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Homework Help: Trace and its square of mixed state density operator using integral

  1. Jul 18, 2012 #1
    1. The problem statement, all variables and given/known data
    I want to show that
    \hat{\rho}_{mixed}=\frac{1}{2\pi}\int_{0}^{2\pi}d \alpha \hat{\rho}(\psi)

    2. Relevant equations
    [tex] tr\left(\psi\right)= \sum_{n}\langle n|\psi|n\rangle[/tex]
    [tex]\hat{\rho}=\sum_{a}\omega_{a}|\psi\rangle\langle \psi|[/tex]

    3. The attempt at a solution
    [tex]tr\left(\hat{\rho}_{mixed}(\psi)\right)= tr\left( \hat{\rho}(\psi)\right)=\sum_{n}\sum_{a}\langle n| \underbrace{\psi_{a}\rangle\langle\psi_{a}}_{=1}|n\rangle=\sum_{n}\langle n|n\rangle=1 [/tex]
    [tex]tr \left(\hat{\rho}_{mixed}^{2}( \psi)\right)= tr\left( \hat{\rho}^{2}(\psi)\right)= tr\left(\hat{\rho}(\psi) \cdot \hat{\rho}(\psi)\right) = \sum_{n}\sum_{a,b} \langle n| \underbrace{\psi_{a}\rangle \langle\psi_{a}}_{=1}| \underbrace{\psi_{b} \rangle\langle\psi_{b}}_{=1}|n\rangle= \sum_{n}\langle n|n\rangle= 1[/tex]

    That`s not correct, at least not the square of the trace.
  2. jcsd
  3. Jul 25, 2012 #2
    Where is my mistake?
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