Optimal control strategies for coupled quantum dots

Semiconductor quantum dots are ideal candidates for quantum information applications in solid-state technology. However, advanced theoretical and experimental tools are required to coherently control, for example, the electronic charge in these systems. Here we demonstrate how quantum optimal control theory provides a powerful way to manipulate the electronic structure of coupled quantum dots with an extremely high fidelity. As alternative control fields we apply both laser pulses as well as electric gates, respectively. We focus on double and triple quantum dots containing a single electron or two electrons interacting via Coulomb repulsion. In the two-electron situation we also briefly demonstrate the challenges of timedependent density-functional theory within the adiabatic local-density approximation to produce comparable results with the numerically exact approach.     

Parametric study on heat transfer enhancement and pressure drop of an internal blade tip-wall with pin-fin arrays

One way to cool gas turbine tips is to design serpentine passages with 180° turns inside the blades to fully utilize the coolant potential. It is therefore a desire to improve the cooling of the blade tips to ensure a long durability and safe operation. In the present work, a two-pass channel with a 180° turn and various arrays of pin-fins mounted internally on the tip-cap is considered. The effects of pin-fin height, diameter and pitches on the heat transfer enhancement and pressure drop are investigated numerically. The nominal ratio of height to diameter (H/D) of the pin-fins is 2, and the ratio of tip clearance to pin-fin height is about 10. The inlet Reynolds numbers based on hydraulic diameter are ranging from 100,000 to 600,000. Details of the three dimensional fluid flow and heat transfer over the pin-finned tips are presented. The overall performances of various tips are compared. It is found that due to the combination of turning, impingement and pin-fin crossflow, the heat transfer coefficient of the pin-finned tips is up to a factor of 2.1 higher than that of the smooth tip. This augmentation is achieved at the expense of a penalty of pressure drop around 30%. Results show that the magnitude of the heat transfer enhancement depends upon pin-fin configuration and arrangement. It is suggested that pin-fins are suitable to enhance the blade tip heat transfer and thus to improve the tip cooling.     

1H and 13C{1H} NMR spectral parameters of sulfur mustards,nitrogen mustards,and lewisites: Computing and Predicting of Reference Spectra for Chemical Identification

The ¹H and ¹³C{¹H} chemical shifts and ¹H spin–spin couplings of sulfur mustards, nitrogen mustards, and lewisites scheduled in the Chemical Weapons Convention, and those of bis(2‐chloromethyl)disulfide, were determined in CDCl₃, CD₂Cl₂, and (CD₃)₂CO. Accurate parameters of this kind of series can be used for evaluating the current molecular modeling programs and the chemical shift and coupling constant prediction possibilities of the programs. Several prediction tests were made with commercial programs, and the results are reported here. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface selective removal of xylan from refined never-dried birch kraft pulp

In this study, the effect of enzyme treatment on refined, never-dried bleached birch kraft pulp was investigated, using an endo-1,4-β-xylanase, that is substantially free from cellulase activity. The xylanase treatment of refined never-dried pulp revealed a rapid initial hydrolysis rate with a time-dependent saturation level in the amount of hydrolyzed pulp carbohydrates. Surprisingly short xylanase treatment times were found to have an impact on the fiber surface structure and on the physicochemical properties of kraft pulp fibers. Xylanase treatment led to mild microscopic differences in the ultrastructure of a never-dried fiber, whereas local topographical differences were distinguishable with atomic force microscopy. Results from the analysis of dissolved carbohydrates and the interfacial properties of the xylanase-treated never-dried fibers thus confirm a selective removal of xylan from the fiber surfaces. The zeta-potential charge and dewatering properties of the pulp slurry, fiber morphology, and strength properties of the paper were affected, which is a concomitant of xylanase treatment. However, the papermaking properties of the fibers were mainly preserved with simultaneous improvement in the dewatering rate of the pulp. Thus, optimized xylanase treatment of refined bleached kraft pulp provides a fiber for papermaking or fiber modification purposes with a selectively modified chemical composition of the fiber surface layer.     

