Isometries and relative entropy preserving maps on density operators

Abstract

In this article we consider certain kinds of metrics and relative entropies on sets of density operators acting on a finite-dimensional complex Hilbert space. The metrics are the ones induced by the von Neumann–Schatten p-norms. We describe the general form of ‘a priori’ nonsurjective isometries of the space of all density operators with respect to those distances. In the second part of this article we study mappings preserving entropic quantities. We determine the structure of ‘a priori’ nonsurjective maps on the set of all density operators which leave a certain measure of relative entropy invariant and also characterize the surjective maps on the set of all invertible density operators with similar invariance properties.

Keywords:

• density operators
• p-norms
• relative entropies
• isometries
• preservers

AMS Subject Classifications::

• 47B49
• 47N50

Acknowledgements

The authors were supported by the Hungarian Scientific Research Fund (OTKA), Grant No. K81166 NK81402. L. Molnár was supported also by the TÁMOP 4.2.1./B-09/1/KONV-2010-0007 project implemented through the New Hungary Development Plan co-financed by the European Social Fund and the European Regional Development Fund.