1,166 research outputs found
Limitations of light delay and storage times in EIT experiments with condensates
We investigate the limitations arising from atomic collisions on the storage
and delay times of probe pulses in EIT experiments. We find that the atomic
collisions can be described by an effective decay rate that limits storage and
delay times. We calculate the momentum and temperature dependence of the decay
rate and find that it is necessary to excite atoms at a particular momentum
depending on temperature and spacing of the energy levels involved in order to
minimize the decoherence effects of atomic collisions.Comment: 4 pages RevTeX, 4 figures. Send correspondence to
[email protected]
Fish schooling as a basis for vertical axis wind turbine farm design
Most wind farms consist of horizontal axis wind turbines (HAWTs) due to the
high power coefficient (mechanical power output divided by the power of the
free-stream air through the turbine cross-sectional area) of an isolated
turbine. However when in close proximity to neighbouring turbines, HAWTs suffer
from a reduced power coefficient. In contrast, previous research on vertical
axis wind turbines (VAWTs) suggests that closely-spaced VAWTs may experience
only small decreases (or even increases) in an individual turbine's power
coefficient when placed in close proximity to neighbours, thus yielding much
higher power outputs for a given area of land. A potential flow model of
inter-VAWT interactions is developed to investigate the effect of changes in
VAWT spatial arrangement on the array performance coefficient, which compares
the expected average power coefficient of turbines in an array to a
spatially-isolated turbine. A geometric arrangement based on the configuration
of shed vortices in the wake of schooling fish is shown to significantly
increase the array performance coefficient based upon an array of 16x16 wind
turbines. Results suggest increases in power output of over one order of
magnitude for a given area of land as compared to HAWTs.Comment: Submitted for publication in BioInspiration and Biomimetics. Note:
The technology described in this paper is protected under both US and
international pending patents filed by the California Institute of Technolog
The Utilization of Stored Soil Moistures for Forage Legumes Supply in the Dry Season in West Timor, Indonesia
Galaxy Candidates in the Zone of Avoidance
Motivated by recent discoveries of nearby galaxies in the Zone of Avoidance,
we conducted a pilot study of galaxy candidates at low Galactic latitude, near
Galactic longitude , where the Supergalactic Plane is crossed by
the Galactic Plane. We observed with the 1m Wise Observatory in the I-band 18
of the `promising' candidates identified by visual examination of Palomar red
plates by Hau et al. (1995). A few candidates were also observed in R or B
bands, or had spectroscopic observations performed at the Isaac Newton
Telescope and at the Wise Observatory. Our study suggests that there are
probably 10 galaxies in this sample. We also identify a probable Planetary
Nebula. The final confirmation of the nature of these sources must await the
availability of full spectroscopic information. The success rate of
in identifying galaxies at Galactic latitude indicates that the
ZOA is a bountiful region to discover new galaxies.Comment: 11 pages; Latex + 5 figures (gif format), Submitted to MNRA
Dark states of dressed Bose-Einstein condensates
We combine the ideas of dressed Bose-Einstein condensates, where an
intracavity optical field allows one to design coupled, multicomponent
condensates, and of dark states of quantum systems, to generate a full quantum
entanglement between two matter waves and two optical waves. While the matter
waves are macroscopically populated, the two optical modes share a single
photon. As such, this system offers a way to influence the behaviour of a
macroscopic quantum system via a microscopic ``knob''.Comment: 6 pages, no figur
Theory of Diluted Magnetic Semiconductor Ferromagnetism
We present a theory of carrier-induced ferromagnetism in diluted magnetic
semiconductors (III_{1-x} Mn_x V) which allows for arbitrary itinerant-carrier
spin polarization and dynamic correlations. Both ingredients are essential in
identifying the system's elementary excitations and describing their
properties. We find a branch of collective modes, in addition to the spin waves
and Stoner continuum which occur in metallic ferromagnets, and predict that the
low-temperature spin stiffness is independent of the strength of the exchange
coupling between magnetic ions and itinerant carriers. We discuss the
temperature dependence of the magnetization and the heat capacity
Is NGC 3108 transforming itself from an early to late type galaxy -- an astronomical hermaphrodite?
