13,858 research outputs found
Vibrations of a Columnar Vortex in a Trapped Bose-Einstein Condensate
We derive a governing equation for a Kelvin wave supported on a vortex line
in a Bose-Einstein condensate, in a rotating cylindrically symmetric parabolic
trap. From this solution the Kelvin wave dispersion relation is determined. In
the limit of an oblate trap and in the absence of longitudinal trapping our
results are consistent with previous work. We show that the derived Kelvin wave
dispersion in the general case is in quantitative agreement with numerical
calculations of the Bogoliubov spectrum and offer a significant improvement
upon previous analytical work.Comment: 5 pages with 1 figur
Quantum Analogue Computing
We briefly review what a quantum computer is, what it promises to do for us,
and why it is so hard to build one. Among the first applications anticipated to
bear fruit is quantum simulation of quantum systems. While most quantum
computation is an extension of classical digital computation, quantum
simulation differs fundamentally in how the data is encoded in the quantum
computer. To perform a quantum simulation, the Hilbert space of the system to
be simulated is mapped directly onto the Hilbert space of the (logical) qubits
in the quantum computer. This type of direct correspondence is how data is
encoded in a classical analogue computer. There is no binary encoding, and
increasing precision becomes exponentially costly: an extra bit of precision
doubles the size of the computer. This has important consequences for both the
precision and error correction requirements of quantum simulation, and
significant open questions remain about its practicality. It also means that
the quantum version of analogue computers, continuous variable quantum
computers (CVQC) becomes an equally efficient architecture for quantum
simulation. Lessons from past use of classical analogue computers can help us
to build better quantum simulators in future.Comment: 10 pages, to appear in the Visions 2010 issue of Phil. Trans. Roy.
Soc.
Velocity, energy and helicity of vortex knots and unknots
In this paper we determine the velocity, the energy and estimate writhe and
twist helicity contributions of vortex filaments in the shape of torus knots
and unknots (toroidal and poloidal coils) in a perfect fluid. Calculations are
performed by numerical integration of the Biot-Savart law. Vortex complexity is
parametrized by the winding number , given by the ratio of the number of
meridian wraps to that of the longitudinal wraps. We find that for vortex
knots and toroidal coils move faster and carry more energy than a reference
vortex ring of same size and circulation, whereas for knots and poloidal
coils have approximately same speed and energy of the reference vortex ring.
Helicity is dominated by the writhe contribution. Finally, we confirm the
stabilizing effect of the Biot-Savart law for all knots and unknots tested,
that are found to be structurally stable over a distance of several diameters.
Our results also apply to quantized vortices in superfluid He.Comment: 17 pages, 8 figures, 2 table
Wind measurement system
A system for remotely measuring vertical and horizontal winds present in discrete volumes of air at selected locations above the ground is described. A laser beam is optically focused in range by a telescope, and the output beam is conically scanned at an angle about a vertical axis. The backscatter, or reflected light, from the ambient particulates in a volume of air, the focal volume, is detected for shifts in wavelength, and from these, horizontal and vertical wind components are computed
Calculation of Elastic Green's Functions for Lattices with Cavities
In this Brief Report, we present an algorithm for calculating the elastic
Lattice Greens Function of a regular lattice, in which defects are created by
removing lattice points. The method is computationally efficient, since the
required matrix operations are on matrices that scale with the size of the
defect subspace, and not with the size of the full lattice. This method allows
the treatment of force fields with multi-atom interactions.Comment: 3 pages. RevTeX, using epsfig.sty. One figur
Influence of FK 506 (tacrolimus) on circulating CD4 <sup>+</sup> t cells expressing cd25 and cd45ra antigens in 19 patients with chronic progressive multiple sclerosis participating in an open label drug safety trial
