Superadditivity of Quantum Relative Entropy for General States

Abstract

The property of superadditivity of the quantum relative entropy states that, in a bipartite system H AB = H A ⊗ H B , for every density operator ρ AB , one has D(ρ AB ||σ A ⊗ σ B )≥ D(ρ A ||σ A ) + D(ρB||σB). In this paper, we provide an extension of this inequality for arbitrary density operators σ AB . More specifically, we prove that α(σ AB )· D(ρ AB ||σ AB )≥D(ρ A ||σ A )+ D(ρ B ||σ B ) holds for all bipartite states ρ AB and σ AB , where α(σ AB ) = 1 + 2||σ A -1/2 ⊗ σ AB σ A -1/2 ⊗ σ B -1/2 - || AB || ∞ .