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Upper bounds for the quantum fisher information?

Physics Asked by tacruc on January 14, 2021

Background:

The quantum fisher information can be used to lower bound the variance, when estimating a parameter $Theta$.
This estimation can be decomposed into three steps.

  1. The initial state $rho_0$
  2. The encoding process, discribed by a CPTP map $M_Theta$
  3. The Messurement ${E_m}$ discribed by a POVM

The QFI allready contains an optimization about all possible POVMs.

My questions:

  1. Is there an upperbound for the optimization over all possible
    initial states $max_{rho_0} QFI_{M_{Theta}}(rho_0)$ ?

  2. Is there an upperbound for the optimization over all seprable
    initial states $max_{rho_0in text{SEP}}
    QFI_{M_{Theta}}(rho_0)$
    ?

  3. Are there other special cases, where bounds are known?

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