Dynamos in Stellar Convection Zones: of Wreaths and Cycles

We live near a magnetic star whose cycles of activity are driven by dynamo action beneath the surface. In the solar convection zone, rotation couples with plasma motions to build highly organized magnetic fields that erupt at the surface and undergo relatively regular cycles of polarity reversal. Despite our proximity to the Sun, the nature of its dynamo remains elusive, but observations of other solar-type stars show that surface magnetism is a nearly ubiquitous feature. In recent time, numerical simulations of convection and dynamo action have taken tremendous strides forward. Global-scale organization and cyclic magnetism are being achieved by several groups in distinctly different solar and stellar simulations. Here I will talk about advances on the numerical front including wreath-building dynamos which may occupy stellar convection zones. I will discuss the interplay between the new simulations, various classes of mean-field models, and current and upcoming solar and stellar observations.Comment: 10 pages, 5 figures, a4 format; proceedings for SOHO 24/GONG 2010 conference: "A new era of seismology of the Sun and solar-like stars," Aix-en-Provence, France, June 27-July 4, 2010 (JPCS

Competing Dimensions of Energy Security: An International Perspective

How well are industrialized nations doing in terms of their energy security? Without a standardized set of metrics, it is difficult to determine the extent that countries are properly responding to the emerging energy security challenges related to climate change, growing dependence on fossil fuels, population growth and economic development. In response, we propose the creation of an Energy Security Index to inform policymakers, investors and analysts about the status of energy conditions. Using the United States and 21 other member countries of the Organization for Economic Cooperation and Development (OECD) as an example, and looking at energy security from 1970 to 2007, our index shows that only four countries¡ªBelgium, Denmark, Japan, and the United Kingdom¡ªhave made progress on multiple dimensions of the energy security problem. The remaining 18 have either made no improvement or are less secure. To make this argument, the first section of the article surveys the scholarly literature on energy security from 2003 to 2008 and argues that an index should address accessibility, affordability, efficiency, and environmental stewardship. Because each of these four components is multidimensional, the second section discusses ten metrics that comprise an Energy Security Index: oil import dependence, percentage of alternative transport fuels, on-road fuel economy for passenger vehicles, energy intensity, natural gas import dependence, electricity prices, gasoline prices, sulfur dioxide emissions, and carbon dioxide emissions. The third section analyzes the relative performance of four countries: Denmark (the top performer), Japan (which performed well), the United States (which performed poorly), and Spain (the worst performer). The article concludes by offering implications for policy. Conflicts between energy security criteria mean that advancement along any one dimension can undermine progress on another dimension. By focusing on a 10-point index, public policy can better illuminate such tradeoffs and can identify compensating policies

Analysing correlated noise on the surface code using adaptive decoding algorithms

Laboratory hardware is rapidly progressing towards a state where quantum error-correcting codes can be realised. As such, we must learn how to deal with the complex nature of the noise that may occur in real physical systems. Single qubit Pauli errors are commonly used to study the behaviour of error-correcting codes, but in general we might expect the environment to introduce correlated errors to a system. Given some knowledge of structures that errors commonly take, it may be possible to adapt the error-correction procedure to compensate for this noise, but performing full state tomography on a physical system to analyse this structure quickly becomes impossible as the size increases beyond a few qubits. Here we develop and test new methods to analyse blue a particular class of spatially correlated errors by making use of parametrised families of decoding algorithms. We demonstrate our method numerically using a diffusive noise model. We show that information can be learnt about the parameters of the noise model, and additionally that the logical error rates can be improved. We conclude by discussing how our method could be utilised in a practical setting blue and propose extensions of our work to study more general error models.Comment: 19 pages, 8 figures, comments welcome; v2 - minor typos corrected some references added; v3 - accepted to Quantu