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I know that this has been disregarded but I could not follow up since I'm new and it's an old post anyway.

I understand the reason that superluminal communication via entanglement won't work. Is that the distant qubit has a totally random reaction or reorientation when the other entangled particle is changed.

However, I fail to see how this would negate the possibility of super luminal communication.

Is there some reason why the qubits couldn't be checked at regular intervals using an atomic clock at both ends and any change is a one and no change is a zero? I suppose you would need multiple entangled sets. One for receiving one for transmitting though. Honestly, you could have a great many entangled to increase bandwidth?

Sorry to pose dumb questions but the discouragement I saw in a previous post really didn't take with me. Since both transceivers are flying through space in some way, I suppose I could see even this would be difficult. I do understand that the atomic clocks would not sink because of this. I just think the machines could easily calculate how to compensate based on the speed the transceiver is moving and the approximate distortion of space due to local mass.

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The problem is that there's no way to definitively say what state each qubit started in if they were entangled, so there's no way to even establish what it means for one qubit to "change." If you know the starting state of your qubit with 100% accurary, it by definition cannot be entangled with someone else's qubit. If you want things done on someone else's qubit to affect your qubit, your qubit must be entangled with the other one, and then you must not have known your qubit's state a priori.

If you want to know what state your qubit starts in, measurement will also ruin your entanglement. You only get one shot at measurement, which might be before or after the other person did something to their qubit, but you'll never be able to distinguish between any of the possibilites because you don't even know if you're measuring the "before" state or the "after" state when all you get is one measurement.

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The root misconception is the idea that an entangled particle somehow reacts when its partner is measured. It doesn't. Nothing changes, either faster than light or slower than light.

The idea that it reacts is known as the "nonlocal wavefunction collapse" and almost no physicist thinks that it actually happens. There's lot of disagreement between the interpretations of quantum mechanics, but this is something that almost all of them agree about (for different reasons, of course).

If one believed that "nonlocal wavefunction collapse" actually happened, then indeed it would be natural to expect superluminal communication to be possible.

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  • $\begingroup$ Agreed. A fun thing to add is when we talk about operations other than measurement. As pointed out in this recent question quantumcomputing.stackexchange.com/q/34128/15820, maximally entangled states have the peculiar property that a local operation on one can instead be interpretted as a local operation on the other. That's what I tried to address in my answer: why, even with this local change, it is impossible to actually know that such a local unitary has been applied $\endgroup$
    – Quantum Mechanic
    Commented Sep 13, 2023 at 19:25
  • $\begingroup$ That's true, but I'm afraid one can read your answer and conclude that there is some action at a distance, but there's also some elaborate conspiracy to prevent you from ever seeing it manifest. That's why I felt compelled to post another answer emphasizing that this is not the case. $\endgroup$
    – Mateus Araújo
    Commented Sep 14, 2023 at 9:48
  • $\begingroup$ Yes - that's why you have my upvote! But it's true that mathematically it can look like something happens at a distance ($U\otimes I$ really produces the same effect as $I\otimes U^*$ when acting on a maximally entangled state), it's just that no information can be transmitted and you'll never be able to measure anything to have happened, so for the purpose of describing reality it is easiest to say that nothing happens $\endgroup$
    – Quantum Mechanic
    Commented Sep 14, 2023 at 15:43

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