiki/Quantum_entanglement_swapping
[3]: https://en.wikipedia.org/wiki/Entanglement_distillation
- Are the photons themselves carrying quantum information?
- Does the photon link result in entangled particles in Delft and Den Haag?
- Can these entangled particles be used for communication without the optical link?
Also, I tried looking this stuff up and ran into something about quantum "repeaters" and a plans for a whole quantum network. Is this research part of working towards that? How far are we now, and what steps are still missing? Thanks!
Edit: Looks like you guys built a multi-node quantum network 2 years ago! I will have to do some more reading.
- Yes and no. The photons emitted and sent through the fiber are entangled with their electron counterparts. So we send simultaneously a photon state (entangled with electron) from Delft, and a photon state (entangled with electron) from Den Haag. Those states interfere in the midpoint (Rijswijk), and upon measurement of one photon (photon now is absorbed/measured/gone) we know that the _electrons_ of the nodes in Delft and Den Haag are entangled.
- The above also answers this question: yes!
- No. They can be used to transfer a quantum state from one place to the other, for example, which _consumes_ the entanglement (one-time use only, per pair of entangled particles). However, still classical feedback signals need to propagate for that to happen, so we still need _a_ link, preferably optical (for speed and distance). Wiki has actually a great page on teleportation: https://en.wikipedia.org/wiki/Quantum_teleportation
I'll answer to a different question on repeaters later in another comment, so check back :) Indeed, multi-node quantum network was an awesome experiment. This takes it to the next level of being able to distribute entanglement over large distances and between quantum nodes that are self-sufficient (no sharing of hardware resources between nodes).
> which means they share a quantum connection enabling instant correlations, no matter the distance
But per your response this is not true, i.e. information transmission is still limited by the speed of light?
So this means there is common knowledge of some random vector 01101010101 but nature decides the vector randomly, not humans, not communication.
You might get clever and say "aha! if I measured or not can be the communication" and that's true. The way you measure that is to see if your particle is in a superposition state or no. You shoot the entangled photon through a double slit and see if a wave-like pattern occurs, in which case we're still in a superposition and our communicator has not measured, or if it's two lines they have measured. "measured or not" thus is our "bit" that has been communicated instantly.
So the answer is kind of yes and know. At face value instant communication is not possible. Adding a quantum superposition detection device, then yes, such a device's readout may be used for Ender's game style ansible communication.
Wait, does this work? Are superposition detection devices theoretically possible? Got any reference with more on this?
IANAQP but I'm pretty sure this is not correct. Basically everyone in the field maintains that any FTL communication is impossible.
The problem is that you almost certainly can't figure whether a given particle is entangled with some faraway particle just by looking at it; you need to look at both. "Quantum networks" rely on knowing beforehand that the particles are entangled. I think you're correct that the key advancement is common knowledge of a random (as far as we know) vector.
I think your "entanglement detector" is a misunderstanding of the double-slit experiment. (You call it a "superposition detector", but really everything is in some sort of superposition all the time.) If you fire one photon through a double slit at a sheet of photo paper, you'll always see one dot on the paper. Even though the single photon is wave-like and even interfering with itself, this is only something that becomes visibly apparent after repeating the experiment many times. So the pattern is not unique to an entangled photon, and you can't test a single photon anyway.
(BTW, you never get a two-line pattern in a decent setup. This is an incredibly common mistake, but it's simply wrong. The interference (which produces fringes) only happens where the separate patterns from the two slits overlap, so if you want a lot of interference, you need them to overlap a lot. So in the no-interference case, you won't get two separate lines with a gap between, you'll get a single merged wash (with probably some fine structure due to diffraction within each of the slits, but that'll also be there when there is interference, on top of the two-slit interference fringes).)
You might think "ok, I'll do this with a bunch of photons, measure/not measure all of their twins, and see if the bunch of them show fringes." This is more-or-less what's done in the delayed-choice quantum eraser experiment, but it doesn't work out in a way that allows communication. What happens is that you always get the no-interference pattern. In order to see interference fringes, you need to split the individual photons' dots up based on the result of the measurement you made on their twins. Based on those measurements (if you made them), you can split the photons up into two groups, which'll have fringes with equal-and-opposite patterns (i.e. each will have bands where the other has gaps [2]).
If you didn't measure the twin photons (or made some other measurement on them instead), you can't split them up, so you won't see the fringes. But that's not because the measurements were different, it's just that you can't split them up afterward to see the fringes. And even if you did measure the twins, you can't split them up until you get a list of which twin got which result -- which can't be sent faster-than-light.
Net result: no, you can't send information via entanglement, you can only get correlation.
[1] https://www.researchgate.net/figure/Electron-Fringe-Pattern-...
[2] https://algassert.com/quantum/2016/01/07/Delayed-Choice-Quan...
In figure 3 of the paper [1] the heralded infidelity of entanglement is reported to be around 45%. That's not good enough for computation, but it's less than 50% which means it makes purification to arbitrarily low infidelity possible. However, the conversion rates would be pretty brutal for such a high infidelity start (e.g. millions of physical pairs consumed per logical pair good enough for use in a fault tolerant computation e.g. a target logical infidelity of 1e-6 or 1e-9).
I know very little about quantum networking. I assume you are going beyond what they did here? How so? [1]
> Recently, as a sort of proof of potential and a first step toward functional quantum networks, a team of researchers with the Illinois‐Express Quantum Network (IEQNET) successfully deployed a long-distance quantum network between two U.S. Department of Energy (DOE) laboratories using local fiber optics.
> The experiment marked the first time that quantum-encoded photons — the particle through which quantum information is delivered — and classical signals were simultaneously delivered across a metropolitan-scale distance with an unprecedented level of synchronization.
