416 research outputs found
Nuclear spin state narrowing via gate--controlled Rabi oscillations in a double quantum dot
We study spin dynamics for two electrons confined to a double quantum dot
under the influence of an oscillating exchange interaction. This leads to
driven Rabi oscillations between the --state and the
--state of the two--electron system. The width of the
Rabi resonance is proportional to the amplitude of the oscillating exchange. A
measurement of the Rabi resonance allows one to narrow the distribution of
nuclear spin states and thereby to prolong the spin decoherence time. Further,
we study decoherence of the two-electron states due to the hyperfine
interaction and give requirements on the parameters of the system in order to
initialize in the --state and to perform a
operation with unit fidelity.Comment: v1:9 pages, 1 figure; v2: 13 pages, 2 figures, added section on
measurement, to appear in Phys. Rev.
Six months functional response to early psychosis intervention program best predicts outcome after three years.
Not all patients respond well to early interventions for their psychosis. The present study's goal was to evaluate whether patients' responses in the first six months of treatment in a specialised three-year programme could predict final outcomes.
206 early psychosis patients were assessed at baseline, using a large set of sociodemographic and clinical variables, and then monitored for 36 months. Among those variables, changes in their Global Assessment of Functioning (GAF) scores during the first six months were used to predict outcomes after three years.
Changes in GAF scores during the first six months were the only variables that predicted every symptom of functional outcome. GAF scores were also always the first or second most important predictor for every outcome. This finding held for both high- and low-functioning patients at baseline.
Predicting poor long-term outcomes after only six months should help clinicians to improve treatments
ESCAP CovCAP survey of heads of academic departments to assess the perceived initial (April/May 2020) impact of the COVID-19 pandemic on child and adolescent psychiatry services.
In April 2020, the European Society for Child and Adolescent Psychiatry (ESCAP) Research Academy and the ESCAP Board launched the first of three scheduled surveys to evaluate the impact of the coronavirus disease 2019 (COVID-19) pandemic on child and adolescent psychiatry (CAP) services in Europe and to assess the abilities of CAP centers to meet the new challenges brought on by the crisis. The survey was a self-report questionnaire, using a multistage process, which was sent to 168 heads of academic CAP services in 24 European countries. Eighty-two responses (56 complete) from 20 countries, representing the subjective judgement of heads of CAP centers, were received between mid-April and mid-May 2020. Most respondents judged the impact of the crisis on the mental health of their patients as medium (52%) or strong (33%). A large majority of CAP services reported no COVID-19 positive cases among their inpatients and most respondents declared no or limited sick leaves in their team due to COVID-19. Outpatient, daycare, and inpatient units experienced closures or reductions in the number of treated patients throughout Europe. In addition, a lower referral rate was observed in most countries. Respondents considered that they were well equipped to handle COVID-19 patients despite a lack of protective equipment. Telemedicine was adopted by almost every team despite its sparse use prior to the crisis. Overall, these first results were surprisingly homogeneous, showing a substantially reduced patient load and a moderate effect of the COVID-19 crisis on psychopathology. The effect on the organization of CAP services appears profound. COVID-19 crisis has accelerated the adoption of new technologies, including telepsychiatry
Universal phase shift and non-exponential decay of driven single-spin oscillations
We study, both theoretically and experimentally, driven Rabi oscillations of
a single electron spin coupled to a nuclear spin bath. Due to the long
correlation time of the bath, two unusual features are observed in the
oscillations. The decay follows a power law, and the oscillations are shifted
in phase by a universal value of ~pi/4. These properties are well understood
from a theoretical expression that we derive here in the static limit for the
nuclear bath. This improved understanding of the coupled electron-nuclear
system is important for future experiments using the electron spin as a qubit.Comment: Main text: 4 pages, 3 figures, Supplementary material: 2 pages, 3
figure
Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: fine structure and spin relaxation
Exciton fine structures in cubic III-V semiconductor GaAs, InAs and GaN
quantum dots are investigated systematically and the exciton spin relaxation in
GaN quantum dots is calculated by first setting up the effective exciton
Hamiltonian. The electron-hole exchange interaction Hamiltonian, which consists
of the long- and short-range parts, is derived within the effective-mass
approximation by taking into account the conduction, heavy- and light-hole
bands, and especially the split-off band. The scheme applied in this work
allows the description of excitons in both the strong and weak confinement
regimes. The importance of treating the direct electron-hole Coulomb
interaction unperturbatively is demonstrated. We show in our calculation that
the light-hole and split-off bands are negligible when considering the exciton
fine structure, even for GaN quantum dots, and the short-range exchange
interaction is irrelevant when considering the optically active doublet
splitting. We point out that the long-range exchange interaction, which is
neglected in many previous works, contributes to the energy splitting between
the bright and dark states, together with the short-range exchange interaction.
