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Homework Help: [QM] Expectation value in spin-1/2 state

  1. Dec 26, 2012 #1
    1. The problem statement, all variables and given/known data

    Basically I need to produce a state for a spin-1/2 particle such that the expectation value of <Jz> = 0 where <Jz> is for a spin-1 particle.

    2. Relevant equations

    Jz = (1 0 0, 0 0 0, 0 0 -1) <--[3x3] matrix

    3. The attempt at a solution

    I don't quite understand how to do this because up till now we have been trying to find expectation values for observables in same spin states.

    Anyways heres my attempt:

    ψ* Jz ψ = <Jz>
    = ψ* (1 0 0, 0 0 0, 0 0 -1) ψ

    the reason I get no further than this is cuz I don't see how i'm supposed to find a spin-1/2 state which would be in the form of a [2x1] matrix and then multiply the [3x3] matrix by a [2x1] matrix, whats the deal here? I'm guessing i've completely misinterpreted something but I can't see what. .
    Last edited: Dec 26, 2012
  2. jcsd
  3. Dec 27, 2012 #2
    can you be more precise with your question.
    going from spin-1 to spin-1/2 doesn't really make sense, can you give us more context
    on what your trying to do.
  4. Dec 27, 2012 #3
    The exact question is as follows:
    Produce a state of spin 1/2 particle such that when measuring <Jz> = 0, normalise that state. Can it be that at the same time <Jz> = 0 and <Jx> = 0?

    It doesn't explicitly tell me that <Jz> is for a spin-one particle but I've just assumed that the J matrices are since thats what our lecturer has used as notation for spin-one particles and S for spin 1/2 particles. Hope this makes the question clearer?
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