Spectral geometric mean and trace characterizations

Abstract

We use nearly parallel pure states to characterize positive linear functionals $ϕ$ on $\mathbb{M}_n$ as positive multiples of the trace if and only if $ϕ(A \natural B) \leq \sqrt{ϕ(A) ϕ(B)}$ for all positive definite matrices $A$ and $B$. Here $A \natural B = (A^{-1} \# B)^{1/2} A (A^{-1} \# B)^{1/2}$ represents the spectral geometric mean. For further clarification, we establish novel characterizations through the inequality $ϕ(A \natural B) \leq ϕ((A+B)/2)$ for all positive definite matrices $A$ and $B$. We also present a trace inequality related to quantum fidelity that applies to all positive definite matrices, and demonstrate that it does not characterize the trace.