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Forgive my overly simplistic view of this. But I have been wondering this for years. Any insight would be greatly appreciated.

So imagine you have two entangled particles, and they are separated and put in box A and box B. Box A is transported across the universe and Box B is left where it is. When someone opens Box B (measures Box B) and say for argument, the particle is measured as spin up, they immediately know that the particle in Box A is spin down.

At a glance this seems like they instantaneously "know" something about the other box, and hence seems like they have communicated instantaneously. But the state of the particle in Box B would have a 50/50 chance of being spin up or spin down.

So to me it seems as if there is nothing deterministic about this system. Every few days I read quasi-bogus pop sci articles indicating that scientists have successfully "teleported" particles or communicated faster than the speed of light.

It seems to me there is no way for information to be actually communicated (deterministically) faster than the speed of light. So how can we engineer anything with entanglement?

Norbert Schuch
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michael b
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1 Answers1

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You are right. Bob can only determine the state of his particle (without measuring it) once he receives information from Alice about the outcome of her measurement - and this information can be transmitted no faster than the speed of light.

Similarly, quantum “teleportation” requires Bob to receive information from Alice, transmitted no faster than the speed of light, which he can then use to reproduce the pre-measurement state of Alice’s particle. And this does not contradict the no-cloning theorem because Alice must make a measurement on her particle in order to provide this information, which changes the state of her particle.

gandalf61
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