A common feature of hierarchical galaxy formation models is the process of
"inverse" morphological transformation: a bulge dominated galaxy accretes a gas
disk, dramatically reducing the system's bulge-to-disk mass ratio. During their
formation, present day galaxies may execute many such cycles across the Hubble
diagram. A good candidate for such a "hermaphrodite" galaxy is NGC 3108: a
dust-lane early-type galaxy which has a large amount of HI gas distributed in a
large scale disk. We present narrow band H_alpha and R-band imaging, and
compare the results with the HI distribution. The emission is in two
components: a nuclear bar and an extended disk component which coincides with
the HI distribution. This suggests that a stellar disk is currently being
formed out of the HI gas. The spatial distributions of the H_alpha and HI
emission and the HII regions are consistent with a barred spiral structure,
extending some 20 kpc in radius. We measure an extinction- corrected SFR of
0.42 Msun/yr. The luminosity function of the HII regions is similar to other
spiral galaxies, with a power law index of -2.1, suggesting that the star
formation mechanism is similar to other spiral galaxies. We measured the
current disk mass and find that it is too massive to have been formed by the
current SFR over the last few Gyr. It is likely that the SFR in NGC 3108 was
higher in the past. With the current SFR, the disk in NGC 3108 will grow to be
~6.2x10^9 Msun in stellar mass within the next 5.5 Gyr. While this is
substantial, the disk will be insignificant compared with the large bulge mass:
the final stellar mass disk-to-bulge ratio will be ~0.02. NGC 3108 will fail to
transform into anything resembling a spiral without a boost in the SFR and
additional supply of gas.Comment: 9 pages, 3 figures, accepted for publication in MNRA
Progress in Three-Dimensional Coherent X-Ray Diffraction Imaging
The Fourier inversion of phased coherent diffraction patterns offers images
without the resolution and depth-of-focus limitations of lens-based tomographic
systems. We report on our recent experimental images inverted using recent
developments in phase retrieval algorithms, and summarize efforts that led to
these accomplishments. These include ab-initio reconstruction of a
two-dimensional test pattern, infinite depth of focus image of a thick object,
and its high-resolution (~10 nm resolution) three-dimensional image.
Developments on the structural imaging of low density aerogel samples are
discussed.Comment: 5 pages, X-Ray Microscopy 2005, Himeji, Japa
Exact Cover with light
We suggest a new optical solution for solving the YES/NO version of the Exact
Cover problem by using the massive parallelism of light. The idea is to build
an optical device which can generate all possible solutions of the problem and
then to pick the correct one. In our case the device has a graph-like
representation and the light is traversing it by following the routes given by
the connections between nodes. The nodes are connected by arcs in a special way
which lets us to generate all possible covers (exact or not) of the given set.
For selecting the correct solution we assign to each item, from the set to be
covered, a special integer number. These numbers will actually represent delays
induced to light when it passes through arcs. The solution is represented as a
subray arriving at a certain moment in the destination node. This will tell us
if an exact cover does exist or not.Comment: 20 pages, 4 figures, New Generation Computing, accepted, 200
Dark-State Polaritons in Electromagnetically Induced Transparency
We identify form-stable coupled excitations of light and matter (``dark-state
polaritons'') associated with the propagation of quantum fields in
Electromagnetically Induced Transparency. The properties of the dark-state
polaritons such as the group velocity are determined by the mixing angle
between light and matter components and can be controlled by an external
coherent field as the pulse propagates. In particular, light pulses can be
decelerated and ``trapped'' in which case their shape and quantum state are
mapped onto metastable collective states of matter. Possible applications of
this reversible coherent-control technique are discussed.Comment: 4 pages, 2 figure
- …