Decoherence in Josephson qubits due to charge and spin fluctuators

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The level of coherent control of quantum circuits in general and Josephson junctions in particular has advanced greatly in the last decade. Major sources of decoherence have been identified and/or suppressed by means of careful circuit engineering. As a consequence, the coherence times of Josephson quantum bits (qubits) reach routinely tens to even hundreds of microseconds. The remaining sources of noise are subtle. They are mostly related to properties of the materials used for the devices or to the failure to reach thermal equilibrium in the circuit surroundings. Investigation of these effects is the main subject of the present proposal. Our aims are twofold. On one hand, we want to contribute to the goal of further enhancing the coherence times of quantum circuits. On the other hand, we aim at a better understanding of material properties, which became accessible due to the advances in qubit technology. Specifically we are going to investigate: (i) The properties of two-level systems (TLS) in Josephson junctions and their signatures in dielectric losses, charge noise, and critical current noise. We will study the mechanisms of qubit relaxation due to the coupling to the TLS and those of the intrinsic dissipation of the TLS in the solid state environment. We will also investigate the effects of interactions between the TSL. In combination with the recent advances in noise spectroscopy with Josephson qubits we hope to be able to advance the understanding of the microscopic nature of the TLS.(ii) The effects of paramagnetic spins on surfaces and interfaces. They appear to be responsible for the observed 1/f noise of the magnetic flux. We will study the interplay of spin-diffusion, spin damping, and spin glass physics. The aim is to understand the statistical properties of the magnetic flux noise, i.e., the frequency and temperature dependence of the 1/f-like power spectrum as well as the non-Gaussian properties (noise of noise).