1,484 research outputs found
The Born and Markov approximations for atom lasers
We discuss the use of the Born and Markov approximations in describing the
dynamics of an atom laser. In particular, we investigate the applicability of
the quantum optical Born-Markov master equation for describing output coupling.
We derive conditions based on the atomic reservoir, and atom dispersion
relations for when the Born-Markov approximations are valid and discuss
parameter regimes where these approximations fail in our atom laser model.
Differences between the standard optical laser model and the atom laser are due
to a combination of factors, including the parameter regimes in which a typical
atom laser would operate, the different reservoir state which is appropriate
for atoms, and the different dispersion relations between atoms and photons. We
present results based on an exact method in the regimes in which the
Born-Markov approximation fails. The exact solutions in some experimentally
relavent parameter regimes give non-exponential loss of atoms from a cavity.Comment: 10 pages, 3 figures. (2 new figues). Exact solutions have been
included in section II. Sections IV and V have been expanded. A new section
discussing the effects of gravity has been include
The sexual health needs of looked after young people; findings from qualitative interviews led through public health and health psychology.
Background - Looked after young people (LAYP) have poorer sexual health outcomes than their peers. However, research seldom examines the health needs of, or intervenes with, this group. The aim of the current study was to identify LAYP's perceived sexual health needs and explore sources of sexual health information, knowledge levels, concerns and service preferences.
Methods - Looked after young people within Fife, Scotland, were recruited through their workers and carers to participate in qualitative semi-structured interviews. Those aged 12–19 years were targeted for recruitment. National Health Services ethics approval was granted. Thematic analysis was conducted using NVivo.
Results - Interviews were conducted with 10 LAYP. Key themes included gaps in knowledge, and gaps between knowledge and behaviour. Being ‘looked after’ may have negative consequences on knowledge, sources of support and safer sex. A flexible, one-to-one service, aimed at several lifestyle issues, not solely sexual health, was preferred by respondents.
Conclusion - Looked after young people require support to bridge the gap between knowledge and behaviour and several theories relevant to the findings have been identified. Participants desired more support around a range of health issues, which may come from school, workers and carers, as well as health professionals. Further research examining effective interventions with this group is crucial to improve outcomes
The steady state quantum statistics of a non-Markovian atom laser
We present a fully quantum mechanical treatment of a single-mode atomic
cavity with a pumping mechanism and an output coupling to a continuum of
external modes. This system is a schematic description of an atom laser. In the
dilute limit where atom-atom interactions are negligible, we have been able to
solve this model without making the Born and Markov approximations. When
coupling into free space, it is shown that for reasonable parameters there is a
bound state which does not disperse, which means that there is no steady state.
This bound state does not exist when gravity is included, and in that case the
system reaches a steady state. We develop equations of motion for the two-time
correlation in the presence of pumping and gravity in the output modes. We then
calculate the steady-state output energy flux from the laser.Comment: 14 pages (twocloumn), 6 figure
Practice pointer: Using the new UK-WHO growth charts
The new UK growth charts for children aged 0-4 years (designed using data from the new WHO standards) describe the optimal pattern of growth for all children, rather than the prevailing pattern in the UK (as with previous charts).
The new charts are suitable for all ethnic groups and set breast feeding as the norm.
UK children match the new charts well for length and height, but after age 6 months fewer children will be below the 2nd centile for weight or show weight faltering, and more will be above the 98th centile.
The new charts look different: they have a separate preterm section, no lines between 0 and 2 weeks, and the 50th percentile is no longer emphasised.
The charts give clear instructions on gestational correction, and there is a new chart for infants born before 32 weeks’ gestation.
The instructions advise on when and how to measure and when a measurement or growth pattern is outside the normal range.
The charts include a “look-up” tool for determining the body mass index centile from height and weight centiles without calculation and aid for predicting adult height.
The charts and supporting educational materials can be downloaded from www.growthcharts.rcpch.ac.u
Alimentos funcionais: a saúde que vem do 'prato'.
bitstream/item/58951/1/alimentos-funcionais-ReciclandoIdeias10.pd
Effects of interatomic collisions on atom laser outcoupling
We present a computational approach to the outcoupling in a simple
one-dimensional atom laser model, the objective being to circumvent
mathematical difficulties arising from the breakdown of the Born and Markov
approximations. The approach relies on the discretization of the continuum
representing the reservoir of output modes, which allows the treatment of
arbitrary forms of outcoupling as well as the incorporation of non-linear terms
in the Hamiltonian, associated with interatomic collisions. By considering a
single-mode trapped condensate, we study the influence of elastic collisions
between trapped and free atoms on the quasi steady-state population of the
trap, as well as the energy distribution and the coherence of the outcoupled
atoms.Comment: 25 pages, 11 figures, to appear in J. Phys.
Coherent Tunneling of Atoms from Bose-condensed Gases at Finite Temperatures
Tunneling of atoms between two trapped Bose-condensed gases at finite
temperatures is explored using a many-body linear response tunneling formalism
similar to that used in superconductors. To lowest order, the tunneling
currents can be expressed quite generally in terms of the single-particle
Green's functions of the isolated Bose gases. A coherent first-order tunneling
Josephson current between two atomic Bose-condensates is found, in addition to
coherent and dissipative contributions from second-order
condensate-noncondensate and noncondensate-noncondensate tunneling. Our work is
a generalization of Meier and Zwerger, who recently treated tunneling between
uniform atomic Bose gases. We apply our formalism to the analysis of an
out-coupling experiment induced by light wave fields, using a simple
Bogoliubov-Popov quasiparticle approximation for the trapped Bose gas. For
tunneling into the vacuum, we recover the results of Japha, Choi, Burnett and
Band, who recently pointed out the usefulness of studying the spectrum of
out-coupled atoms. In particular, we show that the small tunneling current of
noncondensate atoms from a trapped Bose gas has a broad spectrum of energies,
with a characteristic structure associated with the Bogoliubov quasiparticle
u^2 and v^2 amplitudes.Comment: 26 pages, 5 figures, minor changes, to appear in PR
Quantum dynamical theory for squeezing the output of a Bose-Einstein condensate
A linear quantum dynamical theory for squeezing the output of the trapped
Bose-Einstein condensate is presented with the Bogoliubov approximation. We
observe that the non-classical properties, such as sub-Poisson distribution and
quadrature squeezing effect, mutually oscillate between the quantum states of
the applied optical field and the resulting atom laser beam with time. In
particular, it is shown that an initially squeezed optical field will lead to
squeezing in the outcoupled atomic beam at later times.Comment: 6 pages, Latex file, Phys.Rev.A 63(2001)1560
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