The development of laser cooling and trapping techniques for atoms in the previous century enabled us for the first time to study and fully control individual quantum systems in the lab. Apart from progressively refined tests of the laws of quantum mechanics, these capabilities also provide us with the basis for new, quantum-enabled technologies, such as quantum computers, quantum simulators or enhanced sensors.  In recent years, a similar level of control has now also been achieved for artificial macroscopic quantum systems, such as superconducting quantum circuits or micro- and nanomechanical resonators. Unlike atoms, the properties of those systems can be engineered and designed with great flexibility and scaled-up using established nanofabrication techniques. This offers many intriguing new possibilities for quantum technology applications and for exploring complex quantum phenomena under previously inaccessible conditions. 

Our theory efforts are focused on the analysis of control and manipulation techniques for engineered solid-state quantum systems. One of our primary goals is to develop more efficient quantum information processing schemes and robust protocols for operating scalable quantum networks. With a detailed modelling and numerical simulation of such processes, we support and complement the ongoing experimental activities at the WMI in this direction, but also strive to identify and tackle some of the future challenges that implementations of large-scale quantum information processing infrastructures will naturally bring about.

Beyond their potential use for quantum-technology applications, a particular interest in these artificial systems arises from their unique possibilities to engineer both coherent and dissipative interactions at the quantum level and to design model systems and interaction parameters almost at will. These features allow us to explore new physical effects and phenomena that are otherwise not encountered in nature. In this context, we are specifically interested in light-matter interaction effects in 1D waveguides and in the so-called ultrastrong coupling regime, where the coupling strength between a single atom and single photon exceeds the bare energy of that photon. While such conditions are not accessible with real atoms, they can be realized with ‘artificial’ atoms, i.e., superconducting two-level systems, coupled to microwave photons. From a theoretical perspective, the analysis of such systems is particularly challenging since many of the usual approximations and simplification are no longer applicable and new analytic and numerical approaches must be developed.