Quantum coherence, control, and information in mesoscopic systems

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The proposed theoretical research targets the understanding and control of quantum states and their coherence in condensed matter systems such as superconducting nanocircuits and semiconductor quantum dots. It is directly aimed at understanding and proposing experiments. It explores a refined understanding of decoherence - the boundary between classical and quantum dynamics - at low temperatures, when physical correlations have a long lifetime. A specific focus is the study of noise sources such as low frequency noise, which are so far non-standard to theory but ubiquitous in experimentation. In the same context, the physics of quantum measurement using efficient nonlinear detectors and strongly coupled devices will be investigated along models for actual superconducting devices. This type of research allows novel insights in the general topic of quantum measurement and allows to design specific test experiments for the concepts of measurement theory. Quantum logic gates will be optimized using optimum control theory. The optimum solutions will take decoherence into account and explore the boundaries of control under decoherence. Ideas originating from quantum optics and atomic physics will be investigated, specifically from circuit-cavity quantum electrodynamics using linear cavities as well as superconducting quantum interference devices (SQUIDs) as nonlinear cavities. The potential of these tools to creating nonclassical states of microwave fields such as cat states, squeezed states, and entagled pairs will be investigated. The proposed research will extend towards general high-frequency properties of mesosocopic devices such as the quantum kinetics of superconducting single-photon detectors. These high-freuqency properties are key to electronics applications of nanodevices. The theoretical tools combine up-to-date methods of condensed-matter theory, nonequilibrium statistical mechanics, quantum optics, and theory of nuclear magentic resonance.