Recoverable states on von-Neumann algebras
Let $(\mathcal{M},τ)$ and $(\mathcal{N},τ^{\prime})$ be tracial von-Neumann algebras and let $ϕ:\mathcal{M}\to\mathcal{N}$ be a strictly completely positive, trace preserving map. Given a positive, invertible $B\in\mathcal{M}$ with $τ(B)=1$, a state on $\mathcal{M}$ given by a positive $A\in L^1(\mathcal{M}, τ)$ is said to be recoverable if $\mathcal{R}(ϕ(A))=A$ where $\mathcal{R}$ is the Petz recovery map corresponding to $B$ and $ϕ$. In this paper, we study recoverable states and show how an arbitrary state can be made close to a recoverable state via iterates of $\mathcal{R}\circϕ$. We show that there exists a completely positive, trace preserving map $ψ:\mathcal{M}\to\mathcal{M}$ such that $ψ(A)$ is recoverable for all $A$ and $(\mathcal{R}\circϕ)^n\toψ$ in norm as operators on $L^p(\mathcal{M},τ)$ for all $1\,\textless p\,\textless\infty$, and discuss potential applications to quantum information theory. We also show that this convergence holds strongly in $L^1$. Finally, we prove an interesting decomposition theorem for normal states on $\mathcal{M}$.