> Tricks that use entanglement to instantly send information generally involve collapsing the state of the distant particle (by measuring your own), and assuming that the person in charge of that particle will somehow notice. Unfortunately (and this is the answer), there’s no “new-particle-smell” for particles that are still in superpositions and there’s no big flash when that superposition collapses. If you measure an electron’s spin the result is simply “spin up” or “spin down”. That’s all you get. But that doesn’t tell you what the state was before, or if the original state was a superposition of several states, or even if the particle was entangled with something else.

> If you only have access to one particle, then the two situations, 1) the other particle is inaccessible or 2) it has been quietly measured, are indistinguishable in every physical sense. Regardless of whatever else is involved, you can never notice the weirdness of entangled pairs until you directly compare them with each other.

https://www.fierceelectronics.com/electronics/aes-256-joins-...

In any case, doubling the key length would be infinitely simpler than using QKD.

“Instantaneously” kind of means that one event does not happen before or after the other, but something happening before or after should depend on the reference frame, right?

This article from 2009 discusses these kinds of matters, and near the end it talks about absolute simultaneity:

"The kind of nonlocality one encounters in quantum mechanics seems to call for an absolute simultaneity, which would pose a very real and ominous threat to special relativity."

https://www.scientificamerican.com/article/was-einstein-wron...