4,567 research outputs found

    Decoherence suppression via environment preparation

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    To protect a quantum system from decoherence due to interaction with its environment, we investigate the existence of initial states of the environment allowing for decoherence-free evolution of the system. For models in which a two-state system interacts with a dynamical environment, we prove that such states exist if and only if the interaction and self-evolution Hamiltonians share an eigenstate. If decoherence by state preparation is not possible, we show that initial states minimizing decoherence result from a delicate compromise between the environment and interaction dynamics.Comment: 4 pages, 2 figure

    Incidence and severity of childhood pneumonia in the first year of life in a South African birth cohort: the Drakenstein Child Health Study

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    Background Childhood pneumonia causes substantial mortality and morbidity. Accurate measurements of pneumonia incidence are scarce in low-income and middle-income countries, particularly after implementation of pneumococcal conjugate vaccine. We aimed to assess the incidence, severity, and risk factors for pneumonia in the fi rst year of life in children enrolled in a South African birth cohort. Methods This birth cohort study is being done at two sites in Paarl, a periurban area of South Africa. We enrolled pregnant women (>18 years) and followed up mother–infant pairs to 1 year of age. We obtained data for risk factors and respiratory symptoms. Children received 13-valent pneumococcal conjugate vaccine according to national immunisation schedules. We established pneumonia surveillance systems and documented episodes of ambulatory pneumonia and pneumonia warranting hospital admission. We calculated incidence rate ratios for pneumonia with mixed-eff ects Poisson regression. Findings Between May 29, 2012 and May 31, 2014, we enrolled 697 infants who accrued 513 child-years of follow-up. We recorded 141 pneumonia episodes, with an incidence of 0·27 episodes per child-year (95% CI 0·23–0·32). 32 (23%) pneumonia cases were severe pneumonia, with an incidence of 0·06 episodes per child-year (95% CI 0·04–0·08). Two (1%) of 141 pneumonia episodes led to death from pneumonia. Maternal HIV, maternal smoking, male sex, and malnutrition were associated with an increased incidence of pneumonia. Interpretation Pneumonia incidence was high in the fi rst year of life, despite a strong immunisation programme including 13-valent pneumococcal conjugate vaccine. Incidence was associated with pneumonia risk factors that are amenable to interventions. Prevention of childhood pneumonia through public health interventions to address these risk factors should be strengthened. Funding Bill & Melinda Gates Foundation, South African Thoracic Society, Federation of Infectious Diseases Societies of South Africa, and University of Cape Town

    Deviational simulation of phonon transport in graphene ribbons with ab initio scattering

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    We present a deviational Monte Carlo method for solving the Boltzmann-Peierls equation with ab initio 3-phonon scattering, for temporally and spatially dependent thermal transport problems in arbitrary geometries. Phonon dispersion relations and transition rates for graphene are obtained from density functional theory calculations. The ab initio scattering operator is simulated by an energy-conserving stochastic algorithm embedded within a deviational, low-variance Monte Carlo formulation. The deviational formulation ensures that simulations are computationally feasible for arbitrarily small temperature differences, while the stochastic treatment of the scattering operator is both efficient and exhibits no timestep error. The proposed method, in which geometry and phonon-boundary scattering are explicitly treated, is extensively validated by comparison to analytical results, previous numerical solutions and experiments. It is subsequently used to generate solutions for heat transport in graphene ribbons of various geometries and evaluate the validity of some common approximations found in the literature. Our results show that modeling transport in long ribbons of finite width using the homogeneous Boltzmann equation and approximating phonon-boundary scattering using an additional homogeneous scattering rate introduces an error on the order of 10% at room temperature, with the maximum deviation reaching 30% in the middle of the transition regime.Singapore-MIT Alliance for Research and TechnologyAmerican Society for Engineering Education. National Defense Science and Engineering Graduate FellowshipNational Science Foundation (U.S.). Graduate Research Fellowshi

    Reaction mechanism of the direct gas phase synthesis of H2O2 catalyzed by Au3

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    The gas phase reaction of molecular oxygen and hydrogen catalyzed by a Au3cluster to yield H2O2 was investigated theoretically using second order Z-averaged perturbation theory, with the final energies obtained with the fully size extensive completely renormalized CR-CC(2,3) coupled clustertheory. The proposed reaction mechanism is initiated by adsorption and activation of O2 on the Au3cluster. Molecular hydrogen then binds to the Au3O2 global minimum without an energy barrier. The reaction between the activated oxygen and hydrogen molecules proceeds through formation of hydroperoxide (HO2) and a hydrogen atom, which subsequently react to form the product hydrogen peroxide. All reactants, intermediates, and product remain bound to the goldcluster throughout the course of the reaction. The steps in the proposed reaction mechanism have low activation energy barriers below 15kcal∕mol. The overall reaction is highly exothermic by ∼30kcal∕mol

