Superconducting electronic devices have re-emerged as contenders for both classical and quantum computing due to their fast operation speeds, low dissipation and long coherence times. An ultimate demonstration of coherence is lasing. We use one of the fundamental aspects of superconductivity, the acJosephson effect, to demonstrate a laser made from a Josephson junction strongly coupled to a multi-mode superconducting cavity. A dc voltage bias to the junction provides a source of microwave photons, while the circuit's nonlinearity allows for efficient down-conversion of higher order Josephson frequencies down to the cavity's fundamental mode. The simple fabrication and operation allows for easy integration with a range of quantum devices, allowing for efficient on-chip generation of coherent microwave photons at low temperatures.

Spremberg Reservoir is a small and shallow water body where wind sheltering along steep and densely vegetated sections of the shoreline is expected to produce small-scale inhomogeneities of the wind field across the water surface. The potential impacts of these inhomogeneities on the quasi-steady circulation in the main subbasin of the reservoir are investigated in this study by means of a three-dimensional hydrodynamic model. ADCP measurement profiles at three locations are available for comparison. Agreement between characteristic features of the measurements and the simulation data cannot be achieved if uniform winds are applied as boundary conditions, but improves distinctly when sheltering effects are implemented. This is done qualitatively by means of analogy to the separation of flow across a backward fac- Aquat. Sci. 67 (2005) Aquatic Sciences ing step. For south-westerly winds, the modification of the flow tends from a three-dimensional to an essentially horizontal, one-cell circulation. The action of the wind sheltering on the flow is found to rely on (1) the addition of a curl to the background wind field and (2) the weakening of the topological moment along the windward shore. In Spremberg Reservoir the first mechanism appears to prevail for south-westerly winds. The study underlines the need for consideration of small-scale inhomogeneities of the wind field induced by sheltering as a potential source of vorticity in hydrodynamic simulations. Further investigations should be directed towards a physically founded parameterization of the actual wind stress distribution and towards quantifying lake dimensions at which such influences on the circulation pattern tend to be negligible.

Majorana bound states (MBS) differ from the regular zero energy Andreev bound states in their nonlocal properties, since two MBS form a single fermion. We design strategies for detection of this nonlocality by using the phenomenon of Coulomb-mediated Majorana coupling in a setting which still retains falsifiability and does not require locally separated MBS. Focusing on the implementation of MBS based on the quantum spin Hall effect, we also design a way to probe Majoranas without the need to open a magnetic gap in the helical edge states.In the setup that we analyze, long range MBS coupling manifests in the h/e magnetic flux periodicity of tunneling conductance and supercurrent. While h/e is also the periodicity of Aharonov-Bohm effect and persistent current, we show how to ensure its Majorana origin by verifying that switching off the charging energy restores h/2e periodicity conventional for superconducting systems.

We propose a new setup for creating Majorana bound states in a two-dimensional electron gas Josephson junction. Our proposal relies exclusively on a supercurrent parallel to the junction as a mechanism of breaking time-reversal symmetry. We show that combined with spin-orbit coupling, supercurrents induce a Zeeman-like spin splitting. Further, we identify a new conserved quantity-charge-momentum parity-that prevents the opening of the topological gap by the supercurrent in a straight Josephson junction. We propose breaking this conservation law by adding a third superconductor, introducing a periodic potential, or making the junction zigzag-shaped. By comparing the topological phase diagrams and practical limitations of these systems we identify the zigzag-shaped junction as the most promising option.

In many electron spin qubit systems coherent control is impaired by the fluctuating nuclear spin bath of the host material. Previous experiments have shown dynamic nuclear polarization with feedback to significantly prolong the inhomogeneous dephasing time T * 2 by narrowing the distribution of nuclear Overhauser field fluctuations. We present a model relating the achievable narrowing of the Overhauser field to both the pump rate and the noise magnitude and find reasonable agreement with experimental data. It shows that former experiments on gated GaAs quantum dots were limited by the pump rate of the pumping mechanism used. Here we propose an alternative feedback scheme using electron dipole spin resonance. Sequentially applying two ac electric fields with frequencies slightly detuned from the desired Larmor frequency results in a pump curve with a stable fixed point. Our model predicts that T * 2 values on the order of microseconds can be achieved.One limiting factor of spin qubits based on semiconductor quantum dots is the fluctuation of the effective magnetic field arising from nuclear spins. The interaction of these nuclear spins in the host material with the electron spin in the quantum dot induces decoherence. Several techniques have been developed to reduce decoherence.1-8 Narrowing the distribution of the nuclear fluctuations was shown to prolong the inhomogeneously broadened dephasing time for gate defined GaAs quantum dots 5 and for self-assembled InAs quantum dots. 3,7To reduce fluctuations the nuclear field is regulated by dynamic nuclear polarization (DNP). DNP controls the Overhauser field by transferring the electron spin angular momentum to the nuclear spins via the hyperfine interaction. If the effectiveness of the polarization step depends on the current state of the nuclear spin bath, the qubit can be used as a closed feedback loop. In gate defined GaAs double quantum dots, a roughly tenfold enhancement of T * 2 has been achieved by LandauZener-sweep driven electron-nuclear spin flip-flops that were preceded by a free evolution of the qubit to create a feedback effect.5 In self-assembled InAs quantum dots a DNP feedback effect was also observed as the quantum dot resonance exhibited locking to frequency 3,7,9 or repetition rate 10 of the incident laser. Driving GaAs gate defined quantum dots with electron dipole spin resonance (EDSR) showed locking to the spin-resonance condition as well. 11The narrowing effect has been examined theoretically in various studies but these were focussed on microscopic models and optical manipulation.12-15 A phenomenological rate-equation model to describe the effectiveness of DNP in reducing the Overhauser field variance was derived by Vink et al.11 and by Latta et al. 9 The model relates the maximum nuclear field achievable by polarization to the nuclear field variance by treating the polarization as a random variable influenced by DNP and relaxation proportional to the current value. This model was further refined by Yang et al. 16 However, the dominant relax...