Exchange coupling between silicon donors: the crucial role of the central cell and mass anisotropy

Donors in silicon are now demonstrated as one of the leading candidates for implementing qubits and quantum information processing. Single qubit operations, measurements and long coherence times are firmly established, but progress on controlling two qubit interactions has been slower. One reason for this is that the inter donor exchange coupling has been predicted to oscillate with separation, making it hard to estimate in device designs. We present a multivalley effective mass theory of a donor pair in silicon, including both a central cell potential and the effective mass anisotropy intrinsic in the Si conduction band. We are able to accurately describe the single donor properties of valley-orbit coupling and the spatial extent of donor wave functions, highlighting the importance of fitting measured values of hyperfine coupling and the orbital energy of the $1s$ levels. Ours is a simple framework that can be applied flexibly to a range of experimental scenarios, but it is nonetheless able to provide fast and reliable predictions. We use it to estimate the exchange coupling between two donor electrons and we find a smoothing of its expected oscillations, and predict a monotonic dependence on separation if two donors are spaced precisely along the [100] direction.Comment: Published version. Corrected b and B values from previous versio

Negative differential Rashba effect in two-dimensional hole systems

We demonstrate experimentally and theoretically that two-dimensional (2D) heavy hole systems in single heterostructures exhibit a \emph{decrease} in spin-orbit interaction-induced spin splitting with an increase in perpendicular electric field. Using front and back gates, we measure the spin splitting as a function of applied electric field while keeping the density constant. Our results are in contrast to the more familiar case of 2D electrons where spin splitting increases with electric field.Comment: 3 pages, 3 figures. To appear in AP

Surface code architecture for donors and dots in silicon with imprecise and nonuniform qubit couplings

A scaled quantum computer with donor spins in silicon would benefit from a viable semiconductor framework and a strong inherent decoupling of the qubits from the noisy environment. Coupling neighbouring spins via the natural exchange interaction according to current designs requires gate control structures with extremely small length scales. We present a silicon architecture where bismuth donors with long coherence times are coupled to electrons that can shuttle between adjacent quantum dots, thus relaxing the pitch requirements and allowing space between donors for classical control devices. An adiabatic SWAP operation within each donor/dot pair solves the scalability issues intrinsic to exchange-based two-qubit gates, as it does not rely on sub-nanometer precision in donor placement and is robust against noise in the control fields. We use this SWAP together with well established global microwave Rabi pulses and parallel electron shuttling to construct a surface code that needs minimal, feasible local control.Comment: Published version - more detailed discussions, robustness to dephasing pointed out additionall

Addressing spin transitions on 209Bi donors in silicon using circularly-polarized microwaves

Over the past decade donor spin qubits in isotopically enriched $^{28}$Si have been intensely studied due to their exceptionally long coherence times. More recently bismuth donor electron spins have become popular because Bi has a large nuclear spin which gives rise to clock transitions (first-order insensitive to magnetic field noise). At every clock transition there are two nearly degenerate transitions between four distinct states which can be used as a pair of qubits. Here it is experimentally demonstrated that these transitions are excited by microwaves of opposite helicity such that they can be selectively driven by varying microwave polarization. This work uses a combination of a superconducting coplanar waveguide (CPW) microresonator and a dielectric resonator to flexibly generate arbitrary elliptical polarizations while retaining the high sensitivity of the CPW

Maternity protection in formal and informal economy workplaces: the case of Ghana

Maternity protection (MP) in the workplace is a vital element of ensuring women’s and children’s wellbeing. It aims to minimize the difficulties that working women face because of giving birth and to protect the health of mothers and their babies. Most research on MP around the world is carried out in larger organizations in developed countries. This cannot be applied uncritically to developing countries, where work in small firms and the informal economy tends to dominate and enterprises are more influenced by their local communities and environment. This paper explores different forms of MP found in various forms and sizes of enterprise in Ghana. Specifically, it examines 1) how workplaces in the formal and informal economy manage MP and 2) how reciprocity shapes workers’ access to the different forms of MP support. The paper draws on 63 qualitative interviews with employers and women employees across three geographic regions of Ghana. The findings show that there are a range of measures related to pregnancy, maternity leave, family-friendly flexible working conditions, breastfeeding and childcare. While large formal economy organizations mostly offer MP measures corresponding to regulatory entitlements, women working in small informal economy businesses often only benefit from informal MP support. However, neither statutory nor informal supports alone provide adequately for women’s needs. The paper highlights the important roles of reciprocity and relationships built on trust as conditions for family-friendly MP provisions, particularly in the informal economy workplaces. It concludes by identifying lessons that can be learnt by both formal and informal economy businesses

Spin Coherence and 14^{14}N ESEEM Effects of Nitrogen-Vacancy Centers in Diamond with X-band Pulsed ESR

