23,426 research outputs found

### Hybrid HVDC for supply of power to offshore oil platforms

A HVDC hybrid system, comprising a line commutated thyristor HVDC converter and a STATCOM, is proposed in this paper for supplying power to offshore oil platforms that do not have their own generation. The proposed system combines the robust performance, low capital cost and low power loss of a line commutated HVDC converter, with the fast dynamic performance of an equivalent VSC Transmission system. The paper describes the principles and control strategies of the proposed system. PSCAD/EMTDC simulations are presented to demonstrate the robust performance of the system using case studies of various operating conditions such as black-start, load perturbations, AC fault conditions and disturbance caused by the starting of large local induction machines

### An integrated source of broadband quadrature squeezed light

An integrated silicon nitride resonator is proposed as an ultra-compact
source of bright single-mode quadrature squeezed light at 850 nm. Optical
properties of the device are investigated and tailored through numerical
simulations, with particular attention paid to loss associated with interfacing
the device. An asymmetric double layer stack waveguide geometry with inverse
vertical tapers is proposed for efficient and robust fibre-chip coupling,
yielding a simulated total loss of -0.75 dB/facet. We assess the feasibility of
the device through a full quantum noise analysis and derive the output
squeezing spectrum for intra-cavity pump self-phase modulation. Subject to
standard material loss and detection efficiencies, we find that the device
holds promises for generating substantial quantum noise squeezing over a
bandwidth exceeding 1 GHz. In the low-propagation loss regime, approximately -7
dB squeezing is predicted for a pump power of only 50 mW.Comment: 23 pages, 12 figure

### Acoustical modeling study of the open test section of the NASA Langley V/STOL wind tunnel

An acoustic model study was carried out to identify effective sound absorbing treatment of strategically located surfaces in an open wind tunnel test section. Also an aerodynamic study done concurrently, sought to find measures to control low frequency jet pulsations which occur when the tunnel is operated in its open test section configuration. The acoustical modeling study indicated that lining of the raised ceiling and the test section floor immediately below it, results in a substantial improvement. The aerodynamic model study indicated that: (1) the low frequency jet pulsations are most likely caused or maintained by coupling of aerodynamic and aeroacoustic phenomena in the closed tunnel circuit, (2) replacing the hard collector cowl with a geometrically identical but porous fiber metal surface of 100 mks rayls flow resistance does not result in any noticable reduction of the test section noise caused by the impingement of the turbulent flow on the cowl

### Measurement-induced macroscopic superposition states in cavity optomechanics

We present a novel proposal for generating quantum superpositions of
macroscopically distinct states of a bulk mechanical oscillator, compatible
with existing optomechanical devices operating in the readily achievable
bad-cavity limit. The scheme is based on a pulsed cavity optomechanical quantum
non-demolition (QND) interaction, driven by displaced non-Gaussian states, and
measurement-induced feedback, avoiding the need for strong single-photon
optomechanical coupling. Furthermore, we show that single-quadrature cooling of
the mechanical oscillator is sufficient for efficient state preparation, and we
outline a three-pulse protocol comprising a sequence of QND interactions for
squeezing-enhanced cooling, state preparation, and tomography.Comment: 7 pages, 5 figure

### A Hybrid Long-Distance Entanglement Distribution Protocol

We propose a hybrid (continuous-discrete variable) quantum repeater protocol
for distribution of entanglement over long distances. Starting from entangled
states created by means of single-photon detection, we show how entangled
coherent state superpositions, also known as `Schr\"odinger cat states', can be
generated by means of homodyne detection of light. We show that
near-deterministic entanglement swapping with such states is possible using
only linear optics and homodyne detectors, and we evaluate the performance of
our protocol combining these elements.Comment: 4 pages, 3 figure