    Overview of NASA Behavioral Health and Performance Standard Measures

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    NASAs Human Research Program (HRP) is developing a set of Standard Measures for use in spaceflight and spaceflight analog environments to monitor the risks of long-duration missions on human health and performance, including behavioral health, individual and team performance, and social processes. Based on measures selected, developed, and tested under the NASA-funded Behavioral Core Measures project (PI: D.F. Dinges) as well as other projects from NASAs Human Factors & Behavioral Performance research portfolio, NASAs Behavioral Health & Performance (BHP) Laboratory is further evaluating the operational feasibility, acceptability, and validity of a multidisciplinary suite of objective, subjective, behavioral, and biological measures for monitoring monitor behavioral health, individual and team performance, and social processes over time. The inaugural generation of the NASA Behavioral Health & Performance (BHP) Standard Measures includes a neurocognitive test battery, actigraphy, physical proximity sensors, cardiovascular monitors, and subjective self-reports of mood, depression, and various team and social processes and performance outcomes

    Electrode Kinetics of Vanadium Flow Batteries: Contrasting Responses of V\u3csup\u3eII\u3c/sup\u3e-V\u3csup\u3eIII\u3c/sup\u3e and V\u3csup\u3eIV\u3c/sup\u3e-V\u3csup\u3eV\u3c/sup\u3e to Electrochemical Pretreatment of Carbon

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    Electrochemical impedance spectroscopy and cyclic voltammetry were used to investigate the electrode kinetics of VII-VIII and VIV-VV in H2SO4 on glassy carbon, carbon paper, carbon xerogel, and carbon fibers. It was shown that, for all carbon materials investigated, the kinetics of VII-VIII is enhanced by anodic, and inhibited by cathodic, treatment of the electrode; in contrast, the kinetics of VIV-VV is inhibited by anodic, and enhanced by cathodic, treatment. The potential region for each of these effects varied only slightly with carbon material. Rate constants were always greater for VIV-VV than for VII-VIII except when anodized electrodes were compared, which may explain discrepancies in the literature. The observed effects are attributed to oxygen-containing functional-groups on the electrode surface. The considerable differences between the potentials at which enhancement of VII-VIII and inhibition of VIV-VV occur indicates that they do not correspond to a common oxidized state of the electrode. Likewise inhibition of VII-VIII and enhancement of VIV-VV do not correspond to a common reduced state of the electrode. It is possible that enhancement of both VII-VIII and VIV-VV is due to the same (active) state of the electrode

    Kitaev's quantum double model from a local quantum physics point of view

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    A prominent example of a topologically ordered system is Kitaev's quantum double model D(G)\mathcal{D}(G) for finite groups GG (which in particular includes G=Z2G = \mathbb{Z}_2, the toric code). We will look at these models from the point of view of local quantum physics. In particular, we will review how in the abelian case, one can do a Doplicher-Haag-Roberts analysis to study the different superselection sectors of the model. In this way one finds that the charges are in one-to-one correspondence with the representations of D(G)\mathcal{D}(G), and that they are in fact anyons. Interchanging two of such anyons gives a non-trivial phase, not just a possible sign change. The case of non-abelian groups GG is more complicated. We outline how one could use amplimorphisms, that is, morphisms AMn(A)A \to M_n(A) to study the superselection structure in that case. Finally, we give a brief overview of applications of topologically ordered systems to the field of quantum computation.Comment: Chapter contributed to R. Brunetti, C. Dappiaggi, K. Fredenhagen, J. Yngvason (eds), Advances in Algebraic Quantum Field Theory (Springer 2015). Mainly revie

    Rank-based model selection for multiple ions quantum tomography

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    The statistical analysis of measurement data has become a key component of many quantum engineering experiments. As standard full state tomography becomes unfeasible for large dimensional quantum systems, one needs to exploit prior information and the "sparsity" properties of the experimental state in order to reduce the dimensionality of the estimation problem. In this paper we propose model selection as a general principle for finding the simplest, or most parsimonious explanation of the data, by fitting different models and choosing the estimator with the best trade-off between likelihood fit and model complexity. We apply two well established model selection methods -- the Akaike information criterion (AIC) and the Bayesian information criterion (BIC) -- to models consising of states of fixed rank and datasets such as are currently produced in multiple ions experiments. We test the performance of AIC and BIC on randomly chosen low rank states of 4 ions, and study the dependence of the selected rank with the number of measurement repetitions for one ion states. We then apply the methods to real data from a 4 ions experiment aimed at creating a Smolin state of rank 4. The two methods indicate that the optimal model for describing the data lies between ranks 6 and 9, and the Pearson χ2\chi^{2} test is applied to validate this conclusion. Additionally we find that the mean square error of the maximum likelihood estimator for pure states is close to that of the optimal over all possible measurements.Comment: 24 pages, 6 figures, 3 table
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