197 research outputs found
Does matter wave amplification work for fermions?
We discuss the relationship between bosonic stimulation, density
fluctuations, and matter wave gratings. It is shown that enhanced stimulated
scattering, matter wave amplification and atomic four-wave mixing are in
principle possible for fermionic or non-degenerate samples if they are prepared
in a cooperative state. In practice, there are limitations by short coherence
times.Comment: 5 pages, 1 figure
Temperature rise measurement for power-loss comparison of an aluminum electrolytic capacitor between sinusoidal and square-wave current injections
DC-link capacitors are a major factor of degrading reliability of power electric converters because they usually have a shorter lifetime and higher failure rate than those of semiconductor devices or magnetic devices. Characteristics of the capacitors are usually evaluated by a single sinusoidal current waveform. However, actual current flowing out of the converter into the capacitor is a modulated square current waveform. This paper provides experimental comparison of the power loss dissipated in an aluminum electrolytic capacitor between sinusoidal and square-wave current injections. Power loss is estimated by temperature rise of the capacitor. Experimental results confirm that power losses of the square-wave current injection were always lower than those of the sinusoidal current injection by 10–20%. Moreover, the power losses of the square-wave current injection can be estimated by a synthesis of fundamental and harmonic currents based on the Fourier series expansion, which brings a high accuracy less than 1% when more than fifth harmonic current is introduced. This comparison will be useful for estimating power loss and life time of electrolytic capacitors
Spontaneous emission of atoms via collisions of Bose-Einstein condensates
The widely used Gross-Pitaevskii equation treats only coherent aspects of the
evolution of a Bose-Einstein condensate. However, inevitably some atoms scatter
out of the condensate. We have developed a method, based on the field theory
formulation, describing the dynamics of incoherent processes which are due to
elastic collisions. We can therefore treat processes of spontaneous emission of
atoms into the empty modes, as opposed to stimulated processes, which require
non-zero initial occupation.
In this article we study two counter-propagating plane waves of atoms,
calculating the full dynamics of mode occupation, as well as the statistics of
scattered atoms. The more realistic case of Gaussian wavepackets is also
analyzed.Comment: 5 pages, 2 figure
Mach-Zehnder Bragg interferometer for a Bose-Einstein Condensate
We construct a Mach-Zehnder interferometer using Bose-Einstein condensed
rubidium atoms and optical Bragg diffraction. In contrast to interferometers
based on normal diffraction, where only a small percentage of the atoms
contribute to the signal, our Bragg diffraction interferometer uses all the
condensate atoms. The condensate coherence properties and high phase-space
density result in an interference pattern of nearly 100% contrast. In
principle, the enclosed area of the interferometer may be arbitrarily large,
making it an ideal tool that could be used in the detection of vortices, or
possibly even gravitational waves.Comment: 10 pages, 3 figures, Quantum Electronics and Laser Science Conference
1999, Postdeadline papers QPD12-
Sequential superradiant scattering from atomic Bose-Einstein condensates
We theoretically discuss several aspects of sequential superradiant
scattering from atomic Bose-Einstein condensates. Our treatment is based on the
semiclassical description of the process in terms of the Maxwell-Schroedinger
equations for the coupled matter-wave and optical fields. First, we investigate
sequential scattering in the weak-pulse regime and work out the essential
mechanisms responsible for bringing about the characteristic fan-shaped
side-mode distribution patterns. Second, we discuss the transition between the
Kapitza-Dirac and Bragg regimes of sequential scattering in the strong-pulse
regime. Finally, we consider the situation where superradiance is initiated by
coherently populating an atomic side mode through Bragg diffraction, as in
studies of matter-wave amplification, and describe the effect on the sequential
scattering process.Comment: 9 pages, 4 figures. Submitted to Proceedings of LPHYS'06 worksho
Dynamics of Fermionic Four-Wave Mixing
We study the dynamics of a beam of fermions diffracted off a density grating
formed by fermionic atoms in the limit of a large grating. An exact description
of the system in terms of particle-hole operators is developed. We use a
combination of analytical and numerical methods to quantitatively explore the
Raman-Nath and the Bragg regimes of diffraction. We discuss the limits in
diffraction efficiency resulting from the dephasing of the grating due the
distribution of energy states occupied by the fermions. We propose several
methods to overcome these limits, including the novel technique of ``atom
echoes''.Comment: 8 pages, 7 figure
Recoil-Induced-Resonances in Nonlinear, Ground-State, Pump-Probe Spectroscopy
A theory of pump-probe spectroscopy is developed in which optical fields
drive two-photon Raman transitions between ground states of an ensemble of
three-level atoms. Effects related to the recoil the atoms undergo
as a result of their interactions with the fields are fully accounted for in
this theory. The linear absorption coefficient of a weak probe field in the
presence of two pump fields of arbitrary strength is calculated. For subrecoil
cooled atoms, the spectrum consists of eight absorption lines and eight
emission lines. In the limit that , where and
are the Rabi frequencies of the two pump fields, one recovers the
absorption spectrum for a probe field interacting with an effective two-level
atom in the presence of a single pump field. However when , new interference effects arise that allow one to selectively turn on
and off some of these recoil induced resonances.Comment: 30 pages, 8 figures. RevTex. Submitted to Phys. Rev. A, Revised
versio
Momentum transferred to a trapped Bose-Einstein condensate by stimulated light scattering
The response of a trapped Bose-Einstein condensed gas to a density
perturbation generated by a two-photon Bragg pulse is investigated by solving
the time-dependent Gross-Pitaevskii equation. We calculate the total momentum
imparted to the condensate as a function of both the time duration of the pulse
and the frequency difference of the two laser beams. The role of the dynamic
response function in characterizing the time evolution of the system is pointed
out, with special emphasis to the phonon regime. Numerical simulations are
compared with the predictions of local density approximation. The relevance of
our results for the interpretation of current experiments is also discussed.Comment: 7 pages, 3 postscript figure
Coherence properties of an atom laser
We study the coherence properties of an atom laser, which operates by
extracting atoms from a gaseous Bose-Einstein condensate via a two-photon Raman
process, by analyzing a recent experiment. We obtain good agreement with the
experimental data by solving the time-dependent Gross-Pitaevskii equation in
three dimensions both numerically and with a Thomas-Fermi model. The coherence
length is strongly affected by the space-dependent phase developed by the
condensate when the trapping potential is turned off.Comment: 11 pages, 2 Postscript figure
Atomic interaction effects in the superradiant light scattering from a Bose-Einstein condensate
We investigate the effects of the atomic interaction in the Superradiant
Rayleigh scattering from a Bose-Einstein condensate driven by a far-detuned
laser beam. We show that for a homogeneous atomic sample the atomic interaction
has only a dispersive effect, whereas in the inhomogeneous case it may increase
the decay of the matter-wave grating.Comment: 12 pages, 4 figures, presented to the XII International Laser Physics
Workshop, August 24-29, Hamburg, to be published in Laser Physic
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