Constraints for the spectra of generators of quantum dynamical semigroups

Motivated by a spectral analysis of the generator of completely positive trace-preserving semigroup, we analyze a real functional $$ A,B \in M_n(\mathbb{C}) \to r(A,B) = \frac{1}{2}\Bigl(\langle [B,A],BA\rangle + \langle [B,A^\ast],BA^\ast \rangle \Bigr) \in \mathbb{R} $$ where $\langle A,B\rangle := {\rm tr} (A^\ast B)$ is the Hilbert-Schmidt inner product, and $[A,B]:= AB - BA$ is the commutator. In particular we discuss the upper and lower bounds of the form $c_- \|A\|^2 \|B\|^2 \le r(A,B) \le c_+ \|A\|^2 \|B\|^2$ where $\|A\|$ is the Frobenius norm. We prove that the optimal upper and lower bounds are given by $c_\pm = \frac{1 \pm \sqrt{2}}{2}$. If $A$ is restricted to be traceless, the bounds are further improved to be $c_\pm = \frac{1 \pm \sqrt{2(1-\frac{1}{n})}}{2}$. Interestingly, these upper bounds, especially the latter one, provide new constraints on relaxation rates for the quantum dynamical semigroup tighter than previously known constraints in the literature. A relation with Bottcher-Wenzel inequality is also discussed.