> “To have two national labs that are 50 kilometers apart, working on quantum networks with this shared range of technical capability and expertise, is not a trivial thing,” said Panagiotis Spentzouris, head of the Quantum Science Program at Fermilab and lead researcher on the project. “You need a diverse team to attack this very difficult and complex problem.”
[1] https://www.anl.gov/article/quantum-network-between-two-nati...
Like at most i hear about quantum key distribution, but quite frankly the classical equivalents to that are just as good if not better, so what is the actual benefit?
- security - if we use quantum entanglement/teleportation to the extent I've read about how it works, then even if you still need a fiber optic cable connecting the two parties, the data is unreadable if you're not looking at physically the same wave/photons, meaning that man in the middle attack (like the ones with bending an optic cable to break it's internal reflection) is literally impossible. The data in the middle would not be readable without the receiving end entangled device, and the other side would immediately know about the attack, because an identical signal would not be readable either, as it's not the same signal anymore.
- I think the ultimate promise is transferring data without a physical link of any kind in-between. Connect two atoms, manipulate one, read the other - like ansibles in LeGuin/O.S.Card fiction. Instant interplanetary communication (which, I think, fucks up the idea of time too?)
The second is just wrong. It is well known and proven that it's impossible to send information via quantum entanglement. It's true that there are some interpretations of QM where the wave function of the entangled pair collapses instantly the moment one side of the pair is measured. But there is no version of QM where manipulating one side of the pair has any effect whatsoever on the other, except for measurement collapsing the quantum superposition into a random classical state.
The best classical intuition for how entanglement works is that two entangled particles are like two gloves from a pair. If you put them in boxes and separate them, when someone opens a box and finds the left glove, they instantly find out that the other person has the right glove. The difference with quantum entanglement is simply that the universe only decides which glove is which when you open the box, before that they are both in a mix of the states. This makes statistical properties measurably different if you send many pairs of gloves and look at how many times certain things match.
But there really is nothing that you can do with a pair of entangled particles that you couldn't do with the pair of gloves.
I should note for completeness that, because of the different statistical properties, there is a way to send slightly more information using entangled pairs than you can with classical particles. I believe you can send 1.5 bits of information per particle, but I don't remember the exact number. This means that a quantum internet could have higher throughput at the same transmit power, which would have some relevance for very long distance wireless communication, such as communicating with a space probe.
One thing is for sure: you can’t send information faster than light with this or any other kind of quantum communication as two entangled qubits are basically two RNGs that are correlated. You’d just get noise without an additional classical, not FTL, data link (please, somebody with expertise: help!)
And if you still have to rely to classical encryption methods to make sure you know the identity of the other party (to prevent active man in the middle attack), why not just use classical encryption methods for everything else as well, instead of using quantum key distribution?
This is incorrect. QKD can detect passive mitm only. It cannot detect an active mitm.
Which is the main reason its overhyped, since as cool as QKD is, you still need active mitm prevention, so you have to rely on classical crypto anyways.
Yes I get through quantum magic you can theoretically tell if your secret has been intercepted in the quantum state because it would cause a wave form collapse but the wave form wouldn't collapse if they were listening in to your quantum computer squeaking and buzzing and decoding those noises or timings or reading its heat signature etc, or getting your operator drunk and finding out their dog's name or partner's birthday and using it as their password, or kidnapping them and hitting them with things until they voluntarily give you their password etc. All those types of attacks would still work and still be just as undetectable as they are in classical encryption. ie all the most effective forms of attack are still just as effective in a quantum case.
I think it's a very interesting area of research but this whole idea of uncrackable codes is a stretch.
I think over time they will discover a benefit but the hype is obviously not warranted.
As far as i am aware, none of the problems faster on a QC are helped in anyway by quantum internet.
But in (most) distributed models of computing, networks of computers share bits back and forth. The quantum distributed models have computers sharing qubits. So this seems to be a practical implementation of a system that could solve certain problems (specifically some graph labelling problems) more efficiently (specifically, in fewer message-passing rounds).
Perhaps you're confusing "internet" (a network of computers) with "world wide web" (a set of linked documents)
from my perspective this is fascinating area of physics that we need to know more about and will improve our understanding of fundamental physics.
But few years ago I heard of some other interesting uses where quantum properties were used to essentially enable DWDM-like virtual circuit routing with higher capacity - though I would have to look again if it went anywhere or into scrap heap of quantum BS.
Except it doesn't solve the mitm problem, so its not really safer.
Still, it seems like what is needed here is more a quantum LAN, or possibly even just an on board interconnect between quantum processors. The focus on wide area quantum networks feels a bit odd.
(2) easier experiments. Currently, doing a loophole free Bell inequality test is hard enough that people get PhDs for it. With a quantum network that experiment is way easier, because the network solves the hard part (distributing the entanglement). You could probably also use quantum networks for other experimental tasks, like coherently linking telescopes on separate continents, though the bandwidth and computational requirements for that would probably be a bit insane.
There are also some more out there ideas, like if stock markets contain Bell inequalities then you could use a quantum network to build up entanglement that is then consumed to win those games more often which equals $$$. But it's hard to imagine concrete scenarios that would create such an inequality, nevermind one where the expected dollars gained from the quantum strategy exceeded the cost of operating the network.
https://www.nsa.gov/Cybersecurity/Quantum-Key-Distribution-Q...
https://www.ncsc.gov.uk/whitepaper/quantum-security-technolo...
https://en.wikipedia.org/wiki/Snake_oil_(cryptography)
Stick with TLS. If you really think quantum computers are a threat to anything, use a hybrid-PQC key exchange.
My honest professional opinion is a cryptographically-relevant quantum computer will never exist, making classic cryptography superior in every case.