5,371 research outputs found
Hydrogenic Spin Quantum Computing in Silicon: A Digital Approach
We suggest an architecture for quantum computing with spin-pair encoded
qubits in silicon. Electron-nuclear spin-pairs are controlled by a dc magnetic
field and electrode-switched on and off hyperfine interaction. This digital
processing is insensitive to tuning errors and easy to model. Electron
shuttling between donors enables multi-qubit logic. These hydrogenic spin
qubits are transferable to nuclear spin-pairs, which have long coherence times,
and electron spin-pairs, which are ideally suited for measurement and
initialization. The architecture is scalable to highly parallel operation.Comment: 4 pages, 5 figures; refereed and published version with improved
introductio
The Fantastic Four: A plug 'n' play set of optimal control pulses for enhancing nmr spectroscopy
We present highly robust, optimal control-based shaped pulses designed to
replace all 90{\deg} and 180{\deg} hard pulses in a given pulse sequence for
improved performance. Special attention was devoted to ensuring that the pulses
can be simply substituted in a one-to-one fashion for the original hard pulses
without any additional modification of the existing sequence. The set of four
pulses for each nucleus therefore consists of 90{\deg} and 180{\deg}
point-to-point (PP) and universal rotation (UR) pulses of identical duration.
These 1 ms pulses provide uniform performance over resonance offsets of 20 kHz
(1H) and 35 kHz (13C) and tolerate reasonably large radio frequency (RF)
inhomogeneity/miscalibration of (+/-)15% (1H) and (+/-)10% (13C), making them
especially suitable for NMR of small-to-medium-sized molecules (for which
relaxation effects during the pulse are negligible) at an accessible and widely
utilized spectrometer field strength of 600 MHz. The experimental performance
of conventional hard-pulse sequences is shown to be greatly improved by
incorporating the new pulses, each set referred to as the Fantastic Four
(Fanta4).Comment: 28 pages, 19 figure
Coulomb gap in the one-particle density of states in three-dimensional systems with localized electrons
The one-particle density of states (1P-DOS) in a system with localized
electron states vanishes at the Fermi level due to the Coulomb interaction
between electrons. Derivation of the Coulomb gap uses stability criteria of the
ground state. The simplest criterion is based on the excitonic interaction of
an electron and a hole and leads to a quadratic 1P-DOS in the three-dimensional
(3D) case. In 3D, higher stability criteria, including two or more electrons,
were predicted to exponentially deplete the 1P-DOS at energies close enough to
the Fermi level. In this paper we show that there is a range of intermediate
energies where this depletion is strongly compensated by the excitonic
interaction between single-particle excitations, so that the crossover from
quadratic to exponential behavior of the 1P-DOS is retarded. This is one of the
reasons why such exponential depletion was never seen in computer simulations.Comment: 6 pages, 1 figur
Effect of African Potato (Hypoxis Hemerocallidea) on the Pharmacokinetics of Efavirenz
Purpose. The purpose of this study was to evaluate the effect of the African potato (AP) on the pharmacokinetics of efavirenz.
Methods. A single-dose, two-phase sequential study was conducted over 31 days in 10 healthy volunteers. On day 1 of the study, volunteers were administered a 600 mg
efavirenz tablet, and blood samples were collected before dosing and at 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 8, 12, 18, 24, 36 and 48 hours after dosing. From day 16, a traditionally
prepared AP decoction was administered daily until day 30. On day 29, volunteers were administered a single 600 mg dose of efavirenz, as was done on day 1. Plasma samples
were harvested immediately after blood sample collection and frozen at –80ºC until assayed. Plasma concentrations of efavirenz were determined by a validated high performance liquid chromatography (HPLC) method with UV detection, and pharmacokinetic parameters were calculated. Geometric mean ratios of Cmax and AUC0-48 of efavirenz before and after co-administration of 14 successive daily doses of AP were compared.
Results. All subjects completed the study. The geometric mean ratios of Cmax and AUC0-48 were 97.30 and 102.82 with corresponding 90% confidence intervals (CIs) of 78.81 - 120.14
and 89.04 - 118.80, respectively. Conclusion. Pharmacokinetic data generated during this
study indicated that AP did not significantly alter the pharmacokinetics of efavirenz. Hence, co-administration of AP is unlikely to affect the clinical usage of efavirenz. South African Medical Journal Vol. 98 (12) 2008: pp. 945-94
Broadband 180 degree universal rotation pulses for NMR spectroscopy designed by optimal control
Broadband inversion pulses that rotate all magnetization components 180
degrees about a given fixed axis are necessary for refocusing and mixing in
high-resolution NMR spectroscopy. The relative merits of various methodologies
for generating pulses suitable for broadband refocusing are considered. The de
novo design of 180 degree universal rotation pulses using optimal control can
provide improved performance compared to schemes which construct refocusing
pulses as composites of existing pulses. The advantages of broadband universal
rotation by optimized pulses (BURBOP) are most evident for pulse design that
includes tolerance to RF inhomogeneity or miscalibration. We present new
modifications of the optimal control algorithm that incorporate symmetry
principles and relax conservative limits on peak RF pulse amplitude for short
time periods that pose no threat to the probe. We apply them to generate a set
of pulses suitable for widespread use in Carbon-13 spectroscopy on the majority
of available probes
Game On? Smoking Cessation Through the Gamification of mHealth: A Longitudinal Qualitative Study
