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A rudimentary quantum network link between Dutch cities

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Re: A rudimentary quantum network link between Dutch cities

#51

Post by amy-petrik-214 »

ricardobeat wrote: post #50
metropolbadger wrote: post #45

Earlier quoted context omitted.

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?

it's mostly used for crypto if I measure X here then I know the other guy will measure Y, and that is instant. But I can't force a measurement of 0 or 1 for X so as to force the Y (i.e communication).

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.

Re: A rudimentary quantum network link between Dutch cities

#52

Post by bawolff »

vtomole wrote: post #34
bawolff wrote: 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?

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 .

Thanks, that was really intrresting.

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.

Re: A rudimentary quantum network link between Dutch cities

#53

Post by bawolff »

ccppurcell wrote: post #48
bawolff wrote: post #24

Earlier quoted context omitted.

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 messag…

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.

Re: A rudimentary quantum network link between Dutch cities

#54

Post by dwnw »

Strilanc wrote: post #44
dwnw wrote: post #38

Earlier quoted context omitted.

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 neigh…

This is all half-baked and either insecure or unrealizable in the real world we live in.

Re: A rudimentary quantum network link between Dutch cities

#55

Post by ccppurcell »

bawolff wrote: post #53
ccppurcell wrote: post #48

Earlier quoted context omitted.

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 messag…

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.

Re: A rudimentary quantum network link between Dutch cities

#56

Post by TorKlingberg »

amy-petrik-214 wrote: post #51
ricardobeat wrote: post #50

Earlier quoted context omitted.

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?

it's mostly used for crypto if I measure X here then I know the other guy will measure Y, and that is instant. But I can't force a measurement of 0 or 1 for X so as to force the Y (i.e communication). 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 communi…

> 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

Wait, does this work? Are superposition detection devices theoretically possible? Got any reference with more on this?

Re: A rudimentary quantum network link between Dutch cities

#57

Post by feoren »

amy-petrik-214 wrote: post #51
ricardobeat wrote: post #50

Earlier quoted context omitted.

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?

it's mostly used for crypto if I measure X here then I know the other guy will measure Y, and that is instant. But I can't force a measurement of 0 or 1 for X so as to force the Y (i.e communication). 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 communi…

> 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 ... "measured or not" thus is our "bit" that has been communicated instantly.

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.

Re: A rudimentary quantum network link between Dutch cities

#58

Post by vtomole »

bawolff wrote: post #52
vtomole wrote: 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 .

Thanks, that was really intrresting. 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.

Re: A rudimentary quantum network link between Dutch cities

#59

Post by Kon-Peki »

> “The distance over which we create quantum entanglement in this project, via 25 km of deployed underground fiber, is a record for quantum processors,” says Hanson. “This is the first time such quantum processors in different cities are connected.”

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...

Re: A rudimentary quantum network link between Dutch cities

#60

Post by gdavisson »

amy-petrik-214 wrote: post #51
ricardobeat wrote: post #50

Earlier quoted context omitted.

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?

it's mostly used for crypto if I measure X here then I know the other guy will measure Y, and that is instant. But I can't force a measurement of 0 or 1 for X so as to force the Y (i.e communication). 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 communi…

That's not correct; you cannot use a double-slit test to check for entanglement. Running a photon through a double-slit setup always just produces a single dot, not a any sort of pattern. To get a pattern, you need to run a bunch of photons through it and see if a fringe pattern appears [1].

(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...

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