1,639 research outputs found
Indistinguishability and Interference in the Coherent Control of Atomic and Molecular Processes
The subtle and fundamental issue of indistinguishability and interference
between independent pathways to the same target state is examined in the
context of coherent control of atomic and molecular processes, with emphasis
placed on possible "which-way" information due to quantum entanglement
established in the quantum dynamics. Because quantum interference between
independent pathways to the same target state occurs only when the independent
pathways are indistinguishable, it is first shown that creating useful
coherence (as defined in the paper) between nondegenerate states of a molecule
for subsequent quantum interference manipulation cannot be achieved by
collisions between atoms or molecules that are prepared in momentum and energy
eigenstates. Coherence can, however, be transferred from light fields to atoms
or molecules. Using a particular coherent control scenario, it is shown that
this coherence transfer and the subsequent coherent phase control can be
readily realized by the most classical states of light, i.e., coherent states
of light. It is further demonstrated that quantum states of light may suppress
the extent of phase-sensitive coherent control by leaking out some which-way
information while "incoherent interference control" scenarios proposed in the
literature have automatically ensured the indistinguishability of multiple
excitation pathways. The possibility of quantum coherence in photodissociation
product states is also understood in terms of the disentanglement between
photodissociation fragments. Results offer deeper insights into quantum
coherence generation in atomic and molecular processes.Comment: 26 pages, based on one Chapter from first author's Ph.D thesis in
200
Teaching and understanding of quantum interpretations in modern physics courses
Just as expert physicists vary in their personal stances on interpretation in
quantum mechanics, instructors vary on whether and how to teach interpretations
of quantum phenomena in introductory modern physics courses. In this paper, we
document variations in instructional approaches with respect to interpretation
in two similar modern physics courses recently taught at the University of
Colorado, and examine associated impacts on student perspectives regarding
quantum physics. We find students are more likely to prefer realist
interpretations of quantum-mechanical systems when instructors are less
explicit in addressing student ontologies. We also observe contextual
variations in student beliefs about quantum systems, indicating that
instructors who choose to address questions of ontology in quantum mechanics
should do so explicitly across a range of topics.Comment: 18 pages, references, plus 2 pages supplemental materials. 8 figures.
PACS: 01.40.Fk, 03.65.-
Infrared photometry and spectroscopy of the brown dwarf candidate PC0025+0447
We have obtained I, J, H, Kf and L' photometry and infrared spectroscopy (λ/Δλ ≃ 100) from 1.2-2.4 µm of the unusual emission line M star PC0025+0447. The near infrared photometry shows that this star has infrared colors which are comparable to the latest M dwarf stars such as LHS 2924 and LHS 2065. The infrared colors of PC0025+0447 establish that this star is a dwarf and not a distant giant. The infrared spectrum is dominated by strong H_2O absorption bands, and these bands are the
strongest measured in any dwarf. Since H_2O band strength increases with decreasing temperature it is very likely that PC0025-f 0447 is substantially cooler than any of the stars calibrated by Berriman & Reid [MNRAS, 227, 315 (1987)], i.e. T_(eff) 10^9 yr) H burning star, with M ≃ 0.08 M_☉, or a younger brown dwarf
EPR before EPR: a 1930 Einstein-Bohr thought experiment revisited
In 1930 Einstein argued against consistency of the time-energy uncertainty
relation by discussing a thought experiment involving a measurement of mass of
the box which emitted a photon. Bohr seemingly triumphed over Einstein by
arguing that the Einstein's own general theory of relativity saves the
consistency of quantum mechanics. We revisit this thought experiment from a
modern point of view at a level suitable for undergraduate readership and find
that neither Einstein nor Bohr was right. Instead, this thought experiment
should be thought of as an early example of a system demonstrating nonlocal
"EPR" quantum correlations, five years before the famous
Einstein-Podolsky-Rosen paper.Comment: 11 pages, revised, accepted for publication in Eur. J. Phy