BACKGROUND: Finding ways to increase and sustain engagement with mHealth interventions has become a challenge during application development. While gamification shows promise and has proven effective in many fields, critical questions remain concerning how to use gamification to modify health behavior. OBJECTIVE: The objective of this study is to investigate how the gamification of mHealth interventions leads to a change in health behavior, specifically with respect to smoking cessation. METHODS: We conducted a qualitative longitudinal study using a sample of 16 smokers divided into 2 cohorts (one used a gamified intervention and the other used a nongamified intervention). Each participant underwent 4 semistructured interviews over a period of 5 weeks. Semistructured interviews were also conducted with 4 experts in gamification, mHealth, and smoking cessation. Interviews were transcribed verbatim and thematic analysis undertaken. RESULTS: Results indicated perceived behavioral control and intrinsic motivation acted as positive drivers to game engagement and consequently positive health behavior. Importantly, external social influences exerted a negative effect. We identified 3 critical factors, whose presence was necessary for game engagement: purpose (explicit purpose known by the user), user alignment (congruency of game and user objectives), and functional utility (a well-designed game). We summarize these findings in a framework to guide the future development of gamified mHealth interventions. CONCLUSIONS: Gamification holds the potential for a low-cost, highly effective mHealth solution that may replace or supplement the behavioral support component found in current smoking cessation programs. The framework reported here has been built on evidence specific to smoking cessation, however it can be adapted to health interventions in other disease categories. Future research is required to evaluate the generalizability and effectiveness of the framework, directly against current behavioral support therapy interventions in smoking cessation and beyond
Menstrual phase influences cerebrovascular responsiveness in females but may not affect sex differences
Background and aims: Sex differences in the rate and occurrence of cerebrovascular diseases (e.g., stroke) indicate a role for female sex hormones (i.e., oestrogen and progesterone) in cerebrovascular function and regulation. However, it remains unclear how cerebrovascular function differs between the sexes, and between distinct phases of the menstrual cycle. This study aimed to compare cerebrovascular-CO(2) responsiveness in 1) females during the early follicular (EF), ovulatory (O) and mid-luteal (ML) phases of their menstrual cycle; and 2) males compared to females during phases of lower oestrogen (EF) and higher oestrogen (O). Methods: Eleven females (25 ± 5 years) complete experimental sessions in the EF (n = 11), O (n = 9) and ML (n = 11) phases of the menstrual cycle. Nine males (22 ± 3 years) completed two experimental sessions, approximately 2 weeks apart for comparison to females. Middle and posterior cerebral artery velocity (MCAv, PCAv) was measured at rest, during two stages of hypercapnia (2% and 5% CO(2) inhalation) and hypocapnia (voluntary hyperventilation to an end-tidal CO(2) of 30 and 24 mmHg). The linear slope of the cerebral blood velocity response to changes in end-tidal CO(2) was calculated to measure cerebrovascular-CO(2) responsiveness.. Results: In females, MCAv-CO(2) responsiveness to hypocapnia was lower during EF (−.78 ± .45 cm/s/mmHg) when compared to the O phase (−1.17 ± .52 cm/s/mmHg; p < .05) and the ML phase (−1.30 ± .82; p < .05). MCAv-CO(2) responsiveness to hypercapnia and hypo-to-hypercapnia, and PCAv-CO(2) responsiveness across the CO(2) range were similar between menstrual phases (p ≥ .20). MCAv-CO(2) responsiveness to hypo-to hypercapnia was greater in females compared to males (3.12 ± .91 cm/s/mmHg vs. 2.31 ± .46 cm/s/mmHg; p = .03), irrespective of menstrual phase (EF or O). Conclusion: Females during O and ML phases have an enhanced vasoconstrictive capacity of the MCA compared to the EF phase. Additionally, biological sex differences can influence cerebrovascular-CO(2) responsiveness, dependent on the insonated vessel
Non-mean-field theory of anomalously large double-layer capacitance
Mean-field theories claim that the capacitance of the double-layer formed at
a metal/ionic conductor interface cannot be larger than that of the Helmholtz
capacitor, whose width is equal to the radius of an ion. However, in some
experiments the apparent width of the double-layer capacitor is substantially
smaller. We propose an alternate, non-mean-field theory of the ionic
double-layer to explain such large capacitance values. Our theory allows for
the binding of discrete ions to their image charges in the metal, which results
in the formation of interface dipoles. We focus primarily on the case where
only small cations are mobile and other ions form an oppositely-charged
background. In this case, at small temperature and zero applied voltage dipoles
form a correlated liquid on both contacts. We show that at small voltages the
capacitance of the double-layer is determined by the transfer of dipoles from
one electrode to the other and is therefore limited only by the weak
dipole-dipole repulsion between bound ions, so that the capacitance is very
large. At large voltages the depletion of bound ions from one of the capacitor
electrodes triggers a collapse of the capacitance to the much smaller
mean-field value, as seen in experimental data. We test our analytical
predictions with a Monte Carlo simulation and find good agreement. We further
argue that our ``one-component plasma" model should work well for strongly
asymmetric ion liquids. We believe that this work also suggests an improved
theory of pseudo-capacitance.Comment: 19 pages, 14 figures; some Monte Carlo results and a section about
aqueous solutions adde
Doppler Effect of Nonlinear Waves and Superspirals in Oscillatory Media
Nonlinear waves emitted from a moving source are studied. A meandering spiral
in a reaction-diffusion medium provides an example, where waves originate from
a source exhibiting a back-and-forth movement in radial direction. The periodic
motion of the source induces a Doppler effect that causes a modulation in
wavelength and amplitude of the waves (``superspiral''). Using the complex
Ginzburg-Landau equation, we show that waves subject to a convective Eckhaus
instability can exhibit monotonous growth or decay as well as saturation of
these modulations away from the source depending on the perturbation frequency.
Our findings allow a consistent interpretation of recent experimental
observations concerning superspirals and their decay to spatio-temporal chaos.Comment: 4 pages, 4 figure
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