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Expected Value Partial Trance Density Matrix

  1. Jun 7, 2014 #1


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    Hey I am currently studying Quantum Mechanics and I have difficulty grasping a concept.

    I don't understand the following step in the derivation:

    [itex]\langle X_{A} \rangle=tr\left[\left(X_{A}\otimes I_{B}\right)\rho_{AB}\right][/itex]
    [itex]=tr_{A}\left[X_{A} tr_{B}\left[\rho_{AB}\right]\right][/itex]

  2. jcsd
  3. Jun 7, 2014 #2


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    I believe 'tr' stands for 'trace', not 'trance'.
  4. Jun 8, 2014 #3


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    yes of course
  5. Jun 8, 2014 #4


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    Note if you're computing it while listening to techno music :).
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