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How to merge beams with POVMs?

  1. Jun 8, 2014 #1


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    When in a stern-gerlach electrons are not detected the two beams can merge behind. We just have to add the amplitudes.
    this is easy because up> and down> are orthogonal.
    Suppose that we have a POVM setup with 3 output channels: F1 F2 and F3 as operators. They sum to Id but have not to be orthogonal.
    there are three detectors which click with probabilities P1 P2 and P3 (##Pi = Tr(\rho Fi)##)
    Suppose now that we replace detectors 1 and 2 by a merger Have we to add F1 and F2? (add probabilities).
    With ##F1 = M1^\dagger M1## and ##F2 = M2^\dagger M2## we would get
    ##M1^\dagger M1 + M2^\dagger M2## and not ##(M1+M)^\dagger(M+M)##
    This is not the same. so what is the rule when we merge beams?
    Last edited: Jun 9, 2014
  2. jcsd
  3. Jun 9, 2014 #2


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    In theory, it depends on the experimental setup. If you just split the beam and remerge it, you have a coherent process and add amplitudes. If you prepare two different ensembles ρ1 and ρ2 and mix them, you have an incoherent process and add probabilities.

    In practice, I don't think it makes much difference how you model the situation. Decoherence in the first process makes the outcome look identical to the second.
  4. Dec 22, 2015 #3


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    Man'ko gave https://www.researchgate.net/publication/222549915_Inner_composition_law_of_pure_states_as_a_purification_of_impure_states [Broken].
    There is a composition law for density matrices.
    look at (eq 17)
    Last edited by a moderator: May 7, 2017
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