title: Convexity of quantum χ2-divergence.
abstract: The general quantum χ(2)-divergence has recently been introduced by Temme et al. [Temme K, Kastoryano M, Ruskai M, Wolf M, Verstrate F (2010) J Math Phys 51:122201] and applied to quantum channels (quantum Markov processes). The quantum χ(2)-divergence is not unique, as opposed to the classical χ(2)-divergence, but depends on the choice of quantum statistics. It was noticed that the elements in a particular one-parameter family of quantum χ(2)-divergences are convex functions in the density matrices (ρ,σ), thus mirroring the convexity of the classical χ(2)(p,q)-divergence in probability distributions (p,q). We prove that any quantum χ(2)-divergence is a convex function in its two arguments.
authors.affiliation: Institute for International Education, Tohoku University, 41 Kawauchi, Aoba-ku, Sendai 980-8576, Japan. frank.hansen@m.tohoku.ac.jp
authors.forename: Frank
authors.initials: F
authors.lastname: Hansen