Dynamical nuclear spin polarization and the Zamboni effect in gated double quantum dots

A dynamical nuclear polarization scheme is studied in gated double dots. We demonstrate that a small polarization ($\sim 0.5$) is sufficient to enhance the singlet decay time by two orders of magnitude. This enhancement is attributed to an equilibration process between the nuclear reservoirs in the two dots accompanied by reduced fluctuations in the Overhauser fields, that are mediated by the electron-nuclear spin hyperfine interaction.Comment: 4 pages, 3 figure

Decoherence of spin qubits due to a nearby charge fluctuator in gate-defined double dots

The effects of a nearby two-level charge fluctuator on a double-dot two-spin qubit are studied theoretically. Assuming no direct tunneling between the charge fluctuator and the qubit quantum dots, the Coulomb couplings between the qubit orbital states and the fluctuator are calculated within the Hund-Mulliken framework to quadrupole-quadrupole order in a multipole expansion. We identify and quantify the coupling term that entangles the qubit to the fluctuator and analyze qubit decoherence effects that result from the decay of the fluctuator to its reservoir. Our results show that the charge environment can severely impact the performance of spin qubits, and indicate working points at which this decoherence channel is minimized. Our analysis also suggests that an ancillary double-dot can provide a convenient point for single-qubit operations and idle position, adding flexibility in the quantum control of the two-spin qubit.Comment: 19 pages, 9 figure

Electrically controlled quantum gates for two-spin qubits in two double quantum dots

Exchange-coupled singlet-triplet spin qubits in two gate-defined double quantum dots are considered theoretically. Using charge density operators to describe the double-dot orbital states, we calculate the Coulomb couplings between the qubits, and identify optimal bias points for single- and two-qubit operations, as well as convenient idle positions. The same intuitive formulation is used to derive dephasing rates of these qubits due to the fluctuating charge environment, thereby providing the main considerations for a quantum computation architecture that is within current experimental capabilities.Comment: 9 pages, 7 figure

Visualising migration flow data with circular plots

Effective visualisations of migration flows can substantially enhance our understanding of underlying patterns and trends. However, commonly used migration maps that show place-to-place flows as stroked lines drawn atop a geographic map fall short of conveying the complexities of human movement in a clear and compelling manner. We introduce circular migration plots, a new method for visualising and exploring migration flow tables in an intuitively graspable way. Our approach aims to provide detailed quantitative information on the intensities and patterns of migration flows around the globe by using a visualization design that is effective and visually appealing. The key elements of the design are (a) the arrangement of origins and destinations of migration flows in a circular layout, (b) the scaling of individual flows to allow the entire system to be shown simultaneously, (c) the expression of the volume of movement through the width of the flow and its direction through the colour of the origin. Drawing on new estimates of 5-year bilateral migration flows between 196 countries, we demonstrate how to create circular migration plots at regional and country levels using three alternative software packages: Circos, R, and the JavaScript library d3.js. Circular migration plots considerably improve our ability to graphically evaluate complex patterns and trends in migration flow data, and for communicating migration research to scientists in other disciplines and to the general public. Our visualisation method is applicable to other kinds of flow data, including trade and remittances flows

Measuring Type Ia Supernova Distances and Redshifts From Their Multi-band Light Curves

The distance and redshift of a type Ia supernova can be determined simultaneously through its multi-band light curves. This fact may be used for imaging surveys that discover and obtain photometry for large numbers of supernovae; so many that it would be difficult to obtain a spectroscopic redshift for each. Using available supernova-analysis tools we find that there are several conditions in which a viable distance-redshift can be determined. Uncertainties in the effective distance at z~0.3 are dominated by redshift uncertainties coupled with the steepness of the Hubble law. By z~0.5 the Hubble law flattens out and distance-modulus uncertainties dominate. Observations that give S/N=50 at peak brightness and a four-day observer cadence in each of griz-bands are necessary to match the intrinsic supernova magnitude dispersion out to z=1.0. Lower S/N can be tolerated with the addition of redshift priors (e.g. from a host-galaxy photometric redshift), observations in an additional redder band, or by focusing on supernova redshifts that have particular leverage for this measurement. More stringent S/N requirements are anticipated as improved systematics control over intrinsic color, metallicity, and dust is attempted to be drawn from light curves.Comment: 16 pages, 4 figures, Astroparticle Physics, accepte

Efficient parallelization of polyphase arbitrary resampling FIR filters for high-speed applications

This article describes a method for increasing the sampling rate of efficient polyphase arbitrary resampling FIR filters. An FPGA proof of concept prototype of this architecture has been implemented in a Xilinx Kintex-7 FPGA which is able to convert the sampling rate of a signal from 500 MHz to 600 MHz. This article compares this new architecture with other best known efficient resampling architectures implemented on the same FPGA. The area usage on the FPGA shows that our proposed implementation is very proficient in high bandwidth applications without requiring significantly more resources on the FPGA. A theoretical calculation of the resampling error introduced on a modulated data stream is provided to evaluate the new architecture against other existing resampling architectures