Problem with quantum FTL communications

[faramund]

I know its risky to rely on wikipedia, but on the page on quantum entanglement, it claims that possibly instantly, as soon as one of a pair of entangled particles is measured (say A), then the opposite one (say B) takes on the opposite value.

Now to know this, I assume B must somehow know the time when the value of B's particle takes on a value. To labor the point, let's say A measures their particle at 12:00, so at 11:59 B must have known that their particle hadn't been measured, but at 12:00+ a small amount of time, that it had.

So if everything above here is true, can't this be used for faster than light communication.

To do this, if A wants to send a 0, the procedure would be to measure the particle at some pre-specified time (say 12:00), but for 1, to not measure the particle.

So then B, wherever they were, would simply have to check their particle at 12:00+a small amount of time, and then by seeing whether or not the particle had been measured, they would know whether or not A wanted to send a 0 or 1.

To labor the point, the value of the particle is irrelavant - its merely necessary to establish whether the particles are still indefinite, or if they have been measured.

 

[Simon Bridge]

Welcome to PF;
Quantum entanglement cannot be used for FTL communication. No.

Lets say a particle spin-state is entangled.
Someone measures spin-up at home and at that instant the entangled particle is spin down on alpha-centauri.

If the measurement at home had not been made - the apparatus on alpha centauri would still return either spin up or spin down ... it never returns an indeterminate" reading.

Also see:
https://www.physicsforums.com/showthread.php?t=421314
https://www.physicsforums.com/showthread.php?t=483963
https://www.physicsforums.com/showthread.php?t=231008
... it's a very popular topic.

 

[San K]

Now to know this, I assume B must somehow know the time when the value of B's particle takes on a value. To labor the point, let's say A measures their particle at 12:00, so at 11:59 B must have known that their particle hadn't been measured, but at 12:00+ a small amount of time, that it had.

So if everything above here is true, can't this be used for faster than light communication.

To do this, if A wants to send a 0, the procedure would be to measure the particle at some pre-specified time (say 12:00), but for 1, to not measure the particle.

So then B, wherever they were, would simply have to check their particle at 12:00+a small amount of time, and then by seeing whether or not the particle had been measured, they would know whether or not A wanted to send a 0 or 1.

To labor the point, the value of the particle is irrelavant - its merely necessary to establish whether the particles are still indefinite, or if they have been measured.

welcome to PF faramund.

Two things (at the least) that we need to be aware of:

A (or B) cannot tell/determine (in advance) whether B (or A) has done a measurement on their particle.

A (or B) cannot control the spin/outcome (0 or 1) of the particle

 

[meBigGuy]

And, the particle measurements will always correlate.

 

[JayJohn85]

Man my reality thread got deleted and it was those questions that brought me to interesting stuff like bell's theorem but I see it may be a sore point. Anyway can't use it for communication cause:

I assume this quote from wikipedia to be relevant man this stuff is head melting...

The Bell test experiments have been interpreted as showing that the Bell inequalities are violated in favour of QM. The no-communication theorem shows that the observers cannot use the effect to communicate (classical) information to each other faster than the speed of light, but the ‘fair sampling’ and ‘no enhancement’ assumptions require more careful consideration (below). That interpretation follows not from any clear demonstration of super-luminal communication in the tests themselves, but solely from Bell's theory that the correctness of the quantum predictions necessarily precludes any local hidden-variable theory. If that theoretical contention is not correct, then the "tests" of Bell's theory to date do not show anything either way about the local or non-local nature of the phenomena.

 

[Simon Bridge]

Return to post #3.
Try to figure how to use the spin state of entangled particles to send 1 bit of information - just a "yes" or "no".

 

[nikkkom]

So then B, wherever they were, would simply have to check their particle at 12:00+a small amount of time, and then by seeing whether or not the particle had been measured

How would B see "whether or not the particle had been measured"?

B just gets "spin is up" or "spin is down" result.

B can never get a "spin is up and if you'd magically be able to measure me again and again and again without destroying my state, it would be always up (i.e. A has measured my entangled twin's spin and it is down)" result. Particle wouldn't tell that secret :)