766 research outputs found
Multiscale 3D Shape Analysis using Spherical Wavelets
©2005 Springer. The original publication is available at www.springerlink.com:
http://dx.doi.org/10.1007/11566489_57DOI: 10.1007/11566489_57Shape priors attempt to represent biological variations within a population. When variations are global, Principal Component Analysis (PCA) can be used to learn major modes of variation, even from a limited training set. However, when significant local variations exist, PCA typically cannot represent such variations from a small training set. To address this issue, we present a novel algorithm that learns shape variations from data at multiple scales and locations using spherical wavelets and spectral graph partitioning. Our results show that when the training set is small, our algorithm significantly improves the approximation of shapes in a testing set over PCA, which tends to oversmooth data
Transport Properties of Carbon Nanotube C Peapods
We measure the conductance of carbon nanotube peapods from room temperature
down to 250mK. Our devices show both metallic and semiconducting behavior at
room temperature. At the lowest temperatures, we observe single electron
effects. Our results suggest that the encapsulated C molecules do not
introduce substantial backscattering for electrons near the Fermi level. This
is remarkable given that previous tunneling spectroscopy measurements show that
encapsulated C strongly modifies the electronic structure of a nanotube
away from the Fermi level.Comment: 9 pages, 4 figures. This is one of two manuscripts replacing the one
orginally submitted as arXiv:cond-mat/0606258. The other one is
arXiv:0704.3641 [cond-mat
On the electromagnetic properties of active media
Several results concerning active media or metamaterials are proved and
discussed. In particular, we consider the permittivity, permeability, wave
vector, and refractive index, and discuss stability, refraction, gain, and
fundamental limitations resulting from causality
The effect of thermal anisotropies during crystallization in phase-change recording media
types: ArticleCopyright © 2008 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Applied Physics 104 (2008) and may be found at http://dx.doi.org/10.1063/1.2968447The problem discussed is the significance of anisotropies in the thermal parameters of different phases of phase-change materials as used for data storage purposes during recording. The particular phase change in interest is from the amorphous-to-crystalline state. Applying the method of correlation moment analysis produced upper estimators for the time dependence of the width of the crystalline mark and the time at which phase change ceases based on the heat flow process alone. These upper estimators are closed-form analytical expressions that can be used to estimate the recording resolution for any general spatial profile of initial temperature in the medium. This analysis showed that, up to a first order, the specific heat anisotropies have considerably less influence on the heat flow than the thermal conductivity differences. In general, for the material parameters used in phase-change data storage applications, the theory showed that the anisotropy in thermal parameters can be neglected. (C) 2008 American Institute of Physics
Passive-scheme analysis for solving untrusted source problem in quantum key distribution
As a practical method, the passive scheme is useful to monitor the photon
statistics of an untrusted source in a "Plug & Play" quantum key distribution
(QKD) system. In a passive scheme, three kinds of monitor mode can be adopted:
average photon number (APN) monitor, photon number analyzer (PNA) and photon
number distribution (PND) monitor. In this paper, the security analysis is
rigorously given for APN monitor, while for PNA, the analysis including
statistical fluctuation and random noise, is addressed with a confidence level.
The results show that the PNA can achieve better performance than the APN
monitor and can asymptotically