1,264 research outputs found

    Coherent electronic transfer in quantum dot systems using adiabatic passage

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    We describe a scheme for using an all-electrical, rapid, adiabatic population transfer between two spatially separated dots in a triple-quantum dot system. The electron spends no time in the middle dot and does not change its energy during the transfer process. Although a coherent population transfer method, this scheme may well prove useful in incoherent electronic computation (for example quantum-dot cellular automata) where it may provide a coherent advantage to an otherwise incoherent device. It can also be thought of as a limiting case of type II quantum computing, where sufficient coherence exists for a single gate operation, but not for the preservation of superpositions after the operation. We extend our analysis to the case of many intervening dots and address the issue of transporting quantum information through a multi-dot system.Comment: Replaced with (approximately) the published versio

    Quantum dynamics, dissipation, and asymmetry effects in quantum dot arrays

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    We study the role of dissipation and structural defects on the time evolution of quantum dot arrays with mobile charges under external driving fields. These structures, proposed as quantum dot cellular automata, exhibit interesting quantum dynamics which we describe in terms of equations of motion for the density matrix. Using an open system approach, we study the role of asymmetries and the microscopic electron-phonon interaction on the general dynamical behavior of the charge distribution (polarization) of such systems. We find that the system response to the driving field is improved at low temperatures (and/or weak phonon coupling), before deteriorating as temperature and asymmetry increase. In addition to the study of the time evolution of polarization, we explore the linear entropy of the system in order to gain further insights into the competition between coherent evolution and dissipative processes.Comment: 11pages,9 figures(eps), submitted to PR

    Function reconstruction as a classical moment problem: A maximum entropy approach

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    We present a systematic study of the reconstruction of a non-negative function via maximum entropy approach utilizing the information contained in a finite number of moments of the function. For testing the efficacy of the approach, we reconstruct a set of functions using an iterative entropy optimization scheme, and study the convergence profile as the number of moments is increased. We consider a wide variety of functions that include a distribution with a sharp discontinuity, a rapidly oscillatory function, a distribution with singularities, and finally a distribution with several spikes and fine structure. The last example is important in the context of the determination of the natural density of the logistic map. The convergence of the method is studied by comparing the moments of the approximated functions with the exact ones. Furthermore, by varying the number of moments and iterations, we examine to what extent the features of the functions, such as the divergence behavior at singular points within the interval, is reproduced. The proximity of the reconstructed maximum entropy solution to the exact solution is examined via Kullback-Leibler divergence and variation measures for different number of moments.Comment: 20 pages, 17 figure

    Flat-band ferromagnetism in quantum dot superlattices

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    Possibility of flat-band ferromagnetism in quantum dot arrays is theoretically discussed. By using a quantum dot as a building block, quantum dot superlattices are possible. We consider dot arrays on Lieb and kagome lattices known to exhibit flat band ferromagnetism. By performing an exact diagonalization of the Hubbard Hamiltonian, we calculate the energy difference between the ferromagnetic ground state and the paramagnetic excited state, and discuss the stability of the ferromagnetism against the second nearest neighbor transfer. We calculate the dot-size dependence of the energy difference in a dot model and estimate the transition temperature of the ferromagnetic-paramagnetic transition which is found to be accessible within the present fabrication technology. We point out advantages of semiconductor ferromagnets and suggest other interesting possibilities of electronic properties in quantum dot superlattices.Comment: 15 pages, 7 figures (low resolution). High-resolution figures are available at http://www.brl.ntt.co.jp/people/tamura/Research/PublicationPapers.htm

    Bound States and Threshold Resonances in Quantum Wires with Circular Bends

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    We study the solutions to the wave equation in a two-dimensional tube of unit width comprised of two straight regions connected by a region of constant curvature. We introduce a numerical method which permits high accuracy at high curvature. We determine the bound state energies as well as the transmission and reflection matrices, T{\cal T} and R{\cal R} and focus on the nature of the resonances which occur in the vicinity of channel thresholds. We explore the dependence of these solutions on the curvature of the tube and angle of the bend and discuss several limiting cases where our numerical results confirm analytic predictions.Comment: 24 pages, revtex file, one style file and 17 PostScript figures include

    Attitudes about Future Genetic Testing for Posttraumatic Stress Disorder and Addiction among Community-Based Veterans.

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    This study explored attitudes toward hypothetical genetic testing for posttraumatic stress disorder (PTSD) and addiction among veterans. We surveyed a random sample of community-based veterans (n = 700) by telephone. One year later, we asked the veterans to provide a DNA sample for analysis and 41.9% of them returned the DNA samples. Overall, most veterans were not interested in genetic testing neither for PTSD (61.7%) nor for addiction (68.7%). However, bivariate analyses suggested there was an association between having the condition of interest and the likelihood of genetic testing on a 5-point scale (p \u3c 0.001 for PTSD; p = 0.001 for alcohol dependence). While ordinal regressions confirmed these associations, the models with the best statistical fit were bivariate models of whether the veteran would likely test or not. Using logistic regressions, significant predictors for PTSD testing were receiving recent mental health treatment, history of a concussion, younger age, having PTSD, having alcohol dependence, currently taking opioids for pain, and returning the DNA sample during the follow-up. For addiction testing, significant predictors were history of concussion, younger age, psychotropic medication use, having alcohol dependence, and currently taking opioids for pain. Altogether, 25.9% of veterans reported that they would have liked to have known their genetic results before deployment, 15.6% reported after deployment, and 58.6% reported they did not want to know neither before nor after deployment. As advancements in genetic testing continue to evolve, our study suggests that consumer attitudes toward genetic testing for mental disorders are complex and better understanding of these attitudes and beliefs will be crucial to successfully promote utilization

    Demographic, Clinical, and Behavioral Determinants of 7-Year Weight Change Trajectories in Roux-en-Y Gastric Bypass Patients.

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    BACKGROUND: Weight change trajectories after weight-loss surgery may vary significantly. OBJECTIVES: This study evaluated the weight trajectories of Roux-en-Y gastric bypass (RYGB) patients and identified the distinct clinical, behavioral, and demographic features of patients by trajectory. SETTING: Data from 2918 RYGB patients from a comprehensive medical center between January 2004 and November 2016 were included. METHODS: This retrospective, observational study used data for RYGB patients up to year 7 postsurgery. Group-based trajectory models were fitted for percentage weight change. Variables evaluated by trajectory included age, sex, diagnoses, medications, smoking, presurgical body mass index, preoperative weight loss, and early postoperative weight loss. RESULTS: Of 3215 possible patients, 2918 (90.8%) were included (mean age = 46.2 ± 11.2 yr, body mass index = 46.9 ± 7.9 kg/m CONCLUSION: Select clinical, demographic, and behavioral factors may increase or decrease the chance for better weight loss after RYGB
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