78 research outputs found
A proposal for continuous loading of an optical dipole trap with magnetically guided ultra cold atoms
The capture of a moving atom by a non-dissipative trap, such as an optical
dipole trap, requires the removal of the excessive kinetic energy of the atom.
In this article we develop a mechanism to harvest ultra cold atoms from a
guided atom beam into an optical dipole trap by removing their directed kinetic
energy. We propose a continuous loading scheme where this is accomplished via
deceleration by a magnetic potential barrier followed by optical pumping to the
energetically lowest Zeeman sublevel. We theoretically investigate the
application of this scheme to the transfer of ultra cold chromium atoms from a
magnetically guided atom beam into a deep optical dipole trap. We discuss the
realization of a suitable magnetic field configuration. Based on numerical
simulations of the loading process we analyze the feasibility and efficiency of
our loading scheme.Comment: 10 pages, 5 figure
Laser cooling of a magnetically guided ultra cold atom beam
We report on the transverse laser cooling of a magnetically guided beam of
ultra cold chromium atoms. Radial compression by a tapering of the guide is
employed to adiabatically heat the beam. Inside the tapered section heat is
extracted from the atom beam by a two-dimensional optical molasses
perpendicular to it, resulting in a significant increase of atomic phase space
density. A magnetic offset field is applied to prevent optical pumping to
untrapped states. Our results demonstrate that by a suitable choice of the
magnetic offset field, the cooling beam intensity and detuning, atom losses and
longitudinal heating can be avoided. Final temperatures below 65 microkelvin
have been achieved, corresponding to an increase of phase space density in the
guided beam by more than a factor of 30.Comment: 9 pages, 4 figure
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
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
Quantum Dynamics of Atomic Coherence in a Spin-1 Condensate: Mean-Field versus Many-Body Simulation
We analyse and numerically simulate the full many-body quantum dynamics of a
spin-1 condensate in the single spatial mode approximation. Initially, the
condensate is in a ``ferromagnetic'' state with all spins aligned along the
axis and the magnetic field pointing along the z axis. In the course of
evolution the spinor condensate undergoes a characteristic change of symmetry,
which in a real experiment could be a signature of spin-mixing many-body
interactions. The results of our simulations are conveniently visualised within
the picture of irreducible tensor operators.Comment: Accepted for publication for the special issue of "Optics
Communications" on Quantum Control of Light and Matte
Determination of the s-wave Scattering Length of Chromium
We have measured the deca-triplet s-wave scattering length of the bosonic
chromium isotopes Cr and Cr. From the time constants for
cross-dimensional thermalization in atomic samples we have determined the
magnitudes and ,
where . By measuring the rethermalization rate of Cr over a
wide temperature range and comparing the temperature dependence with the
effective-range theory and single-channel calculations, we have obtained strong
evidence that the sign of is positive. Rescaling our Cr
model potential to Cr strongly suggests that is positive,
too.Comment: v3: corrected typo in y-axis scaling of Figs. 3 and
Solitons in Tonks-Girardeau gas with dipolar interactions
The existence of bright solitons in the model of the Tonks-Girardeau (TG) gas
with dipole-dipole (DD) interactions is reported. The governing equation is
taken as the quintic nonlinear Schr\"{o}dinger equation (NLSE) with the
nonlocal cubic term accounting for the DD attraction. In different regions of
the parameter space (the dipole moment and atom number), matter-wave solitons
feature flat-top or compacton-like shapes. For the flat-top states, the NLSE
with the local cubic-quintic (CQ) nonlinearity is shown to be a good
approximation. Specific dynamical effects are studied assuming that the
strength of the DD interactions is ramped up or drops to zero. Generation of
dark-soliton pairs in the gas shrinking under the action of the intensifying DD
attraction is observed. Dark solitons exhibit the particle-like collision
behavior. Peculiarities of dipole solitons in the TG gas are highlighted by
comparison with the NLSE including the local CQ terms. Collisions between the
solitons are studied too. In many cases, the collisions result in merger of the
solitons into a breather, due to strong attraction between them.Comment: 15 pages, 8 figures, accepted by J. Phys. B: At. Mol. Opt. Phy
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
Radiating and non-radiating sources in elasticity
In this work, we study the inverse source problem of a fixed frequency for
the Navier's equation. We investigate that nonradiating external forces. If the
support of such a force has a convex or non-convex corner or edge on their
boundary, the force must be vanishing there. The vanishing property at corners
and edges holds also for sufficiently smooth transmission eigenfunctions in
elasticity. The idea originates from the enclosure method: The energy identity
and new type exponential solutions for the Navier's equation.Comment: 17 page
Effect of Quadratic Zeeman Energy on the Vortex of Spinor Bose-Einstein Condensates
The spinor Bose-Einstein condensate of atomic gases has been experimentally
realized by a number of groups. Further, theoretical proposals of the possible
vortex states have been sugessted. This paper studies the effects of the
quadratic Zeeman energy on the vortex states. This energy was ignored in
previous theoretical studies, although it exists in experimental systems. We
present phase diagrams of various vortex states taking into account the
quadratic Zeeman energy. The vortex states are calculated by the
Gross-Pitaevskii equations. Several new kinds of vortex states are found. It is
also found that the quadratic Zeeman energy affects the direction of total
magnetization and causes a significant change in the phase diagrams.Comment: 6 pages, 5 figures. Published in J. Phys. Soc. Jp
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