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I want to understand positive operator valued measures in QM

  1. Nov 27, 2011 #1

    Fredrik

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    I want to understand positive operator valued measures in QM, in particular why they are considered "observables". Anyone know a good place to start reading about this?

    I know some functional analysis and some measure theory, but I haven't made it all the way to the spectral theorem.
     
    Last edited by a moderator: Nov 28, 2011
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  3. Nov 28, 2011 #2

    A. Neumaier

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    Re: POVMs

    Chapter 3 of
    http://www.theory.caltech.edu/people/preskill/ph229/#lecture
     
  4. Nov 28, 2011 #3

    Fredrik

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    Thank you. That looks like a good enough place to start.
     
  5. Nov 28, 2011 #4

    A. Neumaier

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  6. Nov 30, 2011 #5

    kith

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    What does "positive operator valued" or "projector valued" measure mean? Is this really an accurate naming?

    I'm asking, because the possible outcomes in such measurements are still eigenvalues of a suitable operator, aren't they?
     
  7. Nov 30, 2011 #6

    A. Neumaier

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    No. outcomes can be anything; the POVM only specifies their probabilities:
    (from http://en.wikipedia.org/wiki/POVM )

    This is far superior to the heavily idealizing Born interpretation, which claims that a measurement produces the exact eigenvalue of an operator, which is ridiculous whenever the eigenvalues of the operator in question are irrational numbers. (In particular, discrete spectra of all but the simplest Hamiltonians are irrational, but nobody is able to measure energies to infinitely many digits!)
     
  8. Nov 30, 2011 #7

    dextercioby

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    Yes, it's accurate. First note that <projector operator> is a well defined concept in a Hilbert space. They are operators which have certain particular properties for which they are useful in quantum physics (2 one of them being boundedness and the property to be positively defined). Also <measure> is a concept used in Hilbert spaces through spectral measures for self-adjoint/unitary operators. So one achieves the most general description of self-adjoint operators through projector valued measures (PVM )by the spectral theorem of von Neumann. For positively defined operators, such as the statistical operator ρ of von Neumann, we also have POVMs as most people know them again through the spectral theorem. Actually, POVMs generalize PVMs in the sense discussed in https://wiki.physik.uni-muenchen.de/...8/87/POVMs.pdf [Broken] page 4.
     
    Last edited by a moderator: May 5, 2017
  9. Nov 30, 2011 #8

    A. Neumaier

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    Not quite. An operator contains more information than recorded in the PVM; therefore the latter gives not a full description of the operator, and hence not the most general one:

    If A is a self-adjoint operator with discrete spectrum and not itself a projector then the operators
    A and A^2 have different spectrum but the same associated PVM.
     
  10. Nov 30, 2011 #9
    A good reference to start learning about the spectral theorem is Mathematical Physics by Robert Geroch, in the last few chapters of the book. He doesn't do the operator-valued measure approach, although he mentions it (I think it's still worth reading first, even if you are more interested in operator-valued measures). Very nice exposition, though.
     
  11. Nov 30, 2011 #10

    dextercioby

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    OK, I stand corrected. Nonetheless, A and A2 may not have the same domain...
     
  12. Nov 30, 2011 #11

    A. Neumaier

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    They have the same domain iff A is bounded.

    But A^2 and A^2+1 always have the same domain and the same PVM, but different spectrum.
     
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