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B The sticky elactic in the Bloch sphere

  1. Mar 11, 2016 #1


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    I found a funny model of the qubit written by Aerts in
    Foundations of quantum physics: a general
    realistic and operational realistic and operational approach.

    At the beginning the qubit is at the point P on the Bloch sphere. It will be measured along another direction (two opposite points on the sphere). There is an elastic between these points. A and B. P falls orthogonally on AB at Q and then the elastic breaks somewhere between A and B. So Q is projected to up (A) or down (B) according to the probability formula of QM!
  2. jcsd
  3. Mar 11, 2016 #2


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    Yes; as far as the probabilities go, measurement is like a projection. This is true even for complicated measurements involving many qubits, though there's not a nice sphere-based visualization of those.

    Here's a circuit diagram showing it happening. Notice how the probability of a wire being measured as On (+Z instead of -Z) closely tracks the vertical position on the Bloch sphere representation of the wire's state, and how the measured wire's state is pinned along the Z axis:

    Last edited: Mar 11, 2016
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