We have taken the opportunity of a clinical trial of the potential efficacy and safety of FK 506 (tacrolimus) in chronic progressive multiple sclerosis (MS) to examine the influence of this potent new immunosuppressant on circulating T-lymphocytes in an otherwise healthy non-transplant population. Peripheral blood levels of subsets of CD4+ T lymphocytes expressing the activation molecule interleukin-2 receptor (p55 α chain; CD25) or the CD45RA isoform were determined sequentially in 19 patients that were treated continuously with oral FK 506 (starting dose 0.15 mg/kg/day) for 12 months. No significant change in the proportion of circulating CD25 + CD4+ cells was observed over the study period in which the mean trough plasma FK 506 level rose from 0.3 ±0.2 to 0.5 ±0.4 ng/ml. There was also no significant effect of FK 506 on the percentage of CD45RA + CD4 + cells in the peripheral blood at 12 months compared with pretreatment values. Analysis of a subgroup of 7 patients, who showed a sustained reduction in CD25 + CD4+ cells and a reciprocal increase in CD45RA* CD4 * cells for at least 6 months after start of treatment, did not reveal any difference in disability at one year compared with the treatment group as a whole. The side effects of FK 506 were mild and the overall degree of disability estimated by the mean Kurtzke expanded disability status scale (EDSS) score or the ambulation index did not deteriorate significantly in the 19 patients studied over the 12 months of FK 506 administration. © 1994 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted
Anomalous translational velocity of vortex ring with finite-amplitude Kelvin waves
We consider finite-amplitude Kelvin waves on an inviscid vortex assuming that
the vortex core has infinitesimal thickness. By numerically solving the
governing Biot-Savart equation of motion, we study how the frequency of the
Kelvin waves and the velocity of the perturbed ring depend on the Kelvin wave
amplitude. In particular, we show that, if the amplitude of the Kelvin waves is
sufficiently large, the perturbed vortex ring moves backwards.Comment: 6 pages, 5 figures, v2: minor changes, v3: typos correcte
The 2000 outburst of the recurrent nova CI Aquilae: optical spectroscopy
We present low- and medium resolution spectra of the recurrent nova CI
Aquilae taken at 14 epochs in May and June, 2000. The overall appearance is
similar to other U Sco-type recurrent novae (U Sco, V394 CrA). Medium
resolution (R=7000-10000) hydrogen and iron profiles suggest an early expansion
velocity of 2000-2500 km/s. The H\alpha evolution is followed from Dt = -0.6 d
to +53 d, starting from a nearly Gaussian shape to a double peaked profile
through strong P-Cyg profiles. The interstellar component of the sodium D line
and two diffuse interstellar bands put constraints on the interstellar
reddening which is estimated to be E(B-V)=0.85\pm0.3. The available visual and
CCD-V observations are used to determine t0,t2 and t3. The resulting parameters
are: t0=2451669.5\pm0.1, t2=30\pm1 d, t3=36\pm1 d. The recent lightcurve is
found to be generally similar to that observed in 1917 with departures as large
as 1-2 mag in certain phases. This behaviour is also typical for the U Sco
subclass.Comment: 8 pages, 7 figures, accepted for publication in A&
A Note on Tachyons in the System
The periodic bounce of Born-Infeld theory of -branes is derived, and the
BPS limit of infinite period is discussed as an example of tachyon
condensation. The explicit bounce solution to the Born--Infeld action is
interpreted as an unstable fundamental string stretched between the brane and
its antibrane.Comment: 10 pages, 2 figures. v2: minor changes, acknowledgement added; v3:
explanations and references added. Final version to appear in Mod. Phys.
Lett.
Sampling properties of directed networks
For many real-world networks only a small "sampled" version of the original
network may be investigated; those results are then used to draw conclusions
about the actual system. Variants of breadth-first search (BFS) sampling, which
are based on epidemic processes, are widely used. Although it is well
established that BFS sampling fails, in most cases, to capture the
IN-component(s) of directed networks, a description of the effects of BFS
sampling on other topological properties are all but absent from the
literature. To systematically study the effects of sampling biases on directed
networks, we compare BFS sampling to random sampling on complete large-scale
directed networks. We present new results and a thorough analysis of the
topological properties of seven different complete directed networks (prior to
sampling), including three versions of Wikipedia, three different sources of
sampled World Wide Web data, and an Internet-based social network. We detail
the differences that sampling method and coverage can make to the structural
properties of sampled versions of these seven networks. Most notably, we find
that sampling method and coverage affect both the bow-tie structure, as well as
the number and structure of strongly connected components in sampled networks.
In addition, at low sampling coverage (i.e. less than 40%), the values of
average degree, variance of out-degree, degree auto-correlation, and link
reciprocity are overestimated by 30% or more in BFS-sampled networks, and only
attain values within 10% of the corresponding values in the complete networks
when sampling coverage is in excess of 65%. These results may cause us to
rethink what we know about the structure, function, and evolution of real-world
directed networks.Comment: 21 pages, 11 figure
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