12,268 research outputs found
Long-Term Stability Analysis of Power Systems with Wind Power Based on Stochastic Differential Equations: Model Development and Foundations
In this paper, the variable wind power is incorporated into the dynamic model
for long-term stability analysis. A theory-based method is proposed for power
systems with wind power to conduct long-term stability analysis, which is able
to provide accurate stability assessments with fast simulation speed.
Particularly, the theoretical foundation for the proposed approximation
approach is presented. The accuracy and efficiency of the method are
illustrated by several numerical examples.Comment: The paper has been submitted to IEEE Transactions on Sustainable
Energ
A Framework for Dynamic Stability Analysis of Power Systems with Volatile Wind Power
We propose a framework employing stochastic differential equations to
facilitate the long-term stability analysis of power grids with intermittent
wind power generations. This framework takes into account the discrete dynamics
which play a critical role in the long-term stability analysis, incorporates
the model of wind speed with different probability distributions, and also
develops an approximation methodology (by a deterministic hybrid model) for the
stochastic hybrid model to reduce the computational burden brought about by the
uncertainty of wind power. The theoretical and numerical studies show that a
deterministic hybrid model can provide an accurate trajectory approximation and
stability assessments for the stochastic hybrid model under mild conditions. In
addition, we discuss the critical cases that the deterministic hybrid model
fails and discover that these cases are caused by a violation of the proposed
sufficient conditions. Such discussion complements the proposed framework and
methodology and also reaffirms the importance of the stochastic hybrid model
when the system operates close to its stability limit.Comment: The paper has been accepted by IEEE Journal on Emerging and Selected
Topics in Circuits and System
Systematic Construction of tight-binding Hamiltonians for Topological Insulators and Superconductors
A remarkable discovery in recent years is that there exist various kinds of
topological insulators and superconductors characterized by a periodic table
according to the system symmetry and dimensionality. To physically realize
these peculiar phases and study their properties, a critical step is to
construct experimentally relevant Hamiltonians which support these topological
phases. We propose a general and systematic method based on the quaternion
algebra to construct the tight binding Hamiltonians for all the
three-dimensional topological phases in the periodic table characterized by
arbitrary integer topological invariants, which include the spin-singlet and
the spin-triplet topological superconductors, the Hopf and the chiral
topological insulators as particular examples. For each class, we calculate the
corresponding topological invariants through both geometric analysis and
numerical simulation.Comment: 7 pages (including supplemental material), 1 figure, 1 tabl
Hamiltonian tomography for quantum many-body systems with arbitrary couplings
Characterization of qubit couplings in many-body quantum systems is essential
for benchmarking quantum computation and simulation. We propose a tomographic
measurement scheme to determine all the coupling terms in a general many-body
Hamiltonian with arbitrary long-range interactions, provided the energy density
of the Hamiltonian remains finite. Different from quantum process tomography,
our scheme is fully scalable with the number of qubits as the required rounds
of measurements increase only linearly with the number of coupling terms in the
Hamiltonian. The scheme makes use of synchronized dynamical decoupling pulses
to simplify the many-body dynamics so that the unknown parameters in the
Hamiltonian can be retrieved one by one. We simulate the performance of the
scheme under the influence of various pulse errors and show that it is robust
to typical noise and experimental imperfections.Comment: 9 pages, 4 figures, including supplemental materia
Direct Probe of Topological Order for Cold Atoms
Cold-atom experiments in optical lattices offer a versatile platform to
realize various topological quantum phases. A key challenge in those
experiments is to unambiguously probe the topological order. We propose a
method to directly measure the characteristic topological invariants (order)
based on the time-of-flight imaging of cold atoms. The method is generally
applicable to detection of topological band insulators in one, two, or three
dimensions characterized by integer topological invariants. Using detection of
the Chern number for the 2D anomalous quantum Hall states and the Chern-Simons
term for the 3D chiral topological insulators as examples, we show that the
proposed detection method is practical, robust to typical experimental
imperfections such as limited imaging resolution, inhomogeneous trapping
potential, and disorder in the system.Comment: 10 pages, 5 figures, including Supplemental Material, version
accepted by PRA as a Rapid Communicatio
Probe of Three-Dimensional Chiral Topological Insulators in an Optical Lattice
We propose a feasible experimental scheme to realize a three-dimensional
chiral topological insulator with cold fermionic atoms in an optical lattice,
which is characterized by an integer topological invariant distinct from the
conventional topological insulators and has a remarkable macroscopic
zero-energy flat band. To probe its property, we show that its characteristic
surface states---the Dirac cones---can be probed through time-of-flight imaging
or Bragg spectroscopy and the flat band can be detected via measurement of the
atomic density profile in a weak global trap. The realization of this novel
topological phase with a flat band in an optical lattice will provide a unique
experimental platform to study the interplay between interaction and topology
and open new avenues for application of topological states.Comment: 8 pages, 6 figures, including Supplemental Material, version accepted
by PR
Optimal Contrast Greyscale Visual Cryptography Schemes with Reversing
Visual cryptography scheme (VCS) is an encryption technique that utilizes
human visual system in recovering secret image and it does not require any
complex calculation. However, the contrast of the reconstructed image could be
quite low. A number of reversing-based VCSs (or VCSs with reversing) (RVCS)
have been proposed for binary secret images, allowing participants to perform a
reversing operation on shares (or shadows). This reversing operation can be
easily implemented by current copy machines. Some existing traditional VCS
schemes without reversing (nRVCS) can be extended to RVCS with the same pixel
expansion for binary image, and the RVCS can achieve ideal contrast,
significantly higher than that of the corresponding nRVCS. In the application
of greyscale VCS, the contrast is much lower than that of the binary cases.
