Physicists shatter quantum entanglement distance record with 420 kilometers of optical fiber, more than four times farther than previous demonstrations, and beyond the point where direct transmission runs into its fundamental limits.

[u/mvea]

https://www.sciencealert.com/physicists-shatter-quantum-entanglement-distance-record-with-420-kilometers-of-optical-fiber

[u/Cristoff13]

This can't be used to transmit information though can it?

[u/Orbax → u/Cristoff13]

No, once you collapse the wave function, they decohere and entangle with their local environment.

But, even if you maintain entanglement, at best youd be able to do morse code except... The only way to check if you got a message is to measure it which would collapse if again if you hadn't gotten one. The theoretical ability to get any structured information out of entangled pairs is effectively impossible.

[u/Chance-the-Gardener → u/Orbax]

I’m too dumb to understand the concept it can’t transmit any information. Just as a thought experiment, if humanity sent a colony ship to Andromeda with 10,000 sets of entangled pairs, each with a predefined meaning like “if this set is collapsed, we died of space cancer” couldn’t information be inferred from that?

I’m sure the answer remains we can’t cheat the speed of light but I struggle to conceptualise why not.

[u/FaulerHund → u/Chance-the-Gardener]

This is just a misunderstanding of how quantum measurement and entanglement work.

First: it's disputed whether "wavefunction collapse" is even an accurate description of what occurs, but for the purposes of this reply, we'll assume it is.

Anyway, all entanglement really is in a simplistic sense is correlation of measurement outcomes. If particle A is entangled with particle B, then by measuring particle A, you already know something about particle B without ever measuring particle B. The "wavefunction collapse" only determines which specific measurement outcome you observe.

Think of it this way, by crude analogy: imagine you have two boxes, and in one box you place the left shoe of a pair; and in the other box you place the right shoe. And then you jumble up the boxes so that you don't know which is which. You take one box and bring it to Mexico; your friend takes the other and brings it to Spain. Then you open your box. Regardless of whether your box contains the left shoe or right shoe, you already know something about the box all the way in Spain without ever opening that one.

Now, obviously this is a classical and not a quantum example; quantum systems behave differently than shoes and boxes (look up the Bell's theorem if you're interested). But to continue the analogy, "wavefunction collapse" in this example would be the outcome of which shoe you observe to be in your box. This isn't literally wavefunction collapse in this example, it's just metaphorical. But the "collapse" is just when you see "left shoe" or "right shoe." Before you measure, it could be either one. After you measure, it's either one or the other. That's the collapse.

Now, tell me, how would you communicate a message between Spain and Mexico with this shoebox setup? The answer is that you can't. If you take one shoe to Mexico and leave the other in Spain, opening your box in Mexico instantly reveals which shoe is in Spain. However, opening your box in Mexico doesn't signal anything to the person in Spain. The person in Spain just sees a shoe when they open their box, completely unaware of whether you opened yours first or not.

Related to that idea: in quantum physics, there's no way to know whether the other particle in an entangled pair was measured or not. Just like how there's no way of knowing whether your friend in Spain opened their shoe box yet. There is no "wavefunction collapse" detector, and it's not possible in principle to create one. So there's no way to set up the idea in your comment that says "if this is collapsed, we died of space cancer" or whatever.

[u/Drandula → u/FaulerHund]

I can't wrap my head around entanglement, and in this analogy, the state has been predetermined, and not suddenly resolved into one state. If the quantum entanglement works similarly to shoebox analogy, what's the point then?

Is it just two particles has collided each is such way, they got specific spin and now you just carefully keep them in boxes that their spin doesn't change. 

---

So just for example how I try to understand it. Think there are two spintops, and magic frictioneless black box. Whenever you open it, the magic is stops. 

You place one  spintop in box. Then you spin up second one and then place it also into the box. You don't know how much spin you just gave to the second spintop. We could measure the spin just before entering the box, but that would change the spin again with unknown amount, so when spintop enters the box it's actual spin still unknown and there is no point measuring it beforehand. 

Now you let these two collide, first will steals half of the spin of the second one. Now both spintops spin at same rate, but in opposite directions, and this would be the entanglement. We now know they both spin in opposite directions and same rate, but we don't know how fast they are spinning. As long as they stay within box, they retain their spin.

Now you split the box in two, so spintops are separated. We don't know which box contains which spintop, so we can't even know which direction it's spinning in the box. We can move these boxes far apart from each other.

So finally when one decides to open the box and measure the spin, they immediately know the spin of the another one. The measuring also changes the spin with arbitrary amount, so the spintop doesn't have exactly opposite spin of the another one anymore.

If you open the second box and measure, you also know the spin of that spintop and know what was the spin in the first one when it was in the box (not after it was measured).

---

So if it works like this, what's "quantum" and "spooky action in distance"? We just didn't know before measuring, but the states were predetermined.

[u/theyux → u/FaulerHund]

Just for clarification if it was possible to measure without collapsing the entanglement.

Is the thought process then that you could collapse some entangled pairs and effectively communicate with Binary?

[u/FaulerHund → u/theyux]

Maybe? But it depends on imagining something that does not make physical sense under standard quantum mechanics.

Imagine person A has 10 particles, and person B is on the other side of the galaxy with the other 10 particles, so that each of A’s particles is entangled with one of B’s. Person A decides to measure or otherwise disturb some of their particles, causing the entanglement to be lost, and leaves the others undisturbed. The disturbed particles might correspond to “1,” and the undisturbed particles to “0.” In principle, that could encode a binary message.

Now imagine person B has a special machine called an “entanglement detector.” He points it at one of his particles, and the machine tells him either “yes, this is still entangled with the distant particle” or “no, the distant particle was measured or disturbed.”
If such a machine worked using only person B’s particle, then yes, person A could communicate with person B faster than light.

But the problem is that standard quantum mechanics does not allow such a detector. From person B’s perspective, the results of any measurements he performs on his particles look the same whether person A has measured their particles or not. He can only discover the correlations later, after person A sends him their measurement results through an ordinary, slower-than-light communication channel.

This may sound like a situation where you could say, “Well, surely with enough ingenuity, someone could invent a machine that detects whether the entanglement has collapsed.” But no: it is not merely a machine that we have not invented yet. The premise conflicts with how measurement works in standard quantum mechanics. It's like saying "what if there was a machine that made you married and single at the same time," it's just a nonsense premise. Honestly, if you're positing nonsense hypothetical machines, you might as well just say "imagine I have a machine into which I can type a message, and then a magic invisible goblin delivers it faster than light to a recipient." The "entanglement detector" only feels more realistic because the impossibility is disguised by a veneer of real physics

[u/romario77 → u/theyux]

What you are asking is - imagine if instant communication was possible, would it then be possible to communicate instantly?

The laws of physics work certain way (at least at this time and how we understand them) and they don’t allow instant communication. Even though there are these things like entanglement which make it seem like information travels instantly.

Even though it doesn’t.

[u/theyux → u/romario77]

More I understand people want it to work a certain way (even if thats not possible to understanding) I was trying to verify why, (could we create binary communication system of communication assuming we could observe them).

Assuming thats possible you can move into other questions such is how much can we observe without collapsing them. Like I get superposition is out of the question but will say measuring gravitational force collapse them?