Sure, but you could still create a two way walkie talkie that couldn't be eavesdropped on and would allow faster than light communication this way, theoretically.
You can make it secure from mitm with this but it doesn't allow ftl. You can't use the entangled particles themselves to do the communication since you cant control how the superposition collapses when you measure it. This allows you to get two streams that nobody but you and the party knows but they're fundamentally random and unpredictable, you can't encoded information into it, just use it as a one time pad essentially.
Say you have two sets of two entangled particles, all four particles in similar states. You have one particle from each set, your partner has the corresponding particles in the sets.
You check both particles you have, and:
- if both particles have similar state, then your partner hasn't checked any of their particle states, because neither have collapsed.
- if one particle has collapsed and is in a different state than the other particle, then you know your partner has at least checked the state of one of particle sets, possibly both.
If you partner is 50 light years away and checks it only seconds prior to your check, then they are signalling you at faster-than-light travel, right?
But you have to communicate the states of each other's particles (as far as you can tell random noise) to compare/contrast and glean influence, so it's only suitable for encryption of classically transmitted messages.
Got it, so the state is random for all particles until it is measured by your or your partner, afterwhich the other particle state is known. But the act of measuring or the decaying doesn't tell you when (or if) they measured it because the state never changes, it just becomes known.
It is perhaps less nonsensical (in a "common sense" way) to think of it in terms of the many-worlds interpretation. The moment you have created the photons, you have forked the universe - but observers on both sides are still identical in both worlds, because they haven't done anything that'd allow them to figure out which universe they're in. So for each observer, we treat both "versions" of that observer as one and the same.
Now, when one of the observers measures their photon, they figure out which fork they're in. And then in that universe, the other observer measuring it will also get the same result, of course, proving that they're also in that same universe. And in the other universe, the second observer will get the opposite result, again, proving that they're in that other universe. But neither measurement actually caused anything to happen, nor did they get any information about the opposite observer in their universe as a result of it.
abdullahkhalids has the correct answer. Even if you have the entangled particles already in place, you can't communicate with them.
The real proof is that if you could, we'd already have this technology and high-speed traders would already be using it. If it could exist, we have all the tools to build it; it's far simpler than even a 4-qubit quantum computer, which we've also built. We don't have it because entanglement can't be used that way.