Towards More Accurate Molecular Dynamics Calculation of Thermal Conductivity. Case Study: GaN Bulk Crystals
Significant differences exist among literature for thermal conductivity of
various systems computed using molecular dynamics simulation. In some cases,
unphysical results, for example, negative thermal conductivity, have been
found. Using GaN as an example case and the direct non-equilibrium method,
extensive molecular dynamics simulations and Monte Carlo analysis of the
results have been carried out to quantify the uncertainty level of the
molecular dynamics methods and to identify the conditions that can yield
sufficiently accurate calculations of thermal conductivity. We found that the
errors of the calculations are mainly due to the statistical thermal
fluctuations. Extrapolating results to the limit of an infinite-size system
tend to magnify the errors and occasionally lead to unphysical results. The
error in bulk estimates can be reduced by performing longer time averages using
properly selected systems over a range of sample lengths. If the errors in the
conductivity estimates associated with each of the sample lengths are kept
below a certain threshold, the likelihood of obtaining unphysical bulk values
becomes insignificant. Using a Monte-Carlo approach developed here, we have
determined the probability distributions for the bulk thermal conductivities
obtained using the direct method. We also have observed a nonlinear effect that
can become a source of significant errors. For the extremely accurate results
presented here, we predict a [0001] GaN thermal conductivity of 185 at 300 K, 102 at 500 K, and 74
at 800 K. Using the insights obtained in the work, we have achieved a
corresponding error level (standard deviation) for the bulk (infinite sample
length) GaN thermal conductivity of less than 10 , 5 , and 15 at 300 K, 500 K, and 800 K respectively
Risk Factors for the Development of Cataract in Children with Uveitis
PURPOSE:
To determine the risk factors for the development of cataract in children with uveitis of any etiology.
DESIGN:
Cohort study.
METHODS:
Two hundred forty-seven eyes of 140 children with uveitis were evaluated for the development of vision-affecting cataract. Demographic, clinical, and treatment data were collected between the time of presentation and the first instance cataract was recorded or findings at final follow-up. Main outcome measures included the prevalence of cataract and distribution by type of uveitis, incidence of new onset cataract time to cataract development, and risk factors for the development of cataract.
RESULTS: The prevalence of cataract in our cohort was 44.2% and was highest among eyes with panuveitis (77.1%), chronic anterior uveitis (48.3%), and intermediate uveitis (48.0%). The overall incidence of newly diagnosed cataract was 0.09 per eye-year, with an estimated 69% to develop uveitis-related cataract with time. The main factors related with cataract development were the number of uveitis flares per year (hazard ratio [HR] = 3.06 [95% confidence interval {CI}, 2.15–4.35], P < .001), cystoid macular edema (HR = 2.87 [95% CI, 1.41–5.82], P = .004), posterior synechia at presentation (HR = 2.85 [95% CI, 1.53–5.30], P = .001), and use of local injections of corticosteroids (HR = 2.37 [95% CI, 1.18–4.75], P = .02). Treatments with systemic and topical corticosteroids were not significant risk factors.
CONCLUSIONS:
In this study, we found that development of cataract is common among pediatric eyes with uveitis and is most strongly related to the extent of inflammation recurrences and ocular complications. We suggest that controlling the inflammation, even using higher doses of systemic and topical corticosteroids, is of importance in preventing ocular complications, such as cataract.
Uveitis accounts for 10–15% of blindness in the developed world.1 Although pediatric uveitis is relatively uncommon, accounting for only 5–10% of all uveitis cases,2 it affects young patients, who in most cases are otherwise healthy. Vision loss results from ongoing inflammation that leads to ocular structural changes, such as cataract, corneal opacities, optic neuropathy, and retinal lesions. The most common causes of vision loss in children with uveitis are cataract, glaucoma, and chronic cystoid macular edema (CME).2, 3 In addition, any chronic visual obstruction can result in the development of amblyopia in younger children, with vision loss persisting after the inciting cause is treated.4 Such changes, together with the need for long-term treatment and continuous monitoring, can have a profound impact on their development, independence, and education.
