890 research outputs found
Generating Schr\"{o}dinger-cat states in momentum and internal-state space from Bose-Einstein condensates with repulsive interactions
Resonant Raman coupling between internal levels induced by continuous
illumination of non-collinear laser beams can create double-well momentum-space
potentials for multi-level ``periodically-dressed'' atoms. We develop an
approximate many-body formalism for a weakly interacting, trapped
periodically-dressed Bose gas which illustrates how a tunable exchange
interaction yields correlated many-body ground states. In contrast to the case
of a position-space double well, the ground state of stable
periodically-dressed Bose gases with repulsive interactions tends toward a
Schr\"{o}dinger cat state in the regime where interactions dominate the
momentum-space tunnelling induced by the external trapping potential. The
dependence of the momentum-space tunnelling and exchange interaction on
experimental parameters is derived. We discuss how real-time control of
experimental parameters can be used to create Schr\"{o}dinger cat states either
between momentum or internal states, and how these states could be dynamically
controlled towards highly sensitive interferometry and frequency metrology.Comment: 7 pages, 3 figures. Submitted to PR
Kinetic theory and dynamic structure factor of a condensate in the random phase approximation
We present the microscopic kinetic theory of a homogeneous dilute Bose
condensed gas in the generalized random phase approximation (GRPA), which
satisfies the following requirements: 1) the mass, momentum and energy
conservation laws; 2) the H-theorem; 3) the superfluidity property and 4) the
recovery of the Bogoliubov theory at zero temperature \cite{condenson}. In this
approach, the condensate influences the binary collisional process between the
two normal atoms, in the sense that their interaction force results from the
mediation of a Bogoliubov collective excitation traveling throughout the
condensate. Furthermore, as long as the Bose gas is stable, no collision
happens between condensed and normal atoms. In this paper, we show how the
kinetic theory in the GRPA allows to calculate the dynamic structure factor at
finite temperature and when the normal and superfluid are in a relative motion.
The obtained spectrum for this factor provides a prediction which, compared to
the experimental results, allows to validate the GRPA.
PACS numbers:03.75.Hh, 03.75.Kk, 05.30.-dComment: 6 pages, 1 figures, QFS2004 conferenc
Spinor Dynamics in an Antiferromagnetic Spin-1 Condensate
We observe coherent spin oscillations in an antiferromagnetic spin-1
Bose-Einstein condensate of sodium. The variation of the spin oscillations with
magnetic field shows a clear signature of nonlinearity, in agreement with
theory, which also predicts anharmonic oscillations near a critical magnetic
field. Measurements of the magnetic phase diagram agree with predictions made
in the approximation of a single spatial mode. The oscillation period yields
the best measurement to date of the sodium spin-dependent interaction
coefficient, determining that the difference between the sodium spin-dependent
s-wave scattering lengths is Bohr radii.Comment: 5 pages, 2 figures. Changes: added reference, minor correction
Exotic magnetic orders for high spin ultracold fermions
We study Hubbard models for ultracold bosonic or fermionic atoms loaded into
an optical lattice. The atoms carry a high spin , and interact on site
via strong repulsive Van der Waals forces. Making convenient rearrangements of
the interaction terms, and exploiting their symmetry properties, we derive low
energy effective models with nearest-neighbor interactions, and their
properties. We apply our method to , and 5/2 fermions on two-dimensional
square lattice at quarter, and 1/6 fillings, respectively, and investigate
mean-field equations for repulsive couplings. We find for fermions that
the plaquette state appearing in the highly symmetric SU(4) case does not
require fine tuning, and is stable in an extended region of the phase diagram.
This phase competes with an SU(2) flux state, that is always suppressed for
repulsive interactions in absence of external magnetic field. The SU(2) flux
state has, however, lower energy than the plaquette phase, and stabilizes in
the presence of weak applied magnetic field. For fermions a similar
SU(2) plaquette phase is found to be the ground state without external magnetic
field.Comment: final version, 6 pages, 4 figures, epl forma
From multimode to monomode guided atom lasers: an entropic analysis
We have experimentally demonstrated a high level of control of the mode
populations of guided atom lasers (GALs) by showing that the entropy per
particle of an optically GAL, and the one of the trapped Bose Einstein
condensate (BEC) from which it has been produced are the same. The BEC is
prepared in a crossed beam optical dipole trap. We have achieved isentropic
outcoupling for both magnetic and optical schemes. We can prepare GAL in a
nearly pure monomode regime (85 % in the ground state). Furthermore, optical
outcoupling enables the production of spinor guided atom lasers and opens the
possibility to tailor their polarization
Controlling ultracold atoms in multi-band optical lattices for simulation of Kondo physics
We show that ultracold atoms can be controlled in multi-band optical lattices
through spatially periodic Raman pulses for investigation of a class of
strongly correlated physics related to the Kondo problem. The underlying
dynamics of this system is described by a spin-dependent fermionic or bosonic
Kondo-Hubbard lattice model even if we have only spin-independent atomic
collision interaction. We solve the bosonic Kondo-Hubbard lattice model through
a mean-field approximation, and the result shows a clear phase transition from
the ferromagnetic superfluid to the Kondo-signet insulator at the integer
filling.Comment: 4 pages, 2 figure
Strongly enhanced inelastic collisions in a Bose-Einstein condensate near Feshbach resonances
The properties of Bose-Einstein condensed gases can be strongly altered by
tuning the external magnetic field near a Feshbach resonance. Feshbach
resonances affect elastic collisions and lead to the observed modification of
the scattering length. However, as we report here, this is accompanied by a
strong increase in the rate of inelastic collisions. The observed three-body
loss rate in a sodium Bose-Einstein condensation increased when the scattering
length was tuned to both larger or smaller values than the off-resonant value.
This observation and the maximum measured increase of the loss rate by several
orders of magnitude are not accounted for by theoretical treatments. The strong
losses impose severe limitations for using Feshbach resonances to tune the
properties of Bose-Einstein condensates. A new Feshbach resonance in sodium at
1195 G was observed.Comment: 4 pages, 3 figure
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