# Problem with quantum FTL communications

1. Nov 1, 2013

### 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, lets 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.

2. Nov 1, 2013

### 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:
... it's a very popular topic.

3. Nov 2, 2013

### San K

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

Last edited: Nov 2, 2013
4. Nov 2, 2013

### meBigGuy

And, the particle measurements will always correlate.

5. Nov 8, 2013

### 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.....

6. Nov 8, 2013

### Simon Bridge

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

7. Nov 8, 2013

### nikkkom

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 :)