Therefore, it is more desirable to improve the contrast in the greyscale image
reconstruction. However, when greyscale images are involved, one cannot take
advantage of this reversing operation so easily. Many existing greyscale nRVCS
cannot be directly extended to RVCS. In this paper, we first give a new
greyscale nRVCS with minimum pixel expansion and propose an optimal-contrast
greyscale RVCS (GRVCS) by using basis matrices of perfect black nRVCS. Also, we
propose an optimal GRVCS even though the basis matrices are not perfect black.
Finally, we design an optimal-contrast GRVCS with minimum number of shares held
by each participant. The proposed schemes can satisfy different user
requirement, previous RVCSs for binary images can be viewed as special cases in
the schemes proposed here
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Regeneration of a neoartery through a completely autologous acellular conduit in a minipig model: a pilot study.
BackgroundVascular grafts are widely used as a treatment in coronary artery bypass surgery, hemodialysis, peripheral arterial bypass and congenital heart disease. Various types of synthetic and natural materials were experimented to produce tissue engineering vascular grafts. In this study, we investigated in vivo tissue engineering technology in miniature pigs to prepare decellularized autologous extracellular matrix-based grafts that could be used as vascular grafts for small-diameter vascular bypass surgery.MethodsAutologous tissue conduits (3.9 mm in diameter) were fabricated by embedding Teflon tubings in the subcutaneous pocket of female miniature pigs (n = 8, body weight 25-30 kg) for 4 weeks. They were then decellularized by CHAPS decellularization solution. Heparin was covalently-linked to decellularized tissue conduits by Sulfo-NHS/EDC. We implanted these decellularized, completely autologous extracellular matrix-based grafts into the carotid arteries of miniature pigs, then sacrificed the pigs at 1 or 2 months after implantation and evaluated the patency rate and explants histologically.ResultsAfter 1 month, the patency rate was 100% (5/5) while the inner diameter of the grafts was 3.43 ± 0.05 mm (n = 5). After 2 months, the patency rate was 67% (2/3) while the inner diameter of the grafts was 2.32 ± 0.14 mm (n = 3). Histological staining confirmed successful cell infiltration, and collagen and elastin deposition in 2-month samples. A monolayer of endothelial cells was observed along the inner lumen while smooth muscle cells were dominant in the graft wall.ConclusionA completely autologous acellular conduit with excellent performance in mechanical properties can be remodeled into a neoartery in a minipig model. This proof-of-concept study in the large animal model is very encouraging and indicates that this is a highly feasible idea worthy of further study in non-human primates before clinical translation
Hopf Insulators and Their Topologically Protected Surface States
Three-dimensional (3D) topological insulators in general need to be protected
by certain kinds of symmetries other than the presumed charge
conservation. A peculiar exception is the Hopf insulators which are 3D
topological insulators characterized by an integer Hopf index. To demonstrate
the existence and physical relevance of the Hopf insulators, we construct a
class of tight-binding model Hamiltonians which realize all kinds of Hopf
insulators with arbitrary integer Hopf index. These Hopf insulator phases have
topologically protected surface states and we numerically demonstrate the
robustness of these topologically protected states under general random
perturbations without any symmetry other than the charge conservation
that is implicit in all kinds of topological insulators.Comment: 7 pages (including supplemental material), 4 figure
An experimental proposal to observe non-abelian statistics of Majorana-Shockley fermions in an optical lattice
We propose an experimental scheme to observe non-abelian statistics with cold
atoms in a two dimensional optical lattice. We show that the Majorana-Schockley
modes associated with line defects obey non-abelian statistics and can be
created, braided, and fused, all through adiabatic shift of the local chemical
potentials. The detection of the topological qubit is transformed to local
measurement of the atom number on a single lattice site. We demonstrate the
robustness of the braiding operation by incorporating noise and experiential
imperfections in numerical simulations, and show that the requirement fits well
with the current experimental technology.Comment: 6 page
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