Viable Photocatalysts under Solar‐Spectrum Irradiation: Nonplasmonic Metal Nanoparticles

Supported nanoparticles (NPs) of nonplasmonic transition metals (Pd, Pt, Rh, and Ir) are widely used as thermally activated catalysts for the synthesis of important organic compounds, but little is known about their photocatalytic capabilities. We discovered that irradiation with light can significantly enhance the intrinsic catalytic performance of these metal NPs at ambient temperatures for several types of reactions. These metal NPs strongly absorb the light mainly through interband electronic transitions. The excited electrons interact with the reactant molecules on the particles to accelerate these reactions. The rate of the catalyzed reaction depends on the concentration and energy of the excited electrons, which can be increased by increasing the light intensity or by reducing the irradiation wavelength. The metal NPs can also effectively couple thermal and light energy sources to more efficiently drive chemical transformations.     

Propagation of low-intensity Gaussian fields in photorefractive media with real and imaginary intensity-dependent refractive index components

A theoretical analysis of a low power Gaussian field propagating in unbiased self-defocussing photorefractive media that includes both real and imaginary components of the intensity-dependent refractive index is presented. The analysis, which is in excellent agreement with experimental observations, shows that the imaginary component of the intensity-dependent refractive index can have a focussing effect independent of the focussing or defocussing effect of the real component of the intensity-dependent refractive index. These findings suggest that the imaginary component of the intensity-dependent refractive index is the cause of the previously observed apparent self-focussing in unbiased self-defocussing photorefractive media.     

Shape recovery for sparse‐data tomography

A two‐dimensional sparse‐data tomographic problem is studied. The target is assumed to be a homogeneous object bounded by a smooth curve. A nonuniform rational basis splines (NURBS) curve is used as a computational representation of the boundary. This approach conveniently provides the result in a format readily compatible with computer‐aided design software. However, the linear tomography task becomes a nonlinear inverse problem because of the NURBS‐based parameterization. Therefore, Bayesian inversion with Markov chain Monte Carlo sampling is used for calculating an estimate of the NURBS control points. The reconstruction method is tested with both simulated data and measured X‐ray projection data. The proposed method recovers the shape and the attenuation coefficient significantly better than the baseline algorithm (optimally thresholded total variation regularization), but at the cost of heavier computation.     

The rate of convergence in Orey's theorem for Harris recurrent Markov chains with applications to renewal theory

We derive sufficient conditions for ∝ λ (dx)6Pⁿ(x, ·) - π6 to be of order o(ψ(n)^-1), where Pⁿ (x, A) are the transition probabilities of an aperiodic Harris recurrent Markov chain, π is the invariant probability measure, λ an initial distribution and ψ belongs to a suitable class of non-decreasing sequences. The basic condition involved is the ergodicity of order ψ, which in a countable state space is equivalent to $\text{Σ ψ(n)}\text{P}{}_{\mathrm{i}}\text{{τ}{}_{\mathrm{i}}\text{?n} <∞}$ for some i, where τ_i is the hitting time of the tate i. We also show that for a general Markov chain to be ergodic of order ψ it suffices that a corresponding condition is satisfied by a small set.We apply these results to non-singular renewal measures on $\text{R}$ providing a probabilisite method to estimate the right tail of the renewal measure when the increment distribution F satisfies ∝ tF(dt) 0; > 0 and ∝ ψ(t)(1- F(t))dt< ∞.     

New lower bounds for multiple coverings

A code , where Z ₂ = {0,1}, is said to be a binary μ-fold R-covering code, if for any word there are at least μ distinct codewords which differ from v in at most R coordinates. The size of the smallest binary μ-fold R-covering code of length n is denoted by K(n, R, μ). In this paper we use integer programming and exhaustive search to improve 57 lower bounds on K(n, R, μ) for 6 ≤ n ≤ 16, 1 ≤ R ≤ 4 and 2 ≤ μ ≤ 4.