We carry out theoretical studies on various types of solid state systems: on the one hand, these are quantum coherent systems such as superconducting metals (high Tc cuprates), superfluid bosonic gases (Bose Einstein condensates), Josephson junction arrays, or degenerate Fermi systems. We are interested in novel phases and (quantum) phase transitions where these materials undergo a drastic change in behavior, e.g. ordered versus disordered or mobile versus immobile phases. Furthermore, we study topological excitations such as vortices and systems of vortices; these define an interesting soft-matter system with similarities to polymer physics. These studies are useful in the context of applications of novel materials, e.g., superconducting transport, or serve as a preparation for the future development of quantum information systems. Our second field of interest is mesoscopic physics. Here we study theoretically the transport properties (average current, noise, full counting statistics) in small mesoscopic devices (normal-metallic and/or superconducting) and study basic quantum phenomena such as entanglement, with the goal to understand their potential as devices for quantum information processing.

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