[andai]

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 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.

[metropolbadger]

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).

[ricardobeat]

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?

[amy-petrik-214]

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.

[feoren]

> 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.

[TorKlingberg]

> 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?

[gdavisson]

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...