1,719 research outputs found
Slow quench dynamics of a trapped one-dimensional Bose gas confined to an optical lattice
We analyze the effect of a linear time-variation of the interaction strength
on a trapped one-dimensional Bose gas confined to an optical lattice. The
evolution of different observables such as the experimentally accessible onsite
particle distribution are studied as a function of the ramp time using
time-dependent exact diagonalization and density-matrix renormalization group
techniques. We find that the dynamics of a trapped system typically display two
regimes: for long ramp times, the dynamics are governed by density
redistribution, while at short ramp times, local dynamics dominate as the
evolution is identical to that of an homogeneous system. In the homogeneous
limit, we also discuss the non-trivial scaling of the energy absorbed with the
ramp time.Comment: 4 pages, 4 figures, version published in PR
Colliding wind binaries and gamma-ray binaries : relativistic version of the RAMSES code
Gamma-ray binaries are colliding wind binaries (CWB) composed of a massive
star a non-accreting pulsar with a highly relativistic wind. Particle
acceleration at the shocks results in emission going from extended radio
emission to the gamma-ray band. The interaction region is expected to show
common features with stellar CWB. Performing numerical simulations with the
hydrodynamical code RAMSES, we focus on their structure and stability and find
that the Kelvin-Helmholtz instability (KHI) can lead to important mixing
between the winds and destroy the large scale spiral structure. To investigate
the impact of the relativistic nature of the pulsar wind, we extend RAMSES to
relativistic hydrodynamics (RHD). Preliminary simulations of the interaction
between a pulsar wind and a stellar wind show important similarities with
stellar colliding winds with small relativistic corrections.Comment: Proceeding of the 5th International Symposium on High-Energy
Gamma-Ray Astronomy (Gamma2012). arXiv admin note: text overlap with
arXiv:1212.404
Promoting the use of reliable rate based transport protocols: the Chameleon protocol
Rate-based congestion control, such as TFRC, has not been designed to enable reliability. Indeed, the birth of TFRC protocol has resulted from the need for a congestion-controlled transport protocol in order to carry multimedia traffic. However, certain applications still prefer the use of UDP in order to implement their own congestion control on top of it. The present contribution proposes to design and validate a reliable rate-based protocol based on the combined use of TFRC, SACK and an adapted flow control. We argue that rate-based congestion control is a perfect alternative to window-based congestion control as most of today applications need to interact with the transport layer and should not be only limited to unreliable services. In this paper, we detail the implementation of a reliable rate-based protocol named Chameleon and bring out to the networking community an ns-2 implementation for evaluation purpose
Invisible waveguides on metal plates for plasmonic analogues of electromagnetic wormholes
We introduce two types of toroidal metamaterials which are invisible to
surface plasmon polaritons (SPPs) propagating on a metal surface. The former is
a toroidal handlebody bridging remote holes on the metal surface: It works as a
kind of plasmonic counterpart of electromagnetic wormholes. The latter is a
toroidal ring lying on the metal surface: This bridges two disconnected metal
surfaces i.e. It connects a thin metal cylinder to a flat metal surface with a
hole. Full-wave numerical simulations demonstrate that an electromagnetic field
propagating inside these metamaterials does not disturb the propagation of SPPs
at the metal surface. A multilayered design of these devices is proposed, based
on effective medium theory for a set of reduced parameters: The former
plasmonic analogue of electromagnetic wormhole requires homogeneous isotropic
magnetic layers, while the latter merely requires dielectric layers.Comment: 17 figure
Acoustic cloaking and mirages with flying carpets
Carpets under consideration here, in the context of pressure acoustic waves
propagating in a compressible fluid, do not touch the ground: they levitate in
mid-air (or float in mid-water), which leads to approximate cloaking for an
object hidden underneath, or touching either sides of a square cylinder on, or
over, the ground. The tentlike carpets attached to the sides of a square
cylinder illustrate how the notion of a carpet on a wall naturally generalizes
to sides of other small compact objects. We then extend the concept of flying
carpets to circular cylinders. However, instead of reducing its scattering
cross-section like in acoustic cloaks, we rather mimic that of another
obstacle, say a square rigid cylinder. For instance, show that one can hide any
type of defects under such circular carpets, and yet they still scatter waves
just like a smaller cylinder on its own. Interestingly, all these carpets are
described by non-singular acoustic parameters. To exemplify this important
aspect, we propose a multi-layered carpet consisting of isotropic homogeneous
fluids with constant bulk modulus and varying density which works over a finite
range of wavelengths. We have discussed some applications, with the sonar boats
or radars cases as typical examples. For instance, we would like to render a
pipeline lying on the bottom of the sea or floating in mid-water undetectable
for a boat with a sonar at rest just above it on the surface of the sea.
Another possible application would be protecting parabolic antennas.Comment: 26 pages, 9 figures. Key words: Mathematical methods in physics;
Mathematical Physics, electromagnetic theory; Metamaterials;Anisotropic
optical materials; invisibility; cloa
Blind frame synchronisation for Block code
Publication in the conference proceedings of EUSIPCO, Florence, Italy, 200
CCCP: A CCD Controller for Counting Photons
CCCP, a CCD Controller for Counting Photons, is presented. This new
controller uses a totally new clocking architecture and allows to drive the CCD
in a novel way. Its design is optimized for the driving of EMCCDs at up to
20MHz of pixel rate and fast vertical transfer. Using this controller, the
dominant source of noise of EMCCDs at low flux level and high frame rate, the
Clock Induced Charges, were reduced to 0.001 - 0.0018 electron/pixel/frame
(depending of the electron multiplying gain), making efficient photon counting
possible. CCCP will be deployed in 2009 on the ESO NTT through the 3D-NTT1
project and on the SOAR through the BTFI project.Comment: 10 pages, 10 figures, to appear in "Ground-based and Airborne
Instrumentation for Astronomy II" SPIE conference, Marseille, 23-28 June 200
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