How can information be communicated with entanglement?

Question

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?

Does this answer your question? Why is quantum entanglement considered to be an active link between particles? — Norbert Schuch, Dec 24 '20 at 18:23
This doesn't necessarily answer my question, since I already have a reasonable grasp on the link between the two entangled particles, and the inter-dependency on their collapsed states. — michael b, Dec 24 '20 at 18:28
So what is your question, beyond what is adressed in the linked answers? -- Note that "Does this answer your question?" is SE's default text for duplicate flags. — Norbert Schuch, Dec 25 '20 at 02:05
Since the answer to my title question is no, the follow up questions are: How can we engineer anything with entanglement? Are the communication speed limits linked to any limitations for quantum computing? — michael b, Dec 25 '20 at 03:59
You should focus on one question. Part of the problem is that the title and the question text itself don't ask the same thing (and, in fact, the question text answers the question in the title). — Norbert Schuch, Dec 25 '20 at 04:30

score 4 · Accepted Answer · answered Dec 24 '20 at 18:20

4

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.

answered Dec 24 '20 at 18:20

gandalf61

52,505

Thanks for the answer. So... A couple follow ups. I would assume this means all claims of FTL communications are bogus? Also what does indicate about the limits on quantum computation (if any)? – michael b Dec 24 '20 at 18:30
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Indeed, superluminal communication is impossible – Nihar Karve Dec 25 '20 at 02:21
@MichaelBurt Yes, you should be highly sceptical of any claim of FTL communication of information. The implication is that quantum computing is subject to some of the same limits on performance as classical computing. – gandalf61 Dec 25 '20 at 02:24
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Thanks for your time folks. I probably need to learn a little more about quantum computation as I still struggle with understanding how probabilistic measurements can yield deterministic computation. But that's for another day and possibly another SE question down the road... – michael b Dec 25 '20 at 04:07
@Michael Determinstic evolution yields deterministic computation (if done right)! – Norbert Schuch Dec 25 '20 at 04:33

How can information be communicated with entanglement?

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