Control landscapes for two-level open quantum systems

A quantum control landscape is defined as the physical objective as a function of the control variables. In this paper, the control landscapes for two-level open quantum systems, whose evolution is described by general completely positive trace-preserving maps (i.e., Kraus maps), are investigated in detail. The objective function, which is the expectation value of a target system operator, is defined on the Stiefel manifold representing the space of Kraus maps. Three practically important properties of the objective function are found: (a) the absence of local maxima or minima (i.e., false traps), (b) the existence of multi-dimensional sub-manifolds of optimal solutions corresponding to the global maximum and minimum and (c) the connectivity of each level set. All of the critical values and their associated critical sub-manifolds are explicitly found for any initial system state. Away from the absolute extrema there are no local maxima or minima, and only saddles may exist, whose number and the explicit structure of the corresponding critical sub-manifolds are determined by the initial system state. There are no saddles for pure initial states, one saddle for a completely mixed initial state, and two saddles for partially mixed initial states. In general, the landscape analysis of critical points and optimal manifolds is relevant to explain the relative ease of obtaining good optimal control outcomes in the laboratory, even in the presence of the environment.