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A rudimentary quantum network link between Dutch cities

tudelft.nl

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Re: A rudimentary quantum network link between Dutch cities

#41
post #40

Earlier quoted context omitted.

I don't think it's completely clear (to me) that quantum networking is an oxymoron. I would enthusiastically agree that its very complicated and the real world use cases are incredibly limited. 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 hal…

Lots of handwaving there. Particularly with "and I can (locally) do stuff" Good luck with all of that.

The I can locally do stuff is completely understood theoretically/mathematically. I hand waved because this isn't a forum where those technicalities are particuarly relevant.

Its been well understood since at least 1993

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.71...

Re: A rudimentary quantum network link between Dutch cities

#42
post #34

Earlier quoted context omitted.

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 .

Cool thanks

You're welcome.

Re: A rudimentary quantum network link between Dutch cities

#43

Hi all! I'm one of the co-authors. Honestly it's a dream to end up on HN with my research. As mentioned in the video we made, it has been a long road (6-7 years) to achieve this absolute moonshot of a project. I think we'll look back on this demonstration as the first experiment that truly made a distributed and real-world deployed quantum network. Not only did we use a (quantum) hardware platform capable of quantum…

How hard do you expect it would be to improve the heralded infidelity from 45% to 10%?

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).

1: https://arxiv.org/pdf/2404.03723#page=4

Re: A rudimentary quantum network link between Dutch cities

#44
post #38

Earlier quoted context omitted.

All of the quantum networking stuff is point-to-point. It's not clear to me whether fiber amplifiers are even allowed on these links.

Amplification would absorb one photon and replace it with one or more new photons. Definitely not quantum. 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 co…

> What are we going to do, run direct fiber from every computer to every other computer directly?

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

Re: A rudimentary quantum network link between Dutch cities

#45
post #29

Hi all! I'm one of the co-authors. Honestly it's a dream to end up on HN with my research. As mentioned in the video we made, it has been a long road (6-7 years) to achieve this absolute moonshot of a project. I think we'll look back on this demonstration as the first experiment that truly made a distributed and real-world deployed quantum network. Not only did we use a (quantum) hardware platform capable of quantum…

Layman here! I have no idea what's going on but I have many questions! - 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 t…

All good! That was me 5 years ago :)

- 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).

Re: A rudimentary quantum network link between Dutch cities

#46
post #6

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?

(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.

Re: A rudimentary quantum network link between Dutch cities

#47
post #25
post #19

Earlier quoted context omitted.

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 o…

> Safer mechanisms of distributing and establishing "root" keys for identify verification Except it doesn't solve the mitm problem, so its not really safer.

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

Re: A rudimentary quantum network link between Dutch cities

#48
post #24

Earlier quoted context omitted.

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 c…

I agree that quantum computers are useful. Its quantum internet that seems pointless. As far as i am aware, none of the problems faster on a QC are helped in anyway by quantum internet.

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)

Re: A rudimentary quantum network link between Dutch cities

#49
post #47
post #25

Earlier quoted context omitted.

> Safer mechanisms of distributing and establishing "root" keys for identify verification Except it doesn't solve the mitm problem, so its not really safer.

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.

Re: A rudimentary quantum network link between Dutch cities

#50
post #29

Earlier quoted context omitted.

Layman here! I have no idea what's going on but I have many questions! - 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 t…

All good! That was me 5 years ago :) - 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 th…

The article says

> 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?

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