Strong dependence of the optically active doublet splitting on the anisotropy
of dot shape is reported. Large doublet splittings up to 600 eV, and even
up to several meV for small dot size with large anisotropy, is shown in GaN
quantum dots. The spin relaxation between the lowest two optically active
exciton states in GaN quantum dots is calculated, showing a strong dependence
on the dot anisotropy. Long exciton spin relaxation time is reported in GaN
quantum dots. These findings are in good agreement with the experimental
results.Comment: 22+ pages, 16 figures, several typos in the published paper are
corrected in re
SN 2013df, a double-peaked IIb supernova from a compact progenitor and an extended H envelope
Optical observations of the type IIb SN 2013df from a few days to about 250
days after explosion are presented. These observations are complemented with UV
photometry taken by \textit{SWIFT} up to 60 days post-explosion. The
double-peak optical light curve is similar to those of SNe 1993J and 2011fu
although with different decline and rise rates. From the modelling of the
bolometric light curve, we have estimated that the total mass of synthesised
Ni in the explosion is M, while the ejecta mass is
M and the explosion energy erg. In
addition, we have estimated a lower limit to the progenitor radius ranging from
. The spectral evolution indicates that SN 2013df had a
hydrogen envelope similar to SN 1993J ( M). The line
profiles in nebular spectra suggest that the explosion was asymmetric with the
presence of clumps in the ejecta, while the [O\,{\sc i}]
, luminosities, may indicate that the progenitor
of SN 2013df was a relatively low mass star ( M).Comment: 18 pages, 11 figures, 9 tables, accepted for publication in MNRA
Multibody simulation benchmark for dynamic vehicle-track interaction in switches and crossings: results and method statements
A Benchmark of railway multibody dynamics software application to switches and crossings (S&C) is presented, comparing all major commercially available software and a few independent codes. Two different representative S&C have been implemented, using the Manchester Benchmark passenger vehicle. The final results show that all software offer a reliable and efficient way to understand the kinematic and dynamics forces between the wheels and the track elements. The highest challenges are found when modelling a combination of multiple rails in simultaneous contact with a wheel (check-stock or switch-stock), large longitudinal variations in rail shape (crossings) and high lateral steering forces (diverging cases in tight radius). In those cases, the codes able to account for the exact relative motion of each wheels with respect to each rails independently are the most apt. The most significant variations between software are found in the contact prediction with an influence on the detailed contact tangential and normal forces. The user variability is found to be very small, with the most time-consuming and error prone being the task of handling the input data for the variable rails definition. All software could benefit from improvements to assist the user and ensure higher reliability and efficiency generally
Timely N-Acetyl-Cysteine and Environmental Enrichment Rescue Oxidative Stress-Induced Parvalbumin Interneuron Impairments via MMP9/RAGE Pathway: A Translational Approach for Early Intervention in Psychosis.
