Understanding and Generalizing Unique Decompositions of Generators of Dynamical Semigroups

Abstract

We generalize the result of Gorini, Kossakowski, and Sudarshan [J. Math. Phys. 17:821, 1976] that every generator of a quantum-dynamical semigroup decomposes uniquely into a closed and a dissipative part, assuming the trace of both vanishes. More precisely, we show that given any generator $L$ of a completely positive dynamical semigroup and any matrix $B$ there exists a unique matrix $K$ and a unique completely positive map $\Phi$ such that (i) $L=K(\cdot)+(\cdot)K^*+\Phi$, (ii) the superoperator $\Phi(B^*(\cdot)B)$ has trace zero, and (iii) ${\rm tr}(B^*K)$ is a real number. The key to proving this is the relation between the trace of a completely positive map, the trace of its Kraus operators, and expectation values of its Choi matrix. Moreover, we show that the above decomposition is orthogonal with respect to some $B$-weighted inner product.