2,256 research outputs found
Code-division multiple-access in an optical fiber LAN with amplified bus topology: the SLIM bus
A novel optical fiber network with a bus topology and dark signaling (the SLIM bus) using optical code-division multiple-access (CDMA) is proposed. With a new design of delay line correlator the network is shown to eliminate optical beating noise and overcome the main limitations of incoherent optical CDMA in a star topology
Conditions for strictly purity-decreasing quantum Markovian dynamics
The purity, Tr(rho^2), measures how pure or mixed a quantum state rho is. It
is well known that quantum dynamical semigroups that preserve the identity
operator (which we refer to as unital) are strictly purity-decreasing
transformations. Here we provide an almost complete characterization of the
class of strictly purity-decreasing quantum dynamical semigroups. We show that
in the case of finite-dimensional Hilbert spaces a dynamical semigroup is
strictly purity-decreasing if and only if it is unital, while in the infinite
dimensional case, unitality is only sufficient.Comment: 4 pages, no figures. Contribution to the special issue "Real-time
dynamics in complex quantum systems" of Chemical Physics in honor of Phil
Pechukas. v2: Simplified proof of theorem 1 and validity conditions clarifie
Accuracy of diagnosing atrial fibrillation on electrocardiogram by primary care practitioners and interpretative diagnostic software: analysis of data from screening for atrial fibrillation in the elderly (SAFE) trial
Objective To assess the accuracy of general practitioners, practice nurses, and interpretative software in the use of different types of electrocardiogram to diagnose atrial fibrillation.
Design Prospective comparison with reference standard of assessment of electrocardiograms by two independent specialists.
Setting 49 general practices in central England.
Participants 2595 patients aged 65 or over screened for atrial fibrillation as part of the screening for atrial fibrillation in the elderly (SAFE) study; 49 general practitioners and 49 practice nurses.
Interventions All electrocardiograms were read with the Biolog interpretative software, and a random sample of 12 lead, limb lead, and single lead thoracic placement electrocardiograms were assessed by general practitioners and practice nurses independently of each other and of the Biolog assessment.
Main outcome measures Sensitivity, specificity, and positive and negative predictive values.
Results General practitioners detected 79 out of 99 cases of atrial fibrillation on a 12 lead electrocardiogram (sensitivity 80%, 95% confidence interval 71% to 87%) and misinterpreted 114 out of 1355 cases of sinus rhythm as atrial fibrillation (specificity 92%, 90% to 93%). Practice nurses detected a similar proportion of cases of atrial fibrillation (sensitivity 77%, 67% to 85%), but had a lower specificity (85%, 83% to 87%). The interpretative software was significantly more accurate, with a specificity of 99%, but missed 36 of 215 cases of atrial fibrillation (sensitivity 83%). Combining general practitioners' interpretation with the interpretative software led to a sensitivity of 92% and a specificity of 91%. Use of limb lead or single lead thoracic placement electrocardiograms resulted in some loss of specificity.
Conclusions Many primary care professionals cannot accurately detect atrial fibrillation on an electrocardiogram, and interpretative software is not sufficiently accurate to circumvent this problem, even when combined with interpretation by a general practitioner. Diagnosis of atrial fibrillation in the community needs to factor in the reading of electrocardiograms by appropriately trained peopl
Screening versus routine practice in detection of atrial fibrillation in patients aged 65 or over: Screening versus routine practice in detection cluster randomised controlled trial
Objectives : To assess whether screening improves the detection of atrial fibrillation (cluster randomisation) and to compare systematic and opportunistic screening.
Design : Multicentred cluster randomised controlled trial, with subsidiary trial embedded within the intervention arm.
Setting : 50 primary care centres in England, with further individual randomisation of patients in the intervention practices.
Participants : 14,802 patients aged 65 or over in 25 intervention and 25 control practices.
Interventions : Patients in intervention practices were randomly allocated to systematic screening (invitation for electrocardiography) or opportunistic screening (pulse taking and invitation for electrocardiography if the pulse was irregular). Screening took place over 12 months in each practice from October 2001 to February 2003. No active screening took place in control practices.
Main outcome measure : Newly identified atrial fibrillation.
Results : The detection rate of new cases of atrial fibrillation was 1.63% a year in the intervention practices and 1.04% in control practices (difference 0.59%, 95% confidence interval 0.20% to 0.98%). Systematic and opportunistic screening detected similar numbers of new cases (1.62% v 1.64%, difference 0.02%, −0.5% to 0.5%).
Conclusion : Active screening for atrial fibrillation detects additional cases over current practice. The preferred method of screening in patients aged 65 or over in primary care is opportunistic pulse taking with follow-up
electrocardiography.
