Abstract
In this paper, we deal with two quantum relative entropy preserver problems on the cones of positive (either positive definite or positive semidefinite) operators. The first one is related to a quantum Rényi relative entropy like quantity which plays an important role in classical–quantum channel decoding. The second one is connected to the so-called maximal f-divergences introduced by D. Petz and M. B. Ruskai who considered this quantity as a generalization of the usual Belavkin–Staszewski relative entropy. We emphasize in advance that all the results are obtained for finite-dimensional Hilbert spaces.
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Notes
For the sake of simplicity in the Introduction we take the freedom to define the corresponding divergences only on the set of positive definite operators. The extensions for general (not necessarily invertible) positive semidefinite operators are discussed in Sect. 2.
On the set of positive definite operators the case where f is operator monotone decreasing has been already solved before by Molnár [14].
Here, trace preserving property means that \({\text {Tr}}\Gamma (p)= \sum _{i=1}^{d} p_i\) holds for any probability distribution p.
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Acknowledgements
The authors thank the anonymous referee for carefully reading the manuscript and for a comment which simplified the last part of the proof of Theorem 1. The authors were supported by the “Lendület” Program (LP2012-46/2012) of the Hungarian Academy of Sciences and by the National Research, Development and Innovation Office NKFIH Reg. No. K115383.
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Gaál, M., Nagy, G. Maps on positive operators preserving Rényi type relative entropies and maximal f-divergences. Lett Math Phys 108, 425–443 (2018). https://doi.org/10.1007/s11005-017-1021-4
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DOI: https://doi.org/10.1007/s11005-017-1021-4