101,527 research outputs found
Spatially Distributed Stochastic Systems: equation-free and equation-assisted preconditioned computation
Spatially distributed problems are often approximately modelled in terms of
partial differential equations (PDEs) for appropriate coarse-grained quantities
(e.g. concentrations). The derivation of accurate such PDEs starting from finer
scale, atomistic models, and using suitable averaging, is often a challenging
task; approximate PDEs are typically obtained through mathematical closure
procedures (e.g. mean-field approximations). In this paper, we show how such
approximate macroscopic PDEs can be exploited in constructing preconditioners
to accelerate stochastic simulations for spatially distributed particle-based
process models. We illustrate how such preconditioning can improve the
convergence of equation-free coarse-grained methods based on coarse
timesteppers. Our model problem is a stochastic reaction-diffusion model
capable of exhibiting Turing instabilities.Comment: 8 pages, 6 figures, submitted to Journal of Chemical Physic
Controlling the dynamics of a coupled atom-cavity system by pure dephasing : basics and potential applications in nanophotonics
The influence of pure dephasing on the dynamics of the coupling between a
two-level atom and a cavity mode is systematically addressed. We have derived
an effective atom-cavity coupling rate that is shown to be a key parameter in
the physics of the problem, allowing to generalize the known expression for the
Purcell factor to the case of broad emitters, and to define strategies to
optimize the performances of broad emitters-based single photon sources.
Moreover, pure dephasing is shown to be able to restore lasing in presence of
detuning, a further demonstration that decoherence can be seen as a fundamental
resource in solid-state cavity quantum electrodynamics, offering appealing
perspectives in the context of advanced nano-photonic devices.Comment: 10 pages, 7 figure
Theory of cavity-assisted microwave cooling of polar molecules
We analyze cavity-assisted cooling schemes for polar molecules in the
microwave domain, where molecules are excited on a rotational transition and
energy is dissipated via strong interactions with a lossy stripline cavity, as
recently proposed by A. Andre et al., Nature Physics 2, 636 (2006). We identify
the dominant cooling and heating mechanisms in this setup and study cooling
rates and final temperatures in various parameter regimes. In particular we
analyze the effects of a finite environment temperature on the cooling
efficiency, and find minimal temperature and optimized cooling rate in the
strong drive regime. Further we discuss the trade-off between efficiency of
cavity cooling and robustness with respect to ubiquitous imperfections in a
realistic experimental setup, such as anharmonicity of the trapping potential
Adaptive Replication in Distributed Content Delivery Networks
We address the problem of content replication in large distributed content
delivery networks, composed of a data center assisted by many small servers
with limited capabilities and located at the edge of the network. The objective
is to optimize the placement of contents on the servers to offload as much as
possible the data center. We model the system constituted by the small servers
as a loss network, each loss corresponding to a request to the data center.
Based on large system / storage behavior, we obtain an asymptotic formula for
the optimal replication of contents and propose adaptive schemes related to
those encountered in cache networks but reacting here to loss events, and
faster algorithms generating virtual events at higher rate while keeping the
same target replication. We show through simulations that our adaptive schemes
outperform significantly standard replication strategies both in terms of loss
rates and adaptation speed.Comment: 10 pages, 5 figure
Optimized loading of an optical dipole trap for the production of Chromium BECs
We report on a strategy to maximize the number of chromium atoms transferred
from a magneto-optical trap into an optical trap through accumulation in
metastable states via strong optical pumping. We analyse how the number of
atoms in a chromium Bose Einstein condensate can be raised by a proper handling
of the metastable state populations. Four laser diodes have been implemented to
address the four levels that are populated during the MOT phase. The individual
importance of each state is specified. To stabilize two of our laser diode, we
have developed a simple ultrastable passive reference cavity whose long term
stability is better than 1 MHz
Dissipative Preparation of Antiferromagnetic Order in the Fermi-Hubbard Model
The Fermi-Hubbard model is one of the key models of condensed matter physics,
which holds a potential for explaining the mystery of high-temperature
superconductivity. Recent progress in ultracold atoms in optical lattices has
paved the way to studying the model's phase diagram using the tools of quantum
simulation, which emerged as a promising alternative to the numerical
calculations plagued by the infamous sign problem. However, the temperatures
achieved using elaborate laser cooling protocols so far have been too high to
show the appearance of antiferromagnetic and superconducting quantum phases
directly. In this work, we demonstrate that using the machinery of dissipative
quantum state engineering, one can efficiently prepare antiferromagnetic order
in present-day experiments with ultracold fermions. The core of the approach is
to add incoherent laser scattering in such a way that the antiferromagnetic
state emerges as the dark state of the driven-dissipative dynamics. In order to
elucidate the development of the antiferromagnetic order we employ two
complementary techniques: Monte Carlo wave function simulations for small
systems and a recently proposed variational method for open quantum systems,
operating in the thermodynamic limit. The controlled dissipation channels
described in this work are straightforward to add to already existing
experimental setups.Comment: 9 pages, 5 figure
Quantum discord induced by white noises
We discuss the creation of quantum discord between two two-level atoms
trapped in an optical cavity in a noisy environment. It is shown that nonzero
steady-state quantum discord between atoms can be obtained when the white-noise
field is separately imposed on atoms or cavity mode, while the steady-state
quantum discord reaches zero if both cavity mode and atoms are driven
simultaneously by white-noise fields. In particular, we demonstrate that
white-noise field in different cases can play a variously constructive role in
the generation of quantum discord.Comment: 6 figure
Two-photon and EIT-assisted Doppler cooling in a three-level cascade system
Laser cooling is theoretically investigated in a cascade three-level scheme,
where the excited state of a laser-driven transition is coupled by a second
laser to a top, more stable level, as for alkali-earth atoms. The second laser
action modifies the atomic scattering cross section and produces temperatures
lower than those reached by Doppler cooling on the lower transition. When
multiphoton processes due to the second laser are relevant, an electromagnetic
induced transparency modifies the absorption of the first laser, and the final
temperature is controlled by the second laser parameters. When the intermediate
state is only virtually excited, the dynamics is dominated by the two-photon
process and the final temperature is determined by the spontaneous decay rate
of the top state.Comment: 5 pages, 3 figures. Revised version, accepted for publication in
Phys. Rev A (Rapid Comm.
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