What is actually the usecase for "quantum internet"?
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?
A quantum internet is absolutely necessary for creating a useful quantum computer, the same way the internet (LAN) is needed to create a supercomputer. A supercomputer is essentially many computers connected together. A quantum computer that solves problems we care about will be similar: https://arxiv.org/abs/2212.10609.
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.
One application we care about is using quantum computers to build high resolution telescopes https://arxiv.org/abs/1107.2939. A wide area network is required because the telescopes need to be far apart.
I don't know about use case but in various distributed computing models there are problems that are provably easier for quantum computers. Unlike the classical setting where the best we have is factoring where we don't know an efficient deterministic algorithm and various problems which experimentally seem to be faster for QC (and those results often don't last long as we get better at simulating quantum algorithms classically)
Well I don't really agree that quantum computers are useful! Not yet anyway.
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)
I could see the usecase of a local network between quantum computers in the same room. The part where i get lost is why a wide area quantum network would be useful.
A priori, a quantum network could efficiently solve e.g. leadership election or shortest path type problems. I don't think there's any evidence that they can, but any problem you might want to solve for a wide area network is potentially a use case for quantum. By the way, as I said I'm more or less a QC skeptic in the sense that I don't think we will have scalable QC doing really useful work in our lifetimes. Happy to be wrong though.
(1) distributed computation. If you can network two quantum computers, you essentially have one quantum computer with twice the storage. Quantum networks avoid the need to build one enormous quantum computer.
(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.
As I understand, quantum key distribution cannot be beaten by classical equivalents and they're only good or better because our current quantum computers kinda suck. So the major use case at the moment is proving the tech and developing the infrastructure. The "killer app" of the quantum internet in my mind is as simple as just sending qbits around. Currently every network call involves an observation that collapses the system wavefunction. If you're looking to actually network quantum devices (say, to run distributed quantum computations) then you need quantum infrastructure.
I'm curious too! I'd immediately understand if it allows for speed of light communication wireless, but this is clearly wired, requiring more precision engineering than usual fibre.
I guess, but benefits should be more theoretical. Like i don't think building one will give any insight into ideas for protocols. We already understand how it would work in theory and have for a long time.
Safer mechanisms of distributing and establishing "root" keys for identify verification (so you can then use them easier with normal D-H on normal internet) is one use case I recall from 1990s.
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.
The ideas discussed in 1990s suggested a way to ensure that mitm guaranteed deviation from data transmitted. How well it would work in real life I have no idea
QKD is only safe against MITM if you have pre-shared keys between the parties. At that point you might as well use symmetric cryptography which is immune against hypothetical quantum computers and infinitely more efficient than QKD.
- 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 first one helps with physical attacks on the wire. Not a common issue that people worry about, since there are so many boxes in between that are easier to compromise that it's rarely a significant security increase if you know the wire is perfectly secure.
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.
People have dealt with the second one in sibling comments but I somewhat doubt the first one is true when you take into account sidechannel attacks on the encoding and decoding part of the transmission.
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.
As far as In understand it (not very much) you can listen in on the transmitted keys, but the interaction can be statistically(!) measured and suspicious bits can me omitted (the wiki is quite comprehensible: https://en.wikipedia.org/wiki/Quantum_key_distribution?wprov...).
There are different protocols, some more and some less quantum and most rely on classical, encrypted channels and trusted nodes in addition to the quantum channels.
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!)
As far as I know, they still need classical encryption methods (with something like shared secret key or public key for authentication) to detect active man in the middle attacks where the attacker prevents the parties connecting to each other and then pretending to both parties to be the other party by creating his own "messages" as if they came from the other party. Or at least to have some kind of additional trusted physical medium where it is impossible to prevent the parties communicating directly, capturing their "messages" and then sending your own modified "messages" instead -- perhaps based on some kind of timing etc.
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?
You don't need "classical encryption" for quantum key distribution. With QKD you can provably detect if a MITM attack happened. With classical methods you can never be 100% sure, although how much of that matters in practice is another question.
> You don't need "classical encryption" for quantum key distribution. With QKD you can provably detect if a MITM attack happened.
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.