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Homework Help: Exponential projection operator in Dirac formalism

  1. Feb 25, 2014 #1
    1. The problem statement, all variables and given/known data
    Hey guys.

    So here's the situation:
    Consider the Hilbert space [itex]H_{\frac{1}{2}}[/itex], which is spanned by the orthonormal kets [itex]|j,m_{j}>[/itex] with [itex]j=\frac{1}{2}, m_{j}=(\frac{1}{2},-\frac{1}{2})[/itex]. Let [itex] |+> = |\frac{1}{2}, \frac{1}{2}>[/itex] and [itex]|->=|\frac{1}{2},-\frac{1}{2}>[/itex]. Define the following two projection operators:

    [itex]\hat{P}_{+}=|+><+|[/itex] and [itex]\hat{P}_{-}=|-><-|[/itex].

    Now consider the operator [itex]\hat{O}=e^{i\alpha \hat{P}_{+}+i\beta \hat{P}_{-}}[/itex].

    Compute the following:




    2. Relevant equations

    orthonormality stuff: <+|+> = <-|-> = 1, <-|+> = <+|-> = 0.

    Series representation of e^x:

    [itex]e^{x}=\sum_{k=0}^{\infty} \frac{x^{k}}{k!}[/itex]

    3. The attempt at a solution

    So here's what Ive done. Of course you gota represent the operator O in a nicer way, and this is what I need to know if ive done right:

    [itex]e^{i\alpha \hat{P}_{+}}=I+(i\alpha) \hat{P}_{+} + \frac{(i\alpha)^{2}}{2}\hat{P}_{+}+...=I+\hat{P}_{+}(i\alpha + \frac{(i\alpha)^2}{2}+...)=I+\hat{P}_{+}(e^{i\alpha}-1)[/itex]


    [itex]e^{i\beta \hat{P}_{-}}=I+\hat{P}_{-}(e^{i\beta}-1)[/itex]

    where [itex]I[/itex] is the identity matrix. Now, O is a product of these two:


    because the cross-terms in the product vanish as [itex]\hat{P}_{+}\hat{P}_{-}=0[/itex]

    So that's my expression for O. Using that, I find that

    [itex]<+|\hat{O}|+>=e^{i\alpha}[/itex]. I havent done the rest yet, but is this one right guys...?

    please tell me if ive made any math errors!
  2. jcsd
  3. Feb 25, 2014 #2
    That's right because ([itex]\hat{P}[/itex][itex]_{\pm}[/itex])[itex]^{n}[/itex] = [itex]\hat{P}[/itex][itex]_{\pm}[/itex], otherwise you would have to determine all exponents of the operators.
    You're good so far!
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