This thesis focuses on the Lie-theoretic foundations of controlled open quantum systems. We describe Markovian open quantum system evolutions by Lie semigroups, whose corresponding infinitesimal generators lie in a special type of convex cone - a Lie wedge. The Lie wedge associated to a given control system therefore consists of all generators of the quantum dynamical semigroup that are physically realisable as a result of the interplay between the coherent and incoherent processes the quantum system is subject to. For
n-qubit open quantum systems, we provide a parametrisation of the largest physically relevant Lie algebra (the system algebra), in which these Lie wedges are contained: the Lindblad-Kossakowski Lie algebra. This parametrisation provides several useful benefits. First, it allows us to construct explicit forms of these system Lie wedges and their respective system Lie algebras. Second, we analyse which control scenarios yield Lie wedges that are closed under Baker-Campbell-Hausdorff (BCH) multiplication and therefore generate Markovian semigroups of time-independent quantum channels. Lie wedges of this form are called Lie semialgebras, and we completely solve this open problem by proving that Lie wedges specialise to this form only when the coherent controls have no effect on both the inherent drift Hamiltonian and the incoherent part of the dynamics. Finally, this parametrisation of the Lindblad-Kossakowski Lie algebra points to an intuitive separation between unital and non-unital dissipative dynamics, where the non-unital component of the dynamics is described by affine translation operations. These translation operators are then exploited to construct purely dissipative fixed-point engineering schemes to obtain either pure or mixed states as a system's unique fixed point.
We extend standard Markovian open quantum systems (quantum channels) by allowing for Hamiltonian controls and elucidate their geometry in terms of Lie semigroups. For standard dissipative interactions with the environment and different coherent controls, we particularly specify the tangent cones (Lie wedges) of the respective Lie semigroups of quantum channels. These cones are the counterpart of the infinitesimal generator of a single one-parameter semigroup. They comprise all directions the underlying open quantum system can be steered to and thus give insight into the geometry of controlled open quantum dynamics. Such a differential characterization is highly valuable for approximating reachable sets of given initial quantum states in a plethora of experimental implementations.
The solutions to the celebrated Kossakowski-Lindblad equation extended by coherent controls yield Markovian quantum maps. More precisely, the set of all its solutions forms a semigroup of completely positive trace-preserving maps taking the specific form of a Lie semigroup. Non-trivial symmetries of these semigroups are shown to preclude accessibility in Markovian dissipative systems. This is the open-system analogue to closed systems, where triviality of (quadratic) symmetries of the Hamiltonian part suffices to decide that the system is fully controllable. The findings are placed into a unifying Lie frame of quantum systems and control theory alongside with illustrating examples.