A science geek here with a question about QM.
I've been watching conferences and reading about Quantum Entanglement, and my doubt is whether information is or is not exchanged between two entangled particles.
What I think is let A and B be two particles with entangled spins. It is unknown what the spin of each particle is, but it is known that one is the "opposite" of the other. So when you measure A and see what is its spin you "instantly" know the spin of the other because particle B no longer exists in its superposition state and its spin is now defined by particle A.
Now, is really a message, information, you name it, sent or it is just something that happens from the point of view of the person doing the measurement?
My interpretation is that no information is sent and no FTL communication happens, as it is everything just the revealing of a previously-set property (in the moment of the entanglement) as seen by the viewer.
Answer
There are several misconceptions here:
everything is just the reveal of a previously set property (in the moment of the entanglement) as seen by the viewer
What you're describing here is a local hidden variables model of the experiment, and these are known (through Bell's theorem) to be incompatible with quantum mechanics. Where local-hidden-variable theories conflict with QM, experiment has consistently sided with the quantum mechanical predictions.
That said, if you do have an entangled pair in an 'opposite-spins' state (technically, the singlet state $\left|\uparrow\downarrow\right>-\left|\downarrow\uparrow\right>$, but it's important to know that there are other entangled states that do not share the 'opposite-spins' property), you can try to transmit a message by measuring at A in the $\{\left|\uparrow\right>,\left|\downarrow\right>\}$ basis, and thereby "controlling" the measurements on B, which are constrained in this state to be completely anti-correlated with the measurements in A.
That won't work, for a simple reason: you don't control what measurement outcome you'll get at A ─ you'll get an even mixture of ups and downs ─ and therefore you can't control what's seen at B. You have no way to dial in a message to begin with.
You can try and get beyond that by controlling the type of measurement you apply at A. That also doesn't work, for much the same reasons.
More generally, entanglement cannot be used for communication, period; that is the content of the no-communication theorem. It can still be used to show correlations that go beyond what can be displayed by classical systems (due to the Bell-inequality violations mentioned above), but those only show up once you collate (at light-speed or slower) the results from measurements at the two locations.
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