Pulsed ESR experiments are reported for ensembles of negatively-charged nitrogen-vacancy centers (NV$^-$) in diamonds at X-band magnetic fields (280-400 mT) and low temperatures (2-70 K). The NV$^-$ centers in synthetic type IIb diamonds (nitrogen impurity concentration $<1$~ppm) are prepared with bulk concentrations of $2\cdot 10^{13}$ cm$^{-3}$ to $4\cdot 10^{14}$ cm$^{-3}$ by high-energy electron irradiation and subsequent annealing. We find that a proper post-radiation anneal (1000$^\circ$C for 60 mins) is critically important to repair the radiation damage and to recover long electron spin coherence times for NV$^-$s. After the annealing, spin coherence times of T$_2 = 0.74$~ms at 5~K are achieved, being only limited by $^{13}$C nuclear spectral diffusion in natural abundance diamonds. At X-band magnetic fields, strong electron spin echo envelope modulation (ESEEM) is observed originating from the central $^{14}$N nucleus. The ESEEM spectral analysis allows for accurate determination of the $^{14}$N nuclear hypefine and quadrupole tensors. In addition, the ESEEM effects from two proximal $^{13}$C sites (second-nearest neighbor and fourth-nearest neighbor) are resolved and the respective $^{13}$C hyperfine coupling constants are extracted.Comment: 10 pages, 5 figure

The Effects on Agriculture in Utah of Water Transfers to Oil Shale Development

In Part I the institutional factors affecting water distribution in the Upper Colorado River Basin in general and specifically the Uintah Basin are presented. The historical development of the appropriation doctrine of water allocation is outlined and Utah water policy is examined. These institutional factors are analyzed in light of the prototype oil shale development in the Uintah Basin and potential impact on the area\u27s agricultural sector. Oil shale water estimates are compared with Uintah Basin water availability and examined with regard to population projections and municipal water use. Lastly, Utah water policy and the appropriation doctrine are viewed as restraints to efficient water transfers. In Part II irrigation water is treated as a random variable. Its actual quantity is not known ahead of time. If transfers of water to oil shale production affect the variability of water used in agriculture then there will be impacts in agriculture even if the farmers receive the same average quantity of water as originally. These impacts are analyzed in the context of the expected utility maximization hypothesis, i.e., the farmers are hypothesized to maximize expected utility. The measure of an increase in variability is the mean preserving spread. The analyses seek to determine the impact upon expected (average) real income (utility), expected profits (net farm income), purchased inputs, the price of water, and the price of land. The analyses are conducted for both the case where the farmers are risk neutral and the case where they are risk averse

Mammals of Kenya’s protected areas from 1888 to 2013

Kenya is a world leader in conservation and host to one of the most diverse array of mammals on the planet. As a focus of scientific attention, it is important to be able to assess not only the current state of Kenya’s mammal communities, but also how they have changed over anthropogenic timescales. Comprehensive lists of mammal species from known areas are essential for this goal, and these also provide comparative baselines for assessing changes in mammalian diversity in the future and in the fossil record. Though there is considerable literature available for mammals inhabiting Kenyan protected areas (National Parks and Reserves), species compilation projects vary greatly in scope, completeness, agreement, and accuracy. We combine the information in these databases for Kenya and supplement them with the most up-to-date knowledge available up to November 2013. Comprehensive historical species lists were compiled from specimen lists collected during 1888–1950 in ecosystems that today correspond to 13 different protected areas. We also provide analogous modern species lists based on data collected during 1950–2012. The data sets include both large and small mammals. A master list of a total of 413 species provides ecological information including body mass, diet, feeding and shelter habitat, and activity time. Historical data are based on museum specimens and sighting records, and modern data are based on museum data as well as literature, books, field guides, written accounts, photos, and videos. We used this compilation for an analysis comparing the two data sets (excluding volant and domestic species) for six protected areas with the most complete historical records and have shown in a separate publication that species richness is preserved, but beta diversity, based on pairwise comparisons of sites in this database, is being lost over the entire study area

Componential coding in the condition monitoring of electrical machines Part 2: application to a conventional machine and a novel machine

This paper (Part 2) presents the practical application of componential coding, the principles of which were described in the accompanying Part 1 paper. Four major issues are addressed, including optimization of the neural network, assessment of the anomaly detection results, development of diagnostic approaches (based on the reconstruction error) and also benchmarking of componential coding with other techniques (including waveform measures, Fourier-based signal reconstruction and principal component analysis). This is achieved by applying componential coding to the data monitored from both a conventional induction motor and from a novel transverse flux motor. The results reveal that machine condition monitoring using componential coding is not only capable of detecting and then diagnosing anomalies but it also outperforms other conventional techniques in that it is able to separate very small and localized anomalies