The prevalence of cataract in eyes with uveitis ranges from 20–64%,4, 5, 6, 7 and it is the most common complication of uveitis in children,8 occurring in approximately 35% of children with juvenile idiopathic arthritis (JIA)-associated uveitis9 and increasing ≤80% in adults.10, 11 Cataract progression can be the result of persistent intraocular inflammation,12, 13 can be caused by surgery for uveitis complications (eg, trabeculectomies and repair of retinal detachments), or can be a consequence of uveitis treatment, particularly the use of local or systemic corticosteroids.14, 15, 16, 17 It results in reduced visual acuity and can have a detrimental effect on the development and academic achievements of these children.18
Studies have examined risk factors for the development of cataract among children with JIA-associated uveitis, identifying risk factors such as the presence of posterior synechiae (PS) at presentation,12, 19 the use of systemic corticosteroids,13 topical corticosteroid therapy exceeding 3 drops a day,12 or persistent, uncontrolled active inflammation,3 while early treatment with methotrexate delayed cataract progression.19 However, JIA is a unique cause of uveitis, often localized to the anterior chamber, with frequent intraocular structural changes and the early use of systemic immunosuppressive agents. It may not represent the same risks as other causes of pediatric uveitis.
We examined disease- and treatment-related risk factors for cataract development in children with uveitis of any etiology. We investigated clinical and ophthalmologic characteristics, as well as treatment strategies in relation to the time interval between the first presentation with uveitis and cataract development
On the incidence of weak magnetic fields in DA white dwarfs
Context: About 10% of white dwarfs have magnetic fields with strength in the
range between about 10^5 and 3x10^8 G. It is not known whether the remaining
white dwarfs are not magnetic, or if they have a magnetic field too weak to be
detected with the techniques adopted in the large surveys. Aims. We describe
the results of the first survey specifically devised to clarify the detection
frequency of kG-level magnetic fields in cool DA white dwarfs. Methods: Using
the FORS1 instrument of the ESO VLT, we have obtained Balmer line circular
spectropolarimetric measurements of a small sample of cool (DA6 - DA8) white
dwarfs. Using FORS and UVES archive data, we have also revised numerous white
dwarf field measurements previously published in the literature. Results: We
have discovered an apparently constant longitudinal magnetic field of \sim9.5
kG in the DA6 white dwarf WD2105-820. This star is the first weak-field white
dwarf that has been observed sufficiently to roughly determine the
characteristics of its field. The available data are consistent with a simple
dipolar morphology with magnetic axis nearly parallel to the rotation axis, and
a polar strength of \simeq 56 kG. Our re-evaluation of the FORS archive data
for white dwarfs indicates that longitudinal magnetic fields weaker than 10 kG
had previously been correctly identified in at least three white dwarfs.
Conclusions: We find that the probability of detecting a field of kG strength
in a DA white dwarf is of the order of 10% for each of the cool and hot DA
stars. If there is a lower cutoff to field strength in white dwarfs, or a field
below which all white dwarfs are magnetic, the current precision of
measurements is not yet sufficient to reveal it.Comment: Accepted for publication in Astronomy & Astrophysic
An analysis of spectra in the Red Rectangle nebula
This paper presents an analysis of a series of spectra in the Red Rectangle
nebula. Only the reddest part of the spectra can safely be attributed to light
from the nebula, and indicates Rayleigh scattering by the gas, in conformity
with the large angles of scattering involved and the proximity of the star. In
the blue, light from HD44179, refracted or scattered in the atmosphere,
dominates the spectra. This paper questions the reliability of ground-based
observations of extended objects in the blue.Comment: 25 figure
Big Bang Nucleosynthesis Constraints on Primordial Magnetic Fields
We reanalyze the effect of magnetic fields in BBN, incorporating several
features which were omitted in previous analyses. We find that the effects of
coherent magnetic fields on the weak interaction rates and the electron
thermodynamic functions (\rhoe, \Pe, and \drhoedt ) are unimportant in
comparison to the contribution of the magnetic field energy density in BBN. In
consequence the effect of including magnetic fields in BBN is well approximated
numerically by treating the additional energy density as effective neutrino
number. A conservative upper bound on the primordial magnetic field,
parameterized as , is (). This bound can be stronger than the conventional bound coming from
the Faraday rotation measures of distant quasars if the cosmological magnetic
field is generated by a causal mechanism.Comment: Latex, 20 pages, 3 uuencoded figures appende
Neutrino propagation in a random magnetic field
The active-sterile neutrino conversion probability is calculated for neutrino
propagating in a medium in the presence of random magnetic field fluctuations.
Necessary condition for the probability to be positive definite is obtained.
Using this necessary condition we put constraint on the neutrino magnetic
moment from active-sterile electron neutrino conversion in the early universe
hot plasma and in supernova.Comment: 11 page
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