9,218 research outputs found
Hanbury Brown and Twiss correlations in atoms scattered from colliding condensates
Low energy elastic scattering between clouds of Bose condensed atoms leads to
the well known s-wave halo with atoms emerging in all directions from the
collision zone. In this paper we discuss the emergence of Hanbury Brown and
Twiss coincidences between atoms scattered in nearly parallel directions. We
develop a simple model that explains the observations in terms of an
interference involving two pairs of atoms each associated with the elementary s
wave scattering process.Comment: Minor corrections. reference update
Two-body anticorrelation in a harmonically trapped ideal Bose gas
We predict the existence of a dip below unity in the second-order coherence
function of a partially condensed ideal Bose gas in harmonic confinement,
signaling the anticorrelation of density fluctuations in the sample. The dip in
the second-order coherence function is revealed in a canonical-ensemble
calculation, corresponding to a system with fixed total number of particles. In
a grand-canonical ensemble description, this dip is obscured by the
occupation-number fluctuation catastrophe of the ideal Bose gas. The
anticorrelation is most pronounced in highly anisotropic trap geometries
containing small particle numbers. We explain the fundamental physical
mechanism which underlies this phenomenon, and its relevance to experiments on
interacting Bose gases.Comment: 10 pages, 5 figures. v2: Minor changes and corrections to figures and
text. To appear in PR
Competing interactions in artificial spin chains
The low-energy magnetic configurations of artificial frustrated spin chains
are investigated using magnetic force microscopy and micromagnetic simulations.
Contrary to most studies on two-dimensional artificial spin systems where
frustration arises from the lattice geometry, here magnetic frustration
originates from competing interactions between neighboring spins. By tuning
continuously the strength and sign of these interactions, we show that
different magnetic phases can be stabilized. Comparison between our
experimental findings and predictions from the one-dimensional Anisotropic
Next-Nearest-Neighbor Ising (ANNNI) model reveals that artificial frustrated
spin chains have a richer phase diagram than initially expected. Besides the
observation of several magnetic orders and the potential extension of this work
to highly-degenerated artificial spin chains, our results suggest that the
micromagnetic nature of the individual magnetic elements allows observation of
metastable spin configurations.Comment: 5 pages, 4 figure
Spatial distribution of unidentified infrared bands and extended red emission in the compact galactic HII region Sh 152
We present visible and near IR images of the compact HII region Sh 152. Some
of these images reveal the presence of Extended Red Emission (ERE) around 698
nm and emission from Unidentified Infra Red Bands (UIRBs) at 3.3 and 6.2
micron. Other images show the near infrared (7-12 micron) continuous emission
of the nebula. The ERE emission is found to coincide with the ionized region
and significantly differ from the UIRBs location. Also some evidence is found
in favor of grains as carriers for ERE.Comment: 3 pages, 4 figures, to be published in the proceedings of the
colloquium "The universe as seen by ISO" help in Paris, October 20-23, 1998 ;
available in html format at http://www.obs-hp.fr/preprints.htm
Evaporative cooling in a radio-frequency trap
A theoretical investigation for implementing a scheme of forced evaporative
cooling in radio-frequency (rf) adiabatic potentials is presented. Supposing
the atoms to be trapped by a rf field RF1, the cooling procedure is facilitated
using a second rf source RF2. This second rf field produces a controlled
coupling between the spin states dressed by RF1. The evaporation is then
possible in a pulsed or continuous mode. In the pulsed case, atoms with a given
energy are transferred into untrapped dressed states by abruptly switching off
the interaction. In the continuous case, it is possible for energetic atoms to
adiabatically follow the doubly-dressed states and escape out of the trap. Our
results also show that when the frequencies of the fields RF1 and RF2 are
separated by at least the Rabi frequency associated with RF1, additional
evaporation zones appear which can make this process more efficient.Comment: 12 pages, 11 figure
Three-dimensional Gross-Pitaevskii solitary waves in optical lattices: stabilization using the artificial quartic kinetic energy induced by lattice shaking
In this Letter, we show that a three-dimensional Bose-Einstein solitary wave
can become stable if the dispersion law is changed from quadratic to quartic.
We suggest a way to realize the quartic dispersion, using shaken optical
lattices. Estimates show that the resulting solitary waves can occupy as little
as -th of the Brillouin zone in each of the three directions and
contain as many as atoms, thus representing a \textit{fully
mobile} macroscopic three-dimensional object.Comment: 8 pages, 1 figure, accepted in Phys. Lett.
An Example of Quantum Anomaly in the Physics of Ultra-Cold Gases
In this article, we propose an experimental scheme for observation of a
quantum anomaly---quantum-mechanical symmetry breaking---in a two-dimensional
harmonically trapped Bose gas. The anomaly manifests itself in a shift of the
monopole excitation frequency away from the value dictated by the
Pitaevskii-Rosch dynamical symmetry [L. P. Pitaevskii and A. Rosch, Phys. Rev.
A, 55, R853 (1997)]. While the corresponding classical Gross-Pitaevskii
equation and the derived from it hydrodynamic equations do exhibit this
symmetry, it is---as we show in our paper---violated under quantization. The
resulting frequency shift is of the order of 1% of the carrier, well in reach
for modern experimental techniques. We propose using the dipole oscillations as
a frequency gauge.Comment: Misprints corrected, a discussion on damping added, text is polished
and shortened. 5 pages, 1 figur
CubeSats as pathfinders for planetary detection: the FIRST-S satellite
The idea behind FIRST (Fibered Imager foR a Single Telescope) is to use
single-mode fibers to combine multiple apertures in a pupil plane as such as to
synthesize a bigger aperture. The advantages with respect to a pure imager are
i) relaxed tolerance on the pointing and cophasing, ii) higher accuracy in
phase measurement, and iii) availability of compact, precise, and active
single-mode optics like Lithium Niobate. The latter point being a huge asset in
the context of a space mission. One of the problems of DARWIN or SIM-like
projects was the difficulty to find low cost pathfinders missions. But the fact
that Lithium Niobate optic is small and compact makes it easy to test through
small nanosats missions. Moreover, they are commonly used in the telecom
industry, and have already been tested on communication satellites. The idea of
the FIRST-S demonstrator is to spatialize a 3U CubeSat with a Lithium Niobate
nulling interferometer. The technical challenges of the project are: star
tracking, beam combination, and nulling capabilities. The optical baseline of
the interferometer would be 30 cm, giving a 2.2 AU spatial resolution at
distance of 10 pc. The scientific objective of this mission would be to study
the visible emission of exozodiacal light in the habitable zone around the
closest stars.Comment: SPIE 2014 -- Astronomical telescopes and instrumentation -- Montrea
Observation of atom pairs in spontaneous four wave mixing of two colliding Bose-Einstein Condensates
We study atom scattering from two colliding Bose-Einstein condensates using a
position sensitive, time resolved, single atom detector. In analogy to quantum
optics, the process can also be thought of as spontaneous, degenerate four wave
mixing of de Broglie waves. We find a clear correlation between atoms with
opposite momenta, demonstrating pair production in the scattering process. We
also observe a Hanbury Brown and Twiss correlation for collinear momenta, which
permits an independent measurement of the size of the pair production source
and thus the size of the spatial mode. The back to back pairs occupy very
nearly two oppositely directed spatial modes, a promising feature for future
quantum optics experiments.Comment: A few typos have been correcte
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