Research in schizophrenia (SZ) emphasizes the need for new therapeutic approaches based on antioxidant/anti-inflammatory compounds and psycho-social therapy. A hallmark of SZ is a dysfunction of parvalbumin-expressing fast-spiking interneurons (PVI), which are essential for neuronal synchrony during sensory/cognitive processing. Oxidative stress and inflammation during early brain development, as observed in SZ, affect PVI maturation. We compared the efficacy of N-acetyl-cysteine (NAC) and/or environmental enrichment (EE) provided during juvenile and/or adolescent periods in rescuing PVI impairments induced by an additional oxidative insult during childhood in a transgenic mouse model with gluthation deficit (Gclm KO), relevant for SZ. We tested whether this rescue was promoted by the inhibition of MMP9/RAGE mechanism, both in the mouse model and in early psychosis (EP) patients, enrolled in a double-blind, randomized, placebo-controlled clinical trial of NAC supplementation for 6 months. We show that a sequential combination of NAC+EE applied after an early-life oxidative insult recovers integrity and function of PVI network in adult Gclm KO, via the inhibition of MMP9/RAGE. Six-month NAC treatment in EP patients reduces plasma sRAGE in association with increased prefrontal GABA, improvement of cognition and clinical symptoms, suggesting similar neuroprotective mechanisms. The sequential combination of NAC+EE reverses long-lasting effects of an early oxidative insult on PVI/perineuronal net (PNN) through the inhibition of MMP9/RAGE mechanism. In analogy, patients vulnerable to early-life insults could benefit from a combined pharmacological and psycho-social therapy
Nuclear spin dynamics and Zeno effect in quantum dots and defect centers
We analyze nuclear spin dynamics in quantum dots and defect centers with a
bound electron under electron-mediated coupling between nuclear spins due to
the hyperfine interaction ("J-coupling" in NMR). Our analysis shows that the
Overhauser field generated by the nuclei at the position of the electron has
short-time dynamics quadratic in time for an initial nuclear spin state without
transverse coherence. The quadratic short-time behavior allows for an extension
of the Overhauser field lifetime through a sequence of projective measurements
(quantum Zeno effect). We analyze the requirements on the repetition rate of
measurements and the measurement accuracy to achieve such an effect. Further,
we calculate the long-time behavior of the Overhauser field for effective
electron Zeeman splittings larger than the hyperfine coupling strength and
find, both in a Dyson series expansion and a generalized master equation
approach, that for a nuclear spin system with a sufficiently smooth
polarization the electron-mediated interaction alone leads only to a partial
decay of the Overhauser field by an amount on the order of the inverse number
of nuclear spins interacting with the electron.Comment: 11 pages, 3 figure
Brain connectivity alterations in early psychosis: from clinical to neuroimaging staging.
Early in the course of psychosis, alterations in brain connectivity accompany the emergence of psychiatric symptoms and cognitive impairments, including processing speed. The clinical-staging model is a refined form of diagnosis that places the patient along a continuum of illness conditions, which allows stage-specific interventions with the potential of improving patient care and outcome. This cross-sectional study investigates brain connectivity features that characterize the clinical stages following a first psychotic episode. Structural brain networks were derived from diffusion-weighted MRI for 71 early-psychosis patients and 76 healthy controls. Patients were classified into stage II (first-episode), IIIa (incomplete remission), IIIb (one relapse), and IIIc (two or more relapses), according to the course of the illness until the time of scanning. Brain connectivity measures and diffusion parameters (fractional anisotropy, apparent diffusion coefficient) were investigated using general linear models and sparse linear discriminant analysis (sLDA), studying distinct subgroups of patients who were at specific stages of early psychosis. We found that brain connectivity impairments were more severe in clinical stages following the first-psychosis episode (stages IIIa, IIIb, IIIc) than in first-episode psychosis (stage II) patients. These alterations were spatially diffuse but converged on a set of vulnerable regions, whose inter-connectivity selectively correlated with processing speed in patients and controls. The sLDA suggested that relapsing-remitting (stages IIIb, IIIc) and non-remitting (stage IIIa) patients are characterized by distinct dysconnectivity profiles. Our results indicate that neuroimaging markers of brain dysconnectivity in early psychosis may reflect the heterogeneity of the illness and provide a connectomics signature of the clinical-staging model
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