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Homework Help: Quantum Mechanics Help

  1. Sep 28, 2013 #1
    Quantum Mechanics Help [Probability of a Spin-1/2 Particle]

    Part 1) [itex]\hat{S}[/itex] is a spin-1/2 operator, [itex]\vec n[/itex] is a unit vector, [itex]\vert{\psi_\pm}\rangle[/itex] are normalized eigenvectors of [itex]n\cdot\hat{S}[/itex] with eigenvectors [itex]\pm\frac12[/itex]. Write [itex]\vert{\psi_\pm}\rangle[/itex] in terms of [itex]\vert{+z}\rangle[/itex] and [itex]\vert{-z}\rangle[/itex].

    Part 2) [itex]\vec n_1[/itex] and [itex]\vec n_2[/itex] are unit vectors. A measurement found the projection of spin-1/2 on the direction [itex]\vec n_1[/itex] to be 1/2. Use the results of the previous part to show that a subsequent measurement of the projection of spin on the direction [itex]\vec n_2[/itex] will give 1/2 with probability

    [itex]P=\frac12(1+\vec n_1\cdot\vec n_2)[/itex]

    The attempt at a solution
    First I declare what my normal vector [itex]\vec n[/itex] will be, [itex]\vec n = (sin\theta\cos\phi,\ \sin\theta\sin\phi,\ \cos\theta)[/itex]

    I then solve for [itex]\sum_i\vec n\cdot\hat{S}_i[/itex] and started looking for the eigenstates, which lead to

    [itex]\mu=\pm 1[/itex]

    Setting this up lead me to
    [itex]\langle -z\vert\psi_+\rangle=-e^{i\phi}\frac{\cos\theta-1}{\sin\theta}\langle+z\vert\psi_+\rangle[/itex]
    which, after normalizing, I then find

    Likewise, when I use [itex]\mu=-1[/itex] I find


    **This officially marks the end of part 1**

    For part 2, I assumed that "the projection of spin-1/2 on the direction nāƒ— 1 to be 1/2" was equivalent to
    [itex]\langle \vec n\vert\vec n_1\rangle=\frac12[/itex]

    But supposably this is not the case, which leaves me to just being confused on where to go.
    Last edited: Sep 28, 2013
  2. jcsd
  3. Sep 30, 2013 #2


    User Avatar

    Staff: Mentor

    There is no ##\vec{n}## in part b. The problem mentions the measurement of the projection of the spin in the direction of ##\vec{n}_1##. How do you translate that into mathematical terms?
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