Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Problem with quantum FTL communications

  1. Nov 1, 2013 #1
    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. jcsd
  3. Nov 1, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    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.
  4. Nov 2, 2013 #3
    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
  5. Nov 2, 2013 #4


    User Avatar
    Gold Member

    And, the particle measurements will always correlate.
  6. Nov 8, 2013 #5
    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.....

  7. Nov 8, 2013 #6

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    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".
  8. Nov 8, 2013 #7
    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 :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook