Searching for Lindbladians obeying local conservation laws and showing thermalization

We investigate the possibility of a Markovian quantum master equation (QME) that consistently describes a finite-dimensional system, a part of which is weakly coupled to a thermal bath. To preserve complete positivity and trace, such a QME must be of Lindblad form. For physical consistency, it should additionally preserve local conservation laws and be able to show thermalization. We search for Lindblad equations satisfying these additional criteria. First, we show that the microscopically derived Bloch-Redfield equation violates complete positivity unless in extremely special cases. We then prove that imposing complete positivity and demanding preservation of local conservation laws enforces the Lindblad operators and the lamb-shift Hamiltonian to be local, i.e., to be supported only on the part of the system directly coupled to the bath. We then cast the problem of finding local Lindblad QMEs which can show thermalization into a semidefinite program. We call this the thermalization optimization problem (TOP). For given system parameters and temperature, the solution of the TOP conclusively shows whether the desired type of QME is possible up to a given precision. Whenever possible, it also outputs a form for such a QME. For an XXZ chain of a few qubits, fixing a reasonably high precision, we find that such a QME is impossible over a considerably wide parameter regime when only the first qubit is coupled to the bath. Remarkably, we find that when the first two qubits are attached to the bath, such a QME becomes possible over much of the same parameter regime, including a wide range of temperatures.