On strong superadditivity for a class of quantum channels

Abstract

Given a quantum channel $\Phi $ in a Hilbert space $H$ put $\hat H_{\Phi}(\rho)=\min \limits_{\rho_{av}=\rho}\Sigma_{j=1}^{k}\pi_{j}S(\Phi (\rho_{j}))$, where $\rho_{av}=\Sigma_{j=1}^{k}\pi_{j}\rho_{j}$, the minimum is taken over all probability distributions $\pi =\{\pi_{j}\}$ and states $\rho_{j}$ in $H$, $S(\rho)=-Tr\rho\log\rho$ is the von Neumann entropy of a state $\rho$. The strong superadditivity conjecture states that $\hat H_{\Phi \otimes \Psi}(\rho)\ge \hat H_{\Phi}(Tr_{K}(\rho))+\hat H_{\Psi}(Tr_{H}(\rho))$ for two channels $\Phi $ and $\Psi $ in Hilbert spaces $H$ and $K$, respectively. We have proved the strong superadditivity conjecture for the quantum depolarizing channel in prime dimensions. The estimation of the quantity $\hat H_{\Phi\otimes \Psi}(\rho)$ for the special class of Weyl channels $\Phi $ of the form $\Phi=\Xi \circ \Phi_{dep}$, where $\Phi_{dep}$ is the quantum depolarizing channel and $\Xi $ is the phase damping is given.