1,872 research outputs found
Bose-Einstein Condensation in a Surface Micro Trap
Bose-Einstein condensation has been achieved in a magnetic surface micro trap
with 4x10^5 87Rb atoms. The strongly anisotropic trapping potential is
generated by a microstructure which consists of microfabricated linear copper
conductors at a width ranging from 3 to 30 micrometer. After loading a high
number of atoms from a pulsed thermal source directly into a magneto-optical
trap (MOT) the magnetically stored atoms are transferred into the micro trap by
adiabatic transformation of the trapping potential. The complete in vacuo trap
design is compatible with ultrahigh vacuum below 2x10^(-11) mbar.Comment: 4 pages, 4 figure
Dynamical decoherence in a cavity with a large number of two-level atoms
We consider a large number of two-level atoms interacting with the mode of a
cavity in the rotating-wave approximation (Tavis-Cummings model). We apply the
Holstein-Primakoff transformation to study the model in the limit of the number
of two-level atoms, all in their ground state, becoming very large. The unitary
evolution that we obtain in this approximation is applied to a macroscopic
superposition state showing that, when the coherent states forming the
superposition are enough distant, then the state collapses on a single coherent
state describing a classical radiation mode. This appear as a true dynamical
effect that could be observed in experiments with cavities.Comment: 9 pages, no figures. This submission substitutes paper
quant-ph/0212148 that was withdrawn. Version accepted for publication in
Journal of Physics B: Atomic, Molecular & Optical Physic
Two-species magneto-optical trap with 40K and 87Rb
We trap and cool a gas composed of 40K and 87Rb, using a two-species
magneto-optical trap (MOT). This trap represents the first step towards cooling
the Bose-Fermi mixture to quantum degeneracy. Laser light for the MOT is
derived from laser diodes and amplified with a single high power semiconductor
amplifier chip. The four-color laser system is described, and the
single-species and two-species MOTs are characterized. Atom numbers of 1x10^7
40K and 2x10^9 87Rb are trapped in the two-species MOT. Observation of trap
loss due to collisions between species is presented and future prospects for
the experiment are discussed.Comment: 4 pages, 4 figures; accepted for publication in Physical Review
Feedback control design for subsonic cavity flows
A benchmark problem in active aerodynamic flow control, suppression of strong pressure oscillations induced by flow over a shallow cavity, is addressed in this paper. Proper orthogonal decomposition and Galerkin projection techniques are used to obtain a reduced-order model of the flow dynamics from experimental data. The model is made amenable to control design by means of a control separation technique, which makes the control input appear explicitly in the equations. A prediction model based on quadratic stochastic estimation correlates flow field data with surface pressure measurements, so that the latter can be used to reconstruct the state of the model in real time. The focus of this paper is on the controller design and implementation. A linear-quadratic optimal controller is designed on the basis of the reduced-order model to suppress the cavity flow resonance. To account for the limitation on the magnitude of the control signal imposed by the actuator, the control action is modified by a scaling factor, which plays the role of a bifurcation parameter for the closed-loop system. Experimental results, in qualitative agreement with the theoretical analysis, show that the controller achieves a significant attenuation of the resonant tone with a redistribution of the energy into other frequencies, and exhibits a certain degree of robustness when operating in off-design conditions
Observation of p-wave Threshold Law Using Evaporatively Cooled Fermionic Atoms
We have measured independently both s-wave and p-wave cross-dimensional
thermalization rates for ultracold potassium-40 atoms held in a magnetic trap.
These measurements reveal that this fermionic isotope has a large positive
s-wave triplet scattering length in addition to a low temperature p-wave shape
resonance. We have observed directly the p-wave threshold law which, combined
with the Fermi statistics, dramatically suppresses elastic collision rates at
low temperatures. In addition, we present initial evaporative cooling results
that make possible these collision measurements and are a precursor to
achieving quantum degeneracy in this neutral, low-density Fermi system.Comment: 5 pages, 3 figures, 1 tabl
Static Properties of Trapped Bose-Fermi Mixed Condensate of Alkali Atoms
Static properties of a bose-fermi mixture of trapped potassium atoms are
studied in terms of coupled Gross-Pitaevskii and Thomas-Fermi equations for
both repulsive and attractive bose-fermi interatomic potentials. Qualitative
estimates are given for solutions of the coupled equations, and the parameter
regions are obtained analytically for the boson-density profile change and for
the boson/fermion phase separation. Especially, the parameter ratio
is found that discriminates the region of the large boson-profile change. These
estimates are applied for numerical results for the potassium atoms and checked
their consistency. It is suggested that a small fraction of fermions could be
trapped without an external potential for the system with an attractive
boson-fermion interaction.Comment: 8 pages,5 figure
Excitation spectrum and instability of a two-species Bose-Einstein condensate
We numerically calculate the density profile and excitation spectrum of a
two-species Bose-Einstein condensate for the parameters of recent experiments.
We find that the ground state density profile of this system becomes unstable
in certain parameter regimes, which leads to a phase transition to a new stable
state. This state displays spontaneously broken cylindrical symmetry. This
behavior is reflected in the excitation spectrum: as we approach the phase
transition point, the lowest excitation frequency goes to zero, indicating the
onset of instability in the density profile. Following the phase transition,
this frequency rises again.Comment: 8 pages, 5 figures, uses REVTe
Atomic dynamics in evaporative cooling of trapped alkali atoms in strong magnetic fields
We investigate how the nonlinearity of the Zeeman shift for strong magnetic
fields affects the dynamics of rf field induced evaporative cooling in magnetic
traps. We demonstrate for the 87-Rb and 23-Na F=2 trapping states with wave
packet simulations how the cooling stops when the rf field frequency goes below
a certain limit (for the 85-Rb F=2 trapping state the problem does not appear).
We examine the applicability of semiclassical models for the strong field case
as an extension of our previous work [Phys. Rev. A 58, 3983 (1998)]. Our
results verify many of the aspects observed in a recent Rb experiment
[Phys. Rev. A 60, R1759 (1999)].Comment: 9 pages, RevTex, eps figures embedde
Zero-temperature phase diagram of binary boson-fermion mixtures
We calculate the phase diagram for dilute mixtures of bosons and fermions at
zero temperature. The linear stability conditions are derived and related to
the effective boson-induced interaction between the fermions. We show that in
equilibrium there are three possibilities: a) a single uniform phase, b) a
purely fermionic phase coexisting with a purely bosonic one and c) a purely
fermionic phase coexisting with a mixed phase.Comment: 8 pages, revtex, 3 postscript figures; NORDITA-1999/71 C
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