Trial registration Current Controlled Trials
ISRCTN19633732
Entanglement Dynamics in 1D Quantum Cellular Automata
Several proposed schemes for the physical realization of a quantum computer
consist of qubits arranged in a cellular array. In the quantum circuit model of
quantum computation, an often complex series of two-qubit gate operations is
required between arbitrarily distant pairs of lattice qubits. An alternative
model of quantum computation based on quantum cellular automata (QCA) requires
only homogeneous local interactions that can be implemented in parallel. This
would be a huge simplification in an actual experiment. We find some minimal
physical requirements for the construction of unitary QCA in a 1 dimensional
Ising spin chain and demonstrate optimal pulse sequences for information
transport and entanglement distribution. We also introduce the theory of
non-unitary QCA and show by example that non-unitary rules can generate
environment assisted entanglement.Comment: 12 pages, 8 figures, submitted to Physical Review
Development of selective, ultra-fast multiple co-sensitization to control dye loading in dye-sensitized solar cells
Enhancing the spectral response of dye-sensitized solar cells (DSC) is essential to increasing device efficiency and a key approach to achieve this is co-sensitization (i.e. the use of multiple dyes to absorb light from different parts of the solar spectrum). However, precise control of dye loading within DSC mesoporous metal oxide photo-anodes is non-trivial especially for very rapid processing (minutes). This is further complicated by dyes having very different partition (Kd) and molar extinction (ε) coefficients which strongly influence dye uptake and spectral response, respectively. Here, we present a highly versatile, ultra-fast (ca. 5 min) desorption and re-dyeing method for dye-sensitized solar cells which can be used to precisely control dye loading in photo-electrode films. This method has been successfully applied to re-dye, partially desorb and re-dye and selectively desorb and re-dye photo-electrodes using examples of a Ru-bipy dye (N719) and also organic dyes (SQ1 and D149) giving η up to 8.1% for a device containing the organic dye D149 and re-dyed with the Ru dye N719. The paper also illustrates how this method can be used to rapidly screen large numbers of dyes (and/or dye combinations) and also illustrates how it can also be used to selectively study dye loading
Geometric Suppression of Single-Particle Energy Spacings in Quantum Antidots
Quantum Antidot (AD) structures have remarkable properties in the integer
quantum Hall regime, exhibiting Coulomb-blockade charging and the Kondo effect
despite their open geometry. In some regimes a simple single-particle (SP)
model suffices to describe experimental observations while in others
interaction effects are clearly important, although exactly how and why
interactions emerge is unclear. We present a combination of experimental data
and the results of new calculations concerning SP orbital states which show how
the observed suppression of the energy spacing between states can be explained
through a full consideration of the AD potential, without requiring any effects
due to electron interactions such as the formation of compressible regions
composed of multiple states, which may occur at higher magnetic fields. A full
understanding of the regimes in which these effects occur is important for the
design of devices to coherently manipulate electrons in edge states using AD
resonances.Comment: 4 pages, 2 figure
Geometrothermodynamics of black holes
The thermodynamics of black holes is reformulated within the context of the
recently developed formalism of geometrothermodynamics. This reformulation is
shown to be invariant with respect to Legendre transformations, and to allow
several equivalent representations. Legendre invariance allows us to explain a
series of contradictory results known in the literature from the use of
Weinhold's and Ruppeiner's thermodynamic metrics for black holes. For the
Reissner-Nordstr\"om black hole the geometry of the space of equilibrium states
is curved, showing a non trivial thermodynamic interaction, and the curvature
contains information about critical points and phase transitions. On the
contrary, for the Kerr black hole the geometry is flat and does not explain its
phase transition structure.Comment: Revised version, to be published in Gen.Rel.Grav.(Mashhoon's
Festschrift
Formation and Evaporation of Charged Black Holes
We investigate the dynamical formation and evaporation of a spherically
symmetric charged black hole. We study the self-consistent one loop order
semiclassical back-reaction problem. To this end the mass-evaporation is
modeled by an expectation value of the stress-energy tensor of a neutral
massless scalar field, while the charge is not radiated away. We observe the
formation of an initially non extremal black hole which tends toward the
extremal black hole , emitting Hawking radiation. If also the discharge
due to the instability of vacuum to pair creation in strong electric fields
occurs, then the black hole discharges and evaporates simultaneously and decays
regularly until the scale where the semiclassical approximation breaks down. We
calculate the rates of the mass and the charge loss and estimate the life-time
of the decaying black holes.Comment: 23 pages, 7 eps figures, RevTex, accepted for publication in Phys.
Rev.
The Future Evolution of White Dwarf Stars Through Baryon Decay and Time Varying Gravitational Constant
Motivated by the possibility that the fundamental ``constants'' of nature
could vary with time, this paper considers the long term evolution of white
dwarf stars under the combined action of proton decay and variations in the
gravitational constant. White dwarfs are thus used as a theoretical laboratory
to study the effects of possible time variations, especially their implications
for the future history of the universe. More specifically, we consider the
gravitational constant to vary according to the parametric relation , where the time scale is the same order as
the proton lifetime. We then study the long term fate and evolution of white
dwarf stars. This treatment begins when proton decay dominates the stellar
luminosity, and ends when the star becomes optically thin to its internal
radiation.Comment: 12 pages, 10 figures, accepted to Astrophysics and Space Scienc
- …