An electrostatically defined serial triple quantum dot charged with few electrons

A serial triple quantum dot (TQD) electrostatically defined in a GaAs/AlGaAs heterostructure is characterized by using a nearby quantum point contact as charge detector. Ground state stability diagrams demonstrate control in the regime of few electrons charging the TQD. An electrostatic model is developed to determine the ground state charge configurations of the TQD. Numerical calculations are compared with experimental results. In addition, the tunneling conductance through all three quantum dots in series is studied. Quantum cellular automata processes are identified, which are where charge reconfiguration between two dots occurs in response to the addition of an electron in the third dot.Comment: 12 pages, 9 figure

The origin of switching noise in GaAs/AlGaAs lateral gated devices

We have studied the origin of switching (telegraph) noise at low temperature in lateral quantum structures defined electrostatically in GaAs/AlGaAs heterostructures by surface gates. The noise was measured by monitoring the conductance fluctuations around $e^2/h$ on the first step of a quantum point contact at around 1.2 K. Cooling with a positive bias on the gates dramatically reduces this noise, while an asymmetric bias exacerbates it. We propose a model in which the noise originates from a leakage current of electrons that tunnel through the Schottky barrier under the gate into the doped layer. The key to reducing noise is to keep this barrier opaque under experimental conditions. Bias cooling reduces the density of ionized donors, which builds in an effective negative gate voltage. A smaller negative bias is therefore needed to reach the desired operating point. This suppresses tunnelling from the gate and hence the noise. The reduction in the density of ionized donors also strengthens the barrier to tunneling at a given applied voltage. Support for the model comes from our direct observation of the leakage current into a closed quantum dot, around $10^{-20} \mathrm{A}$ for this device. The current was detected by a neighboring quantum point contact, which showed monotonic steps in time associated with the tunneling of single electrons into the dot. If asymmetric gate voltages are applied, our model suggests that the noise will increase as a consequence of the more negative gate voltage applied to one of the gates to maintain the same device conductance. We observe exactly this behaviour in our experiments.Comment: 8 pages, 7 figure

Theory of electronic transport through a triple quantum dot in the presence of magnetic field

Theory of electronic transport through a triangular triple quantum dot subject to a perpendicular magnetic field is developed using a tight binding model. We show that magnetic field allows to engineer degeneracies in the triple quantum dot energy spectrum. The degeneracies lead to zero electronic transmission and sharp dips in the current whenever a pair of degenerate states lies between the chemical potential of the two leads. These dips can occur with a periodicity of one flux quantum if only two levels contribute to the current or with half flux quantum if the three levels of the triple dot contribute. The effect of strong bias voltage and different lead-to-dot connections on Aharonov-Bohm oscillations in the conductance is also discussed

Solid-state Marx based two-switch voltage modulator for the On-Line Isotope Mass Separator accelerator at the European Organization for Nuclear Research

A new circuit topology is proposed to replace the actual pulse transformer and thyratron based resonant modulator that supplies the 60 kV target potential for the ion acceleration of the On-Line Isotope Mass Separator accelerator, the stability of which is critical for the mass resolution downstream separator, at the European Organization for Nuclear Research. The improved modulator uses two solid-state switches working together, each one based on the Marx generator concept, operating as series and parallel switches, reducing the stress on the series stacked semiconductors, and also as auxiliary pulse generator in order to fulfill the target requirements. Preliminary results of a 10 kV prototype, using 1200 V insulated gate bipolar transistors and capacitors in the solid-state Marx circuits, ten stages each, with an electrical equivalent circuit of the target, are presented, demonstrating both the improved voltage stability and pulse flexibility potential wanted for this new modulator

Psychological resilience in sport performers: a review of stressors and protective factors

Psychological resilience is important in sport because athletes must utilize and optimize a range of mental qualities to withstand the pressures that they experience. In this paper, we discuss psychological resilience in sport performers via a review of the stressors athletes encounter and the protective factors that help them withstand these demands. It is hoped that synthesizing what is known in these areas will help researchers gain a deeper profundity of resilience in sport, and also provide a rigorous and robust foundation for the development of a sport-specific measure of resilience. With these points in mind, we divided the narrative into two main sections. In the first section, we review the different types of stressors encountered by sport performers under three main categories: competitive, organizational, and personal. Based on our recent research examining psychological resilience in Olympics champions (Fletcher & Sarkar, 2012), in the second section we discuss the five main families of psychological factors (viz. positive personality, motivation, confidence, focus, perceived social support) that protect the best athletes from the potential negative effect of stressors. It is anticipated that this review will help sport psychology researchers examine the interplay between stressors and protective factors which will, in turn, focus the analytical lens on the processes underlying psychological resilience in athletes

Actual and ideal roles of school staff to support students with special needs: Current needs and strategies for improvement

To optimise school-based service delivery for students with disabilities, it is important to understand roles and needs of school staff. This study aimed to clarify ideal and actual roles of school staff (teachers, special educators, administrators) working with students with special needs, and to identify potential strategies to support actual roles. Ninety-five school personnel (64% teachers) from 3 different elementary schools and school boards in Quebec completed a 14-question survey. Open-ended responses were coded and analysed thematically. Common actual roles included task adaptation, offering individualized support, being available, and teamwork. Respondents felt roles could improve through in-context professional support, continuing education, teamwork opportunities extending to partnerships with families, and access to resources. Clarifying roles and expectations within a tiered-model to best support students also emerged, emphasizing the importance of sharing responsibilities across all service providers. Findings can guide implementation strategies and processes for providing effective services, enabling inclusion for students

Hemispherical differences in the shape and topography of asteroid (101955) Bennu

We investigate the shape of near-Earth asteroid (101955) Bennu by constructing a high-resolution (20 cm) global digital terrain model from laser altimeter data. By modeling the northern and southern hemispheres separately, we find that longitudinal ridges previously identified in the north extend into the south but are obscured there by surface material. In the south, more numerous large boulders effectively retain surface materials and imply a higher average strength at depth to support them. The north has fewer large boulders and more evidence of boulder dynamics (toppling and downslope movement) and surface flow. These factors result in Bennu’s southern hemisphere being rounder and smoother, whereas its northern hemisphere has higher slopes and a less regular shape. We infer an originally asymmetric distribution of large boulders followed by a partial disruption, leading to wedge formation in Bennu’s history

Coherent control of three-spin states in a triple quantum dot

Spin qubits involving individual spins in single quantum dots or coupled spins in double quantum dots have emerged as potential building blocks for quantum information processing applications. It has been suggested that triple quantum dots may provide additional tools and functionalities. These include the encoding of information to either obtain protection from decoherence or to permit all-electrical operation, efficient spin busing across a quantum circuit, and to enable quantum error correction utilizing the three-spin Greenberger-Horn-Zeilinger quantum state. Towards these goals we demonstrate for the first time coherent manipulation between two interacting three-spin states. We employ the Landau-Zener-St\"uckelberg approach for creating and manipulating coherent superpositions of quantum states. We confirm that we are able to maintain coherence when decreasing the exchange coupling of one spin with another while simultaneously increasing its coupling with the third. Such control of pairwise exchange is a requirement of most spin qubit architectures but has not been previously demonstrated.Comment: 12 pages, 13 figures, and 2 table