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Quantum teleportation question

  1. Nov 15, 2012 #1
    Hi everyone.
    I had a question to ask about quantum teleportation and quantum entanglement. I read that recently, some scientists managed to make an altercation to one quantum particle (a photon, i think) and the same change occured to the other particle that was 143 km away. but there were parts that I couldn't understand. Firstly, I could not find anything on the web about "how" they managed to make an altercation to the photon. In one video I saw, it said that the spin was changed, still without mentioning how. Secondly, because actually measuring the particle would remove its entanglement and the superposition state, the scientists had another photon in a different quantum state and then measured the quantum state of the first photo relative to the second photon. What does this mean?
  2. jcsd
  3. Nov 15, 2012 #2


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    Good questions!

    With entangled photons pairs, usually measured respectively by Alice and Bob, it works like this. Alice is separated from Bob by some distance (143 km in this case). Alice chooses to measure photon polarization (spin) at some angle, say 45 degrees - the outcome will be either +/- (or 0/1, or +1/-1, you can label it anyway you like as long as it is a bit of information). That guarantees that if Bob also measures at 45 degrees, he will get the opposite result. It therefore appears as if Alice's choice of angle to measure influences Bob's results, even though he is far away. (Please note that this is something of an illusion, because you could just as easily say it was Bob's decision that influenced Alice's result.)

    Usually the above experiment is expanded to include a test of what is called a "Bell Inequality". It is this test that truly demonstrates entanglement is occurring. The Bell test is done at a variety of specific angle pairings.
  4. Nov 15, 2012 #3


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    Well, this depends on what you mean by "altercation". In quantum teleportation, you can change the wavefunction of one photon by measuring the other photon. Whether this change is a real change in the photon depends on how you interpret the reality of the wavefunction. You can't actually flip the spin of one photon and expect the spin of the other photon to flip as well. You can only choose which angle to measure the spin at, and see which direction it decided by itself to point in. Somehow, the other photon decides to point in the opposite direction.
  5. Nov 15, 2012 #4


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    In English, that would mean there's an argument going on. I'm pretty sure he mean alteration, not altercation. :smile:
  6. Nov 15, 2012 #5
    Haha yep you're right. My bad :p I did mean alteration

    But according to quantum physics, doesn't entanglement of the two particles 'disappear' (I'm not sure if this is the right term but I mean the particle that existed in a superposition state becomes defined and thus exists at only one place) when you measure it/observe it? So how did the scientists that I measured manage to observe it? This again goes back to my original question. To expand this, they had one quantum particle (unentangled) (we will call this a) and a pair of entangled particles (b and c). They apparently measured b relative to a so as to be able to measure it but without removing the entangled state. I don't see what this means.
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