Which is still extremely useful for things like probes.

Instant comm's to Mars rover or even voyager.

Of course no ones figured out how to make use of entanglement for information transfer.

Pops up a lot in Sci-Fi, reading Three-Bodied Problem now and the aliens sent "sophons" basically probes the size of protons at near the speed of light to earth from Alpha Centauri, they arrived in a few years while their invasion fleet will take 400 years to get here. The sophons spy and interfere with earth communicating with aliens via quantum entanglement.

> Of course no ones figured out how to make use of entanglement for information transfer.

Saying "no one's figured it out" is misleading because it creates the impression that it's an engineering problem to be solved.

According to our best understanding of quantum mechanics, the laws of physics fundamentally do not permit faster-than-light communication, whether through entanglement or any other mechanism.

Well, of course they don't. How would you even begin to build a simulator for something like that?

Communications overhead between the nodes is bad enough as it is.

Our understanding of the model is thought to be quite poor. Further, we should not ever mistake the model for reality or attempt to fit data to predictions. The cosmos might not be anthropomorphic.

The no communication theorem is not the no communication hypothesis, it's actually a math proof. You start from the principles of quantum mechanics (unitarity, etc), define what you mean by entanglement (non-factoring states), and prove it's not possible to use those states to win the communication game. This is an aspect of quantum mechanics that we understand extremely well.

I was trying to imply that its probably the best avenue for research, and also we may never figure it out, might be a dead end.

However I would say the reason it is used so much in sci-fi is it currently the one way we can cause action at a distance faster than light, which is so tantalizingly close to communication.

I wouldn't say we're close to FTL transfer of information in any meaningful way. In particular,

> we can cause action at a distance

is not true, either.

>is not true, either.

I didn't think this was settled and depends on your interpretation of QM: https://en.wikipedia.org/wiki/Action_at_a_distance

…and on your precise definition of "action and a distance", yes.

But there is something like standard textbook quantum mechanics which most physicists subscribe to, i.e. the Copenhagen interpretation, and while I'm not saying this is the interpretation everyone should follow and believe in (in fact, there are good reasons to be dissatisfied with this interpretation), the term "action at a distance" was coined in the context of the Copenhagen interpretation, so I think should therefore also be interpreted in this context.

Now, Copenhagen is a local and non-realist theory, the latter of which meaning among other things that the wave function is not believed to be an object of reality. Therefore, in this interpretation, the collapse of the wave function is merely a tool for predicting the outcome of a measurement and not something that's believed to actually happen. The same thing goes for an "instantaneous collapse" at a distance — this action does not exist in reality.

I know there is probably going to be someone who objects to me saying that Copenhagen is non-realist and local. But those people will have to explain to me how they would bring the actual existence of the wave function and its collapse into agreement with relativity and, in particular, with different observers who come to different conclusions about when the wave function collapses. For instance, for one observer, it might be Alice who causes the wave function to collapse and, thus, Bob to measure a certain outcome but then there is always going to be another observer who will say the exact opposite, namely that Bob caused the collapse. So, essentially, if you really think the wave function is an object of reality, all hell breaks lose.

Needless to say, even if you completely disregard these issues and still want to think of Copenhagen as a realist but then necessarily non-local theory (in which case I'd say you should really give your theory a different name), this is all just splitting hairs because the only thing that matters is that there's no FTL transfer of information. This is the only experimentally verifiable statement we can make.

There is no reason to think quantum entanglement could be used for FTL communication. The theory which gives us entanglement also proves (mathematically) it can't be used for communication.

Sure, but you could still create a two way walkie talkie that couldn't be eavesdropped on and would allow faster than light communication this way, theoretically.

You can make it secure from mitm with this but it doesn't allow ftl. You can't use the entangled particles themselves to do the communication since you cant control how the superposition collapses when you measure it. This allows you to get two streams that nobody but you and the party knows but they're fundamentally random and unpredictable, you can't encoded information into it, just use it as a one time pad essentially.

But that in itself is a signal, right?

Say you have two sets of two entangled particles, all four particles in similar states. You have one particle from each set, your partner has the corresponding particles in the sets.

You check both particles you have, and:

- if both particles have similar state, then your partner hasn't checked any of their particle states, because neither have collapsed.

- if one particle has collapsed and is in a different state than the other particle, then you know your partner has at least checked the state of one of particle sets, possibly both.

If you partner is 50 light years away and checks it only seconds prior to your check, then they are signalling you at faster-than-light travel, right?

But you have to communicate the states of each other's particles (as far as you can tell random noise) to compare/contrast and glean influence, so it's only suitable for encryption of classically transmitted messages.

Think faster-than-light XOR ;)

Got it, so the state is random for all particles until it is measured by your or your partner, afterwhich the other particle state is known. But the act of measuring or the decaying doesn't tell you when (or if) they measured it because the state never changes, it just becomes known.

It's not random; it's a superposition.

It is perhaps less nonsensical (in a "common sense" way) to think of it in terms of the many-worlds interpretation. The moment you have created the photons, you have forked the universe - but observers on both sides are still identical in both worlds, because they haven't done anything that'd allow them to figure out which universe they're in. So for each observer, we treat both "versions" of that observer as one and the same.

Now, when one of the observers measures their photon, they figure out which fork they're in. And then in that universe, the other observer measuring it will also get the same result, of course, proving that they're also in that same universe. And in the other universe, the second observer will get the opposite result, again, proving that they're in that other universe. But neither measurement actually caused anything to happen, nor did they get any information about the opposite observer in their universe as a result of it.

Nope, you couldn't. From all the experiments and theory we have on entanglement, you can't use it to communicate faster than light.

abdullahkhalids has the correct answer. Even if you have the entangled particles already in place, you can't communicate with them.

The real proof is that if you could, we'd already have this technology and high-speed traders would already be using it. If it could exist, we have all the tools to build it; it's far simpler than even a 4-qubit quantum computer, which we've also built. We don't have it because entanglement can't be used that way.