### More Torsion in the Homology of the Matching Complex

A matching on a set $X$ is a collection of pairwise disjoint subsets of $X$
of size two. Using computers, we analyze the integral homology of the matching
complex $M_n$, which is the simplicial complex of matchings on the set $\{1,
>..., n\}$. The main result is the detection of elements of order $p$ in the
homology for $p \in \{5,7,11,13\}$. Specifically, we show that there are
elements of order 5 in the homology of $M_n$ for $n \ge 18$ and for $n \in
{14,16}$. The only previously known value was $n = 14$, and in this particular
case we have a new computer-free proof. Moreover, we show that there are
elements of order 7 in the homology of $M_n$ for all odd $n$ between 23 and 41
and for $n=30$. In addition, there are elements of order 11 in the homology of
$M_{47}$ and elements of order 13 in the homology of $M_{62}$. Finally, we
compute the ranks of the Sylow 3- and 5-subgroups of the torsion part of
$H_d(M_n;Z)$ for $13 \le n \le 16$; a complete description of the homology
already exists for $n \le 12$. To prove the results, we use a
representation-theoretic approach, examining subcomplexes of the chain complex
of $M_n$ obtained by letting certain groups act on the chain complex.Comment: 35 pages, 10 figure

### The role of binaries in the enrichment of the early Galactic halo. I. r-process-enhanced metal-poor stars

The detailed chemical composition of most metal-poor halo stars has been
found to be highly uniform, but a minority of stars exhibit dramatic
enhancements in their abundances of heavy neutron-capture elements and/or of
carbon. The key question for Galactic chemical evolution models is whether
these peculiarities reflect the composition of the natal clouds, or if they are
due to later mass transfer of processed material from a binary companion. If
the former case applies, the observed excess of certain elements was implanted
within selected clouds in the early ISM from a production site at interstellar
distances. Our aim is to determine the frequency and orbital properties of
binaries among these chemically peculiar stars. This information provides the
basis for deciding whether mass transfer from a binary companion is necessary
and sufficient to explain their unusual compositions. This paper discusses our
study of a sample of 17 moderately (r-I) and highly (r-II) r-process-element
enhanced VMP and EMP stars. High-resolution, low signal-to-noise spectra of the
stars were obtained at roughly monthly intervals over 8 years with the FIES
spectrograph at the Nordic Optical Telescope. From these spectra, radial
velocities with an accuracy of ~100 m/s were determined by cross-correlation
against an optimized template. 14 of the programme stars exhibit no significant
RV variation over this period, while 3 are binaries with orbits of typical
eccentricity for their periods, resulting in a normal binary frequency of
~18+-6% for the sample. Our results confirm our preliminary conclusion from
2011, based on partial data, that the chemical peculiarity of the r-I and r-II
stars is not caused by any putative binary companions. Instead, it was
imprinted on the natal molecular clouds of these stars by an external, distant
source. Models of the ISM in early galaxies should account for such mechanisms.Comment: 14 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic

### Imprint of Gravitational Lensing by Population III Stars in Gamma Ray Burst Light Curves

We propose a novel method to extract the imprint of gravitational lensing by
Pop III stars in the light curves of Gamma Ray Bursts (GRBs). Significant
portions of GRBs can originate in hypernovae of Pop III stars and be
gravitationally lensed by foreground Pop III stars or their remnants. If the
lens mass is on the order of $10^2-10^3M_\odot$ and the lens redshift is
greater than 10, the time delay between two lensed images of a GRB is $\approx
1$s and the image separation is $\approx 10 \mu$as. Although it is difficult to
resolve the two lensed images spatially with current facilities, the light
curves of two images are superimposed with a delay of $\approx 1$ s. GRB light
curves usually exhibit noticeable variability, where each spike is less than
1s. If a GRB is lensed, all spikes are superimposed with the same time delay.
Hence, if the autocorrelation of light curve with changing time interval is
calculated, it should show the resonance at the time delay of lensed images.
Applying this autocorrelation method to GRB light curves which are archived as
the {\it BATSE} catalogue, we demonstrate that more than half light curves can
show the recognizable resonance, if they are lensed. Furthermore, in 1821 GRBs
we actually find one candidate of GRB lensed by a Pop III star, which may be
located at redshift 20-200. The present method is quite straightforward and
therefore provides an effective tool to search for Pop III stars at redshift
greater than 10. Using this method, we may find more candidates of GRBs lensed
by Pop III stars in the data by the {\it Swift} satellite.Comment: 13 pages, 13 figures, accepted for publication in Ap

### Observation of twin beam correlations and quadrature entanglement by frequency doubling in a two-port resonator

We demonstrate production of quantum correlated and entangled beams by second
harmonic generation in a nonlinear resonator with two output ports. The output
beams at wavelength 428.5 nm exhibit 0.9 dB of nonclassical intensity
correlations and 0.3 dB of entanglement.Comment: 5 pages, 7 figure

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