119 research outputs found
Condensate fraction of cold gases in non-uniform external potential
Exact calculation of the condensate fraction in multi-dimensional
inhomogeneous interacting Bose systems which do not possess continuous
symmetries is a difficult computational problem. We have developed an iterative
procedure which allows to calculate the condensate fraction as well as the
corresponding eigenfunction of the one-body density matrix. We successfully
validate this procedure in diffusion Monte Carlo simulations of a Bose gas in
an optical lattice at zero temperature. We also discuss relation between
different criteria used for testing coherence in cold Bose systems, such as
fraction of particles that are superfluid, condensed or are in the
zero-momentum state.Comment: 4 pages, 2 figure
Manipulation of ultracold atoms in dressed adiabatic radio frequency potentials
We explore properties of atoms whose magnetic hyperfine sub-levels are
coupled by an external magnetic radio frequency (rf) field. We perform a
thorough theoretical analysis of this driven system and present a number of
systematic approximations which eventually give rise to dressed adiabatic radio
frequency potentials. The predictions of this analytical investigation are
compared to numerically exact results obtained by a wave packet propagation. We
outline the versatility and flexibility of this new class of potentials and
demonstrate their potential use to build atom optical elements such as
double-wells, interferometers and ringtraps. Moreover, we perform simulations
of interference experiments carried out in rf induced double-well potentials.
We discuss how the nature of the atom-field coupling mechanism gives rise to a
decrease of the interference contrast
Thermodynamical Properties of a Rotating Ideal Bose Gas
In a recent experiment, a Bose-Einstein condensate was trapped in an
anharmonic potential which is well approximated by a harmonic and a quartic
part. The condensate was set into such a fast rotation that the centrifugal
force in the corotating frame overcompensates the harmonic part in the plane
perpendicular to the rotation axis. Thus, the resulting trap potential became
Mexican-hat shaped. We present an analysis for an ideal Bose gas which is
confined in such an anharmonic rotating trap within a semiclassical
approximation where we calculate the critical temperature, the condensate
fraction, and the heat capacity. In particular, we examine in detail how these
thermodynamical quantities depend on the rotation frequency.Comment: Author Information under
http://www.theo-phys.uni-essen.de/tp/ags/pelster_dir
Phase-sensitive detection of Bragg scattering at 1D optical lattices
We report on the observation of Bragg scattering at 1D atomic lattices. Cold
atoms are confined by optical dipole forces at the antinodes of a standing wave
generated by the two counter-propagating modes of a laser-driven high-finesse
ring cavity. By heterodyning the Bragg-scattered light with a reference beam,
we obtain detailed information on phase shifts imparted by the Bragg scattering
process. Being deep in the Lamb-Dicke regime, the scattered light is not
broadened by the motion of individual atoms. In contrast, we have detected
signatures of global translatory motion of the atomic grating.Comment: 4 pages, 4 figure
Self-consistent fragmented excited states of trapped condensates
Self-consistent excited states of condensates are solutions of the
Gross-Pitaevskii (GP) equation and have been amply discussed in the literature
and related to experiments. By introducing a more general mean-field which
includes the GP one as a special case, we find a new class of self-consistent
excited states. In these states macroscopic numbers of bosons reside in
different one-particle functions, i.e., the states are fragmented. Still, a
single chemical potential is associated with the condensate. A numerical
example is presented, illustrating that the energies of the new, fragmented,
states are much lower than those of the GP excited states, and that they are
stable to variations of the particle number and shape of the trap potential.Comment: (11 pages 2 figures, submitted to PRL
Evolutional Entanglement in Nonequilibrium Processes
Entanglement in nonequilibrium systems is considered. A general definition
for entanglement measure is introduced, which can be applied for characterizing
the level of entanglement produced by arbitrary operators. Applying this
definition to reduced density matrices makes it possible to measure the
entanglement in nonequilibrium as well as in equilibrium statistical systems.
An example of a multimode Bose-Einstein condensate is discussed.Comment: 10 pages, Late
Superfluid drag of two-species Bose-Einstein condensates in optical lattices
We study two-species Bose-Einstein condensates in quasi two-dimensional
optical lattices of varying geometry and potential depth. Based on the
numerically exact Bloch and Wannier functions obtained using the plane-wave
expansion method, we quantify the drag (entrainment coupling) between the
condensate components. This drag originates from the (short range)
inter-species interaction and increases with the kinetic energy. As a result of
the interplay between interaction and kinetic energy effects, the
superfluid-drag coefficient shows a non-monotonic dependence on the lattice
depth. To make contact with future experiments, we quantitatively investigate
the drag for mass ratios corresponding to relevant atomic species.Comment: 6 pages, 4 figures. Accepted in its original form but minor changes
have been don
Injection locking of a low cost high power laser diode at 461 nm
Stable laser sources at 461 nm are important for optical cooling of strontium
atoms. In most existing experiments this wavelength is obtained by frequency
doubling infrared lasers, since blue laser diodes either have low power or
large emission bandwidths. Here, we show that injecting less than 10 mW of
monomode laser radiation into a blue multimode 500 mW high power laser diode is
capable of slaving at least 50% of the power to the desired frequency. We
verify the emission bandwidth reduction by saturation spectroscopy on a
strontium gas cell and by direct beating of the slave with the master laser. We
also demonstrate that the laser can efficiently be used within the Zeeman
slower for optical cooling of a strontium atomic beam.Comment: 2nd corrected version (minor revisions); Manuscript accepted for
publication in Review of Scientific Instruments; 5 pages, 6 figure
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