GKSL Generators and Digraphs: Computing Invariant States

TL;DR

This paper explores the structure of quantum dynamical semigroups generated by digraph-induced GKSL operators, explicitly characterizing invariant states and extending the class of generators beyond traditional constraints.

Contribution

It introduces pair block diagonal generators, characterizes when GKSL equations define proper generators with arbitrary Lindblad operators, and links generator properties to digraph structures for invariant state analysis.

Findings

Explicit computation of all invariant states for certain semigroups.

Extension of GKSL generator class to include non-traceless Lindblad operators.

Connection between digraph properties and the structure of invariant states.

Abstract

In recent years, digraph induced generators of quantum dynamical semigroups have been introduced and studied, particularly in the context of unique relaxation and invariance. In this article we define the class of pair block diagonal generators, which allows for additional interaction coefficients but preserves the main structural properties. Namely, when the basis of the underlying Hilbert space is given by the eigenbasis of the Hamiltonian (for example the generic semigroups), then the action of the semigroup leaves invariant the diagonal and off-diagonal matrix spaces. In this case, we explicitly compute all invariant states of the semigroup. In order to define this class we provide a characterization of when the Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) equation defines a proper generator when arbitrary Lindblad operators are allowed (in particular, they do not need to be traceless as demanded by the GKSL Theorem). Moreover, we consider the converse construction to show that every generator naturally gives rise to a digraph, and that under certain assumptions the properties of this digraph can be exploited to gain knowledge of both the number and the structure of the invariant states of the corresponding semigroup.