1,548 research outputs found
Two-fluid hydrodynamics of a Bose gas including damping from normal fluid transport coefficients
We extend our recent work on the two-fluid hydrodynamics of the condensate
and non-condensate in a trapped Bose gas by including the dissipation
associated with viscosity and thermal conduction. For purposes of illustration,
we consider the hydrodynamic modes in the case of a uniform Bose gas. A finite
thermal conductivity and shear viscosity give rise to a damping of the first
and second sound modes in addition to that found previously due to the lack of
diffusive equilibrium between the condensate and non-condensate. The
relaxational mode associated with this equilibration process is strongly
coupled to thermal fluctuations and reduces to the usual thermal diffusion mode
above the Bose-Einstein transition. In contrast to the standard Landau
two-fluid hydrodynamics, we predict a damped mode centered at zero frequency,
in addition to the usual second sound doublet.Comment: 18 pages, revtex, 4 postscript figures, Submitted to the Canadian
Journal of Physics for the Boris Stoicheff Festschrift issu
A Dynamical Self-Consistent Finite Temperature Kinetic Theory: The ZNG Scheme
We review a self-consistent scheme for modelling trapped weakly-interacting
quantum gases at temperatures where the condensate coexists with a significant
thermal cloud. This method has been applied to atomic gases by Zaremba, Nikuni,
and Griffin, and is often referred to as ZNG. It describes both
mean-field-dominated and hydrodynamic regimes, except at very low temperatures
or in the regime of large fluctuations. Condensate dynamics are described by a
dissipative Gross-Pitaevskii equation (or the corresponding quantum
hydrodynamic equation with a source term), while the non-condensate evolution
is represented by a quantum Boltzmann equation, which additionally includes
collisional processes which transfer atoms between these two subsystems. In the
mean-field-dominated regime collisions are treated perturbatively and the full
distribution function is needed to describe the thermal cloud, while in the
hydrodynamic regime the system is parametrised in terms of a set of local
variables. Applications to finite temperature induced damping of collective
modes and vortices in the mean-field-dominated regime are presented.Comment: Unedited version of chapter to appear in Quantum Gases: Finite
Temperature and Non-Equilibrium Dynamics (Vol. 1 Cold Atoms Series). N.P.
Proukakis, S.A. Gardiner, M.J. Davis and M.H. Szymanska, eds. Imperial
College Press, London (in press). See
http://www.icpress.co.uk/physics/p817.htm
Effects of temperature upon the collapse of a Bose-Einstein condensate in a gas with attractive interactions
We present a study of the effects of temperature upon the excitation
frequencies of a Bose-Einstein condensate formed within a dilute gas with a
weak attractive effective interaction between the atoms. We use the
self-consistent Hartree-Fock Bogoliubov treatment within the Popov
approximation and compare our results to previous zero temperature and
Hartree-Fock calculations The metastability of the condensate is monitored by
means of the excitation frequency. As the number of atoms in the
condensate is increased, with held constant, this frequency goes to zero,
signalling a phase transition to a dense collapsed state. The critical number
for collapse is found to decrease as a function of temperature, the rate of
decrease being greater than that obtained in previous Hartree-Fock
calculations.Comment: 4 pages LaTeX, 3 eps figures. To appear as a letter in J. Phys.
Beam splitting and Hong-Ou-Mandel interference for stored light
Storing and release of a quantum light pulse in a medium of atoms in the
tripod configuration are studied. Two complementary sets of control fields are
defined, which lead to independent and complete photon release at two stages.
The system constitutes a new kind of a flexible beam splitter in which the
input and output ports concern photons of the same direction but well separated
in time. A new version of Hong-Ou-Mandel interference is discussed.Comment: 8 pages, 3 figure
Controlled light storage in a double lambda system
It is shown theoretically that after light storing in a medium of four-level
atoms it is possible to release a new pulse of a different frequency, the
process being steered by another driving beam. It is also possible to store one
pulse and to release two different ones, with their time separation and heights
being controlled.Comment: 7 pages,3 figure
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