822 research outputs found
Density-Matrix approach to a Strongly Coupled Two-Component Bose-Einstein Condensate
The time evolution equations for average values of population and relative
phase of a strongly coupled two component BEC is derived analytically. The two
components are two hyper-fine states coupled by an external laser that drives
fast Rabi oscillations between these states. Specifically, this derivation
incorporates the two-mode model proposed in [1] for the strongly coupled
hyper-fine states of Rb. The fast Rabi cycle is averaged out and rate equations
are derived that represents the slow dynamics of the system. These include the
collapse and revival of Rabi oscillations and their subsequent dependence on
detuning and trap displacement as reported in experiments of [1]. A proposal to
create stable vortices is also given.Comment: 11 Latex pages, 2 figures (Figure 3 was removed and the text chnaged
accordingly
Time varying gravitational constant G via the entropic force
If the uncertainty principle applies to the Verlinde entropic idea, it leads
to a new term in the Newton's second law of mechanics in the Planck's scale.
This curious velocity dependence term inspires a frictional feature of the
gravity. In this short letter we address that this new term modifies the
effective mass and the Newtonian constant as the time dependence quantities.
Thus we must have a running on the value of the effective mass on the particle
mass near the holographic screen and the . This result has a nigh
relation with the Dirac hypothesis about the large numbers hypothesis (L.N.H.)
[1]. We propose that the corrected entropic terms via Verlinde idea can be
brought as a holographic evidence for the authenticity of the Dirac idea.Comment: Accepted for publication in "Communications in Theoretical Physics
(CTP)",Major revisio
Input-output theory for fermions in an atom cavity
We generalize the quantum optical input-output theory developed for optical
cavities to ultracold fermionic atoms confined in a trapping potential, which
forms an "atom cavity". In order to account for the Pauli exclusion principle,
quantum Langevin equations for all cavity modes are derived. The dissipative
part of these multi-mode Langevin equations includes a coupling between cavity
modes. We also derive a set of boundary conditions for the Fermi field that
relate the output fields to the input fields and the field radiated by the
cavity. Starting from a constant uniform current of fermions incident on one
side of the cavity, we use the boundary conditions to calculate the occupation
numbers and current density for the fermions that are reflected and transmitted
by the cavity
Non-supersymmetric Attractors in Born-Infeld Black Holes with a Cosmological Constant
We investigate the attractor mechanism for spherically symmetric extremal
black holes in Einstein-Born-Infeld-dilaton theory of gravity in
four-dimensions, in the presence of a cosmological constant. We look for
solutions analytic near the horizon by using perturbation method. It is shown
that the values of the scalar fields at the horizon are only dependent on the
charges carried by the black hole and are irrelevant in their asymptotic
values. This analysis supports the validity of non-supersymmetric attractors in
the presence of higher derivative interactions in the gauge fields part and in
non-asymptotically flat spacetime.Comment: 18 pages, no figu
On The Stability of Non-Supersymmetric Attractors in String Theory
We study non-supersymmetric attractors obtained in Type IIA compactifications
on Calabi Yau manifolds. Determining if an attractor is stable or unstable
requires an algebraically complicated analysis in general. We show using group
theoretic techniques that this analysis can be considerably simplified and can
be reduced to solving a simple example like the STU model. For attractors with
D0-D4 brane charges, determining stability requires expanding the effective
potential to quartic order in the massless fields. We obtain the full set of
these terms. For attractors with D0-D6 brane charges, we find that there is a
moduli space of solutions and the resulting attractors are stable. Our analysis
is restricted to the two derivative action.Comment: 20 pages, Late
Job Scheduling Using successive Linear Programming Approximations of a Sparse Model
EuroPar 2012In this paper we tackle the well-known problem of scheduling a collection of parallel jobs on a set of processors either in a cluster or in a multiprocessor computer. For the makespan objective, i.e., the completion time of the last job, this problem has been shown to be NP-Hard and several heuristics have already been proposed to minimize the execution time. We introduce a novel approach based on successive linear programming (LP) approximations of a sparse model. The idea is to relax an integer linear program and use lp norm-based operators to force the solver to find almost-integer solutions that can be assimilated to an integer solution. We consider the case where jobs are either rigid or moldable. A rigid parallel job is performed with a predefined number of processors while a moldable job can define the number of processors that it is using just before it starts its execution. We compare the scheduling approach with the classic Largest Task First list based algorithm and we show that our approach provides good results for small instances of the problem. The contributions of this paper are both the integration of mathematical methods in the scheduling world and the design of a promising approach which gives good results for scheduling problems with less than a hundred processors
Ground State and Quasiparticle Spectrum of a Two Component Bose-Einstein Condensate
We consider a dilute atomic Bose-Einstein condensate with two non-degenerate
internal energy levels. The presence of an external radiation field can result
in new ground states for the condensate which result from the lowering of the
condensate energy due to the interaction energy with the field. In this
approach there are no instabilities in the quasiparticle spectrum as was
previously found by Goldstein and Meystre (Phys. Rev. A \QTR{bf}{55}, 2935
(1997)).Comment: 20 pages, 2 figures RevTex. Submitted to Phys. Rev. A; Revised
versio
Adiabatic Output Coupling of a Bose Gas at Finite Temperatures
We develop a general theory of adiabatic output coupling from trapped atomic
Bose-Einstein Condensates at finite temperatures. For weak coupling, the output
rate from the condensate, and the excited levels in the trap, settles in a time
proportional to the inverse of the spectral width of the coupling to the output
modes. We discuss the properties of the output atoms in the quasi-steady-state
where the population in the trap is not appreciably depleted. We show how the
composition of the output beam, containing condensate and thermal component,
may be controlled by changing the frequency of the output coupler. This
composition determines the first and second order coherence of the output beam.
We discuss the changes in the composition of the bose gas left in the trap and
show how nonresonant output coupling can stimulate either the evaporation of
thermal excitations in the trap or the growth of non-thermal excitations, when
pairs of correlated atoms leave the condensate.Comment: 22 pages, 6 Figs. To appear in Physical Review A All the typos from
the previous submission have been fixe
Temperature Variation of Ultra Slow Light in a Cold Gas
A model is developed to explain the temperature dependence of the group
velocity as observed in the experiments of Hau et al (Nature {\bf397}, 594
(1999)). The group velocity is quite sensitive to the change in the spatial
density. The inhomogeneity in the density and its temperature dependence are
primarily responsible for the observed behavior.Comment: 12 pages, 4 figure
Barrier effects on the collective excitations of split Bose-Einstein condensates
We investigate the collective excitations of a single-species Bose gas at T=0
in a harmonic trap where the confinement undergoes some splitting along one
spatial direction. We mostly consider onedimensional potentials consisting of
two harmonic wells separated a distance 2 z_0, since they essentially contain
all the barrier effects that one may visualize in the 3D situation. We find,
within a hydrodynamic approximation, that regardless the dimensionality of the
system, pairs of levels in the excitation spectrum, corresponding to
neighbouring even and odd excitations, merge together as one increases the
barrier height up to the current value of the chemical potential. The
excitation spectra computed in the hydrodynamical or Thomas-Fermi limit are
compared with the results of exactly solving the time-dependent
Gross-Pitaevskii equation. We analyze as well the characteristics of the
spatial pattern of excitations of threedimensional boson systems according to
the amount of splitting of the condensate.Comment: RevTeX, 12 pages, 13 ps figure
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