241 research outputs found
Continued-fraction representation of the Kraus map for non-Markovian reservoir damping
Quantum dissipation is studied for a discrete system that linearly interacts
with a reservoir of harmonic oscillators at thermal equilibrium. Initial
correlations between system and reservoir are assumed to be absent. The
dissipative dynamics as determined by the unitary evolution of system and
reservoir is described by a Kraus map consisting of an infinite number of
matrices. For all Laplace-transformed Kraus matrices exact solutions are
constructed in terms of continued fractions that depend on the pair correlation
functions of the reservoir. By performing factorizations in the Kraus map a
perturbation theory is set up that conserves in arbitrary perturbative order
both positivity and probability of the density matrix. The latter is determined
by an integral equation for a bitemporal matrix and a finite hierarchy for
Kraus matrices. In lowest perturbative order this hierarchy reduces to one
equation for one Kraus matrix. Its solution is given by a continued fraction of
a much simpler structure as compared to the non-perturbative case. In lowest
perturbative order our non-Markovian evolution equations are applied to the
damped Jaynes-Cummings model. From the solution for the atomic density matrix
it is found that the atom may remain in the state of maximum entropy for a
significant time span that depends on the initial energy of the radiation
field
Modified atomic decay rate near absorptive scatterers at finite temperature
The change in the decay rate of an excited atom that is brought about by
extinction and thermal-radiation effects in a nearby dielectric medium is
determined from a quantummechanical model. The medium is a collection of
randomly distributed thermally-excited spherical scatterers with absorptive
properties. The modification of the decay rate is described by a set of
correction functions for which analytical expressions are obtained as sums over
contributions from the multipole moments of the scatterers. The results for the
modified decay rate as a function of the distance between the excited atom and
the dielectric medium show the influence of absorption, scattering and
thermal-radiation processes. Some of these processes are found to be mutually
counteractive. The changes in the decay rate are compared to those following
from an effective-medium theory in which the discrete scatterers are replaced
by a continuum
Damping and dispersion of oscillating modes of a multicomponent ionic mixture in a magnetic field
The collective-mode spectrum of a multicomponent magnetized ionic mixture for
small wave number k is studied with the use of magnetohydrodynamics and formal
kinetic theory. Apart from the usual thermal and diffusive modes, the spectrum
contains a set of four oscillating modes. By evaluating the k^2 contributions
to the eigenfrequencies, the damping and the dispersion of these oscillating
modes are determined. The long-range nature of the Coulomb interactions is
shown to imply that Burnett terms with higher-order gradients in the linear
phenomenological laws have to be taken into account in order to obtain a full
description of all damping and dispersion effects.Comment: 25 page
Predicting asthma in preschool children at high risk presenting in primary care: development of a clinical asthma prediction score
Cross-linguistic influence in simultaneous and early sequential bilingual children: A meta-analysis
Although cross-linguistic influence at the level of morphosyntax is one of the most intensively studied topics in child bilingualism, the circumstances under which it occurs remain unclear. In this meta-analysis, we measured the effect size of cross-linguistic influence and systematically assessed its predictors in 750 simultaneous and early sequential bilingual children in 17 unique language combinations across 26 experimental studies. We found a significant small to moderate average effect size of cross-linguistic influence, indicating that cross-linguistic influence is part and parcel of bilingual development. Language dominance, operationalized as societal language, was a significant predictor of cross-linguistic influence, whereas surface overlap, language domain and age were not. Perhaps an even more important finding was that definitions and operationalisations of cross-linguistic influence and its predictors varied considerably between studies. This could explain the absence of a comprehensive theory in the field. To solve this issue, we argue for a more uniform method of studying cross-linguistic influence
Atomic decay near a quantized medium of absorbing scatterers
The decay of an excited atom in the presence of a medium that both scatters
and absorbs radiation is studied with the help of a quantum-electrodynamical
model. The medium is represented by a half space filled with a randomly
distributed set of non-overlapping spheres, which consist of a linear
absorptive dielectric material. The absorption effects are described by means
of a quantized damped-polariton theory. It is found that the effective
susceptibility of the bulk does not fully account for the medium-induced change
in the atomic decay rate. In fact, surface effects contribute to the
modification of the decay properties as well. The interplay of scattering and
absorption in the total decay rate is discussed.Comment: 20 pages, 1 figur
Field quantization in inhomogeneous anisotropic dielectrics with spatio-temporal dispersion
A quantum damped-polariton model is constructed for an inhomogeneous
anisotropic linear dielectric with arbitrary dispersion in space and time. The
model Hamiltonian is completely diagonalized by determining the creation and
annihilation operators for the fundamental polariton modes as specific linear
combinations of the basic dynamical variables. Explicit expressions are derived
for the time-dependent operators describing the electromagnetic field, the
dielectric polarization and the noise term in the latter. It is shown how to
identify bath variables that generate the dissipative dynamics of the medium.Comment: 24 page
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