Optimal Bounds on Functions of Quantum States under Quantum Channels

Abstract

Let ${\rho }_1,{\rho }_2$ be quantum states and $(\rho_1,\rho_2) \mapsto D(\rho_1, \rho_2)$ be a scalar function such as the trace norm, the fidelity, and the relative entropy, etc. We determine optimal bounds for $D(\rho_1, \Phi(\rho_2))$for$\Phi \in \mathcal{S}$ for different class of functions $D(\cdot ,\cdot )$, where $\mathcal{S}$ is the set of unitary quantum channels, the set of mixed unitary channels, the set of unital quantum channels, and the set of all quantum channels.