386 research outputs found
Global agenda for livestock research: Proceedings of a consultation
The objectives of this consultation are to identify priority requirements for improved livestock productivity and development of the sustainable production systems in the developing regions; to identify cross-regional priorities, major constraints, the researchable areas and issues in livestock production most likely to bring distinct benefits and impacts; and to assist identification of participants and planning for future regional research workshops. The report summarises the priority researchable issues suggested during the consultation to be considered for ILRI's broader agenda for livestock and related research. These issues include livestock production systems and feed development and improvement
Semiclassical theory of cavity-assisted atom cooling
We present a systematic semiclassical model for the simulation of the
dynamics of a single two-level atom strongly coupled to a driven high-finesse
optical cavity. From the Fokker-Planck equation of the combined atom-field
Wigner function we derive stochastic differential equations for the atomic
motion and the cavity field. The corresponding noise sources exhibit strong
correlations between the atomic momentum fluctuations and the noise in the
phase quadrature of the cavity field. The model provides an effective tool to
investigate localisation effects as well as cooling and trapping times. In
addition, we can continuously study the transition from a few photon quantum
field to the classical limit of a large coherent field amplitude.Comment: 10 pages, 8 figure
Hybrid phase-space simulation method for interacting Bose fields
We introduce an approximate phase-space technique to simulate the quantum
dynamics of interacting bosons. With the future goal of treating Bose-Einstein
condensate systems, the method is designed for systems with a natural
separation into highly occupied (condensed) modes and lightly occupied modes.
The method self-consistently uses the Wigner representation to treat highly
occupied modes and the positive-P representation for lightly occupied modes. In
this method, truncation of higher-derivative terms from the Fokker-Planck
equation is usually necessary. However, at least in the cases investigated
here, the resulting systematic error, over a finite time, vanishes in the limit
of large Wigner occupation numbers. We tested the method on a system of two
interacting anharmonic oscillators, with high and low occupations,
respectively. The Hybrid method successfully predicted atomic quadratures to a
useful simulation time 60 times longer than that of the positive-P method. The
truncated Wigner method also performed well in this test. For the prediction of
the correlation in a quantum nondemolition measurement scheme, for this same
system, the Hybrid method gave excellent agreement with the exact result, while
the truncated Wigner method showed a large systematic error.Comment: 13 pages; 6 figures; references added; figures correcte
The time-reversal test for stochastic quantum dynamics
The calculation of quantum dynamics is currently a central issue in
theoretical physics, with diverse applications ranging from ultra-cold atomic
Bose-Einstein condensates (BEC) to condensed matter, biology, and even
astrophysics. Here we demonstrate a conceptually simple method of determining
the regime of validity of stochastic simulations of unitary quantum dynamics by
employing a time-reversal test. We apply this test to a simulation of the
evolution of a quantum anharmonic oscillator with up to
(Avogadro's number) of particles. This system is realisable as a Bose-Einstein
condensate in an optical lattice, for which the time-reversal procedure could
be implemented experimentally.Comment: revtex4, two figures, four page
Global agenda for livestock research. Proceedings of the consultation for the South-East Asia region
The papers presented here by the group covering the states of the Association of South East Asian Nations (ASEAN) had detailed discussions on defining the livestock priorities, research requirements, and ways to achieve these with a focus on Indonesia, Malaysia, the Philippines and Thailand. Discussions were conducted in three sequential phases. In the first phase, each country identified the priority agro-ecological zones (AEZs) and, within each, priority production systems in which livestock research was required. The results are presented in a tabular form. It was interesting that there was a consensual focus on the upland and lowland rainfed AEZs. Among the production systems, rice/buffalo and rice/cattle (beef and dairy), maize/buffalo and cattle systems, and the integration of ruminants with plantation crops (coconut, oil palm and rubber) were prominent. Phase two focused on specific researchable areas within production systems and within each country. They are summarised in detail in table 1. The most prominent priority research areas were feeding and nutrition, genetic improvement, methodologies for crop-animal interactions, and the promotion of the adoption of appropriate technologies. Phase three discussed the research requirements within individual countries. Notwithstanding the variable research capacity in NARS and the availability of funds, the general consensus among the countries was a requirement for the following: adequate funding for livestock research, training institutional strengthening, and germplasm conservation and use
