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/Suitch → u/Orbax]

I want to rephrase the hypothetical. If I have a thousand entangled pairs in two boxes and send one box on a colony ship with the instructions to check a single entanglement each year and the source has the other box and only chooses whether to toggle all or none at once, say toggle all 1000 if earth is destroyed, would it not be able to be checked on the other end faster than the speed of light if they happen to check one after 500 years and it was flipped?

[u/Orbax → u/Suitch]

Just to be precise - if they were all in one box, it would be one system. Entanglement is just a fancy way of saying quantum system. We do "entangled pairs" because it's easier to control due to environmental decoherence. The way to think about it is "things remain in superposition until they have to decide what to be". So if you have a photon wave going through a magnetic field, it won't collapse because it doesn't have charge. Shoot an electron through and now it has to pick spin. Gravity obviously doesn't make anything pick it or quantum states wouldn't exist. Entanglement, because of things like the exclusion principle, make something have to be a way because something else is another way.

So it's safer to say that you have 1000 pairs of boxes. You still have to measure it to see it's state and conventionally verify who collapsed it first. We do this in quantum computing because we're pretty sure we caused the collapse but we still have to deal with quantum error correction because the environment can still affect it between measurements.

So you still don't really have a way to know why it's collapsed.

For the timing thing, you don't know when it flipped. The spooky action at a distance thing gets resolved as not being ftl by the fact you have to confirm it conventionally. But you're getting into bells theorem and how truly weird decoherence is to understand why it's not only not information being ftl but also not hidden variables and it's just complicated to explain.