101 research outputs found
Depolarisation cooling of an atomic cloud
We propose a cooling scheme based on depolarisation of a polarised cloud of
trapped atoms. Similar to adiabatic demagnetisation, we suggest to use the
coupling between the internal spin reservoir of the cloud and the external
kinetic reservoir via dipolar relaxation to reduce the temperature of the
cloud. By optical pumping one can cool the spin reservoir and force the cooling
process. In case of a trapped gas of dipolar chromium atoms, we show that this
cooling technique can be performed continuously and used to approach the
critical phase space density for BECComment: 8 pages, 5 figure
Conditions for one-dimensional supersonic flow of quantum gases
One can use transsonic Bose-Einstein condensates of alkali atoms to establish
the laboratory analog of the event horizon and to measure the acoustic version
of Hawking radiation. We determine the conditions for supersonic flow and the
Hawking temperature for realistic condensates on waveguides where an external
potential plays the role of a supersonic nozzle. The transition to supersonic
speed occurs at the potential maximum and the Hawking temperature is entirely
determined by the curvature of the potential
Observation of dipole-dipole interaction in a degenerate quantum gas
We have investigated the expansion of a Bose-Einstein condensate (BEC) of
strongly magnetic chromium atoms. The long-range and anisotropic magnetic
dipole-dipole interaction leads to an anisotropic deformation of the expanding
Cr-BEC which depends on the orientation of the atomic dipole moments. Our
measurements are consistent with the theory of dipolar quantum gases and show
that a Cr-BEC is an excellent model system to study dipolar interactions in
such gases.Comment: 4 pages, 2 figure
Production of a chromium Bose-Einstein condensate
The recent achievement of Bose-Einstein condensation of chromium atoms [1]
has opened longed-for experimental access to a degenerate quantum gas with
long-range and anisotropic interaction. Due to the large magnetic moment of
chromium atoms of 6 {}B, in contrast to other Bose- Einstein condensates
(BECs), magnetic dipole-dipole interaction plays an important role in a
chromium BEC. Many new physical properties of degenerate gases arising from
these magnetic forces have been predicted in the past and can now be studied
experimentally. Besides these phenomena, the large dipole moment leads to a
breakdown of standard methods for the creation of a chromium BEC. Cooling and
trapping methods had to be adapted to the special electronic structure of
chromium to reach the regime of quantum degeneracy. Some of them apply
generally to gases with large dipolar forces. We present here a detailed
discussion of the experimental techniques which are used to create a chromium
BEC and alow us to produce pure condensates with up to {} atoms in an
optical dipole trap. We also describe the methods used to determine the
trapping parameters.Comment: 17 pages, 9 figure
Ground state and elementary excitations of single and binary Bose-Einstein condensates of trapped dipolar gases
We analyze the ground-state properties and the excitation spectrum of
Bose-Einstein condensates of trapped dipolar particles. First, we consider the
case of a single-component polarized dipolar gas. For this case we discuss the
influence of the trapping geometry on the stability of the condensate as well
as the effects of the dipole-dipole interaction on the excitation spectrum. We
discuss also the ground state and excitations of a gas composed of two
antiparallel dipolar components.Comment: 12 pages, 9 eps figures, final versio
Acoustic black holes for relativistic fluids
We derive a new acoustic black hole metric from the Abelian Higgs model. In
the non-relativistic limit, while the Abelian Higgs model becomes the
Ginzburg-Landau model, the metric reduces to an ordinary Unruh type. We
investigate the possibility of using (type I and II) superconductors as the
acoustic black holes. We propose to realize experimental acoustic black holes
by using spiral vortices solutions from the Navier-stokes equation in the
non-relativistic classical fluids.Comment: 16 pages. typos corrected, contents expande
Realization of a single Josephson junction for Bose-Einstein condensates
We report on the realization of a double-well potential for Rubidium-87
Bose-Einstein condensates. The experimental setup allows the investigation of
two different dynamical phenomena known for this system - Josephson
oscillations and self-trapping. We give a detailed discussion of the
experimental setup and the methods used for calibrating the relevant
parameters. We compare our experimental findings with the predictions of an
extended two-mode model and find quantitative agreement
Critical number of atoms in an attractive Bose-Einstein condensate on an optical plus harmonic traps
The stability of an attractive Bose-Einstein condensate on a joint
one-dimensional optical lattice and an axially-symmetric harmonic trap is
studied using the numerical solution of the time-dependent mean-field
Gross-Pitaevskii equation and the critical number of atoms for a stable
condensate is calculated. We also calculate this critical number of atoms in a
double-well potential which is always greater than that in an axially-symmetric
harmonic trap. The critical number of atoms in an optical trap can be made
smaller or larger than the corresponding number in the absence of the optical
trap by moving a node of the optical lattice potential along the axial
direction of the harmonic trap. This variation of the critical number of atoms
can be observed experimentally and compared with the present calculation.Comment: Latex with 7 eps figures, Accepted in Journal of Physics
Classical versus quantum dynamics of the atomic Josephson junction
We compare the classical (mean-field) dynamics with the quantum dynamics of
atomic Bose-Einstein condensates in double-well potentials. The quantum
dynamics are computed using a simple scheme based upon the Raman-Nath
equations. Two different methods for exciting a non-equilbrium state are
considered: an asymmetry between the wells which is suddenly removed, and a
periodic time oscillating asymmetry. The first method generates wave packets
that lead to collapses and revivals of the expectation values of the
macroscopic variables, and we calculate the time scale for these revivals. The
second method permits the excitation of a single energy eigenstate of the
many-particle system, including Schroedinger cat states. We also discuss a band
theory interpretation of the energy level structure of an asymmetric
double-well, thereby identifying analogies to Bloch oscillations and Bragg
resonances. Both the Bloch and Bragg dynamics are purely quantum and are not
contained in the mean-field treatment.Comment: 31 pages, 14 figure
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