522 research outputs found
Two-step Doppler cooling of a three-level ladder system with an intermediate metastable level
Doppler laser cooling of a three-level ladder system using two near-resonant
laser fields is analyzed in the case of the intermediate level being metastable
while the upper level is short-lived. Analytical as well as numerical results
for e.g. obtainable scattering rates and achievable temperatures are presented.
When appropriate, comparisons with two-level single photon Doppler laser
cooling is made. These results are relevant to recent experimental Doppler
laser cooling investigations addressing intercombination lines in alkali-earth
metal atoms and quadrupole transitions in alkali-earth metal ions.Comment: accepted by Phys Rev
Evaluation of the ultimate performances of a Ca+ single-ion frequency standard
We numerically evaluate the expected performances of an optical frequency
standard at 729 nm based on a single calcium ion. The frequency stability is
studied through the Allan deviation and its dependence on the excitation method
(single Rabi pulse or two Ramsey pulses schemes) and the laser linewidth are
discussed. The minimum Allan deviation that can be expected is estimated to
with the
integration time. The frequency shifts induced by the environmental conditions
are evaluated to minimize the uncertainty of the proposed standard by chosing
the most suited environment for the ion. If using the odd isotope
Ca and a vessel cooled to 77 K, the expected relative shift is with an uncertainty of , mainly due to
the quadrupole shift induced by the unknown static electric field gradient .Comment: soumis le 27/07/04 a Physics Letters
Dark resonances as a probe for the motional state of a single ion
Single, rf-trapped ions find various applications ranging from metrology to
quantum computation. High-resolution interrogation of an extremely weak
transition under best observation conditions requires an ion almost at rest. To
avoid line-broadening effects such as the second order Doppler effect or rf
heating in the absence of laser cooling, excess micromotion has to be
eliminated as far as possible. In this work the motional state of a confined
three-level ion is probed, taking advantage of the high sensitivity of observed
dark resonances to the trapped ion's velocity. Excess micromotion is controlled
by monitoring the dark resonance contrast with varying laser beam geometry. The
influence of different parameters such as the cooling laser intensity has been
investigated experimentally and numerically
Terahertz frequency standard based on three-photon coherent population trapping
A scheme for a THz frequency standard based on three-photon coherent
population trapping in stored ions is proposed. Assuming the propagation
directions of the three lasers obey the phase matching condition, we show that
stability of few 10 at one second can be reached with a precision
limited by power broadening to in the less favorable case. The
referenced THz signal can be propagated over long distances, the useful
information being carried by the relative frequency of the three optical
photons.Comment: article soumis a PRL le 21 mars 2007, accepte le 10 mai, version 2
(24/05/2007
Cosmological SPH simulations with four million particles: statistical properties of X-ray clusters in a low-density universe
We present results from a series of cosmological SPH (smoothed particle
hydrodynamics) simulations coupled with the P3M
(Particle-Particle-Particle-Mesh) solver for the gravitational force. The
simulations are designed to predict the statistical properties of X-ray
clusters of galaxies as well as to study the formation of galaxies. We have
seven simulation runs with different assumptions on the thermal state of the
intracluster gas. Following the recent work by Pearce et al., we modify our SPH
algorithm so as to phenomenologically incorporate the galaxy formation by
decoupling the cooled gas particles from the hot gas particles. All the
simulations employ 128^3 particles both for dark matter and for gas components,
and thus constitute the largest systematic catalogues of simulated clusters in
the SPH method performed so far. These enable us to compare the analytical
predictions on statistical properties of X-ray clusters against our direct
simulation results in an unbiased manner. We find that the luminosities of the
simulated clusters are quite sensitive to the thermal history and also to the
numerical resolution of the simulations, and thus are not reliable. On the
other hand, the mass-temperature relation for the simulated clusters is fairly
insensitive to the assumptions of the thermal state of the intracluster gas,
robust against the numerical resolution, and in fact agrees well with the
analytic prediction. Therefore the prediction for the X-ray temperature
function of clusters on the basis of the Press-Schechter mass function and the
virial equilibrium is fairly reliable.Comment: Accepted for publication in The Astrophysical Journal. 18 pages with
7 embedded figure
Is aberrant affective cognition an endophenotype for affective disorders? - A monozygotic twin study
Evaluating case studies of community-oriented integrated care.
This paper summarises a ten-year conversation within London Journal of Primary Care about the nature of community-oriented integrated care (COIC) and how to develop and evaluate it. COIC means integration of efforts for combined disease-treatment and health-enhancement at local, community level. COIC is similar to the World Health Organisation concept of a Community-Based Coordinating Hub - both require a local geographic area where different organisations align their activities for whole system integration and develop local communities for health. COIC is a necessary part of an integrated system for health and care because it enables multiple insights into 'wicked problems', and multiple services to integrate their activities for people with complex conditions, at the same time helping everyone to collaborate for the health of the local population. The conversation concludes seven aspects of COIC that warrant further attention
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