Relativistic diffusion processes and random walk models
The nonrelativistic standard model for a continuous, one-parameter diffusion
process in position space is the Wiener process. As well-known, the Gaussian
transition probability density function (PDF) of this process is in conflict
with special relativity, as it permits particles to propagate faster than the
speed of light. A frequently considered alternative is provided by the
telegraph equation, whose solutions avoid superluminal propagation speeds but
suffer from singular (non-continuous) diffusion fronts on the light cone, which
are unlikely to exist for massive particles. It is therefore advisable to
explore other alternatives as well. In this paper, a generalized Wiener process
is proposed that is continuous, avoids superluminal propagation, and reduces to
the standard Wiener process in the non-relativistic limit. The corresponding
relativistic diffusion propagator is obtained directly from the nonrelativistic
Wiener propagator, by rewriting the latter in terms of an integral over
actions. The resulting relativistic process is non-Markovian, in accordance
with the known fact that nontrivial continuous, relativistic Markov processes
in position space cannot exist. Hence, the proposed process defines a
consistent relativistic diffusion model for massive particles and provides a
viable alternative to the solutions of the telegraph equation.Comment: v3: final, shortened version to appear in Phys. Rev.
The Star Formation History of the Large Magellanic Cloud
Using WFPC2 aboard the Hubble Space Telescope, we have created deep
color-magnitude diagrams in the V and I passbands for approximately 100,000
stars in a field at the center of the LMC bar and another in the disk. The
main--sequence luminosity functions (LFs) from 19 mag < V < 23.5 mag, the red
clump and horizontal branch morphologies, and the differential Hess diagram of
the two fields all strongly imply that the disk and bar have significantly
different star-formation histories (SFHs). The disk's SFH has been relatively
smooth and continuous over the last 15 Gyr while the bar's SFH was dominated by
star formation episodes at intermediate ages. Comparison of the LF against
predictions based on Padova theoretical stellar evolution models and an assumed
age-metallicity relationship allows us to identify the dominant stellar
populations in the bar with episodes of star formation that occurred from 4 to
6 and 1 to 2 Gyr ago. These events accounted, respectively, for approximately
25% and 15% of its stellar mass. The disk field may share a mild enhancement in
SF for the younger episode, and thus we identify the 4 to 6 Gyr episode with
the formation of the LMC bar.Comment: 14 pages, 5 figures, Latex, also available at
http://www.ps.uci.edu/physics/smeckerhane.html. Accepted for publication in
Ap
Monte Carlo techniques for real-time quantum dynamics
The stochastic-gauge representation is a method of mapping the equation of
motion for the quantum mechanical density operator onto a set of equivalent
stochastic differential equations. One of the stochastic variables is termed
the "weight", and its magnitude is related to the importance of the stochastic
trajectory. We investigate the use of Monte Carlo algorithms to improve the
sampling of the weighted trajectories and thus reduce sampling error in a
simulation of quantum dynamics. The method can be applied to calculations in
real time, as well as imaginary time for which Monte Carlo algorithms are
more-commonly used. The method is applicable when the weight is guaranteed to
be real, and we demonstrate how to ensure this is the case. Examples are given
for the anharmonic oscillator, where large improvements over stochastic
sampling are observed.Comment: 28 pages, submitted to J. Comp. Phy
Cat States and Single Runs for the Damped Harmonic Oscillator
We discuss the fate of initial states of the cat type for the damped harmonic
oscillator, mostly employing a linear version of the stochastic Schr\"odinger
equation. We also comment on how such cat states might be prepared and on the
relation of single realizations of the noise to single runs of experiments.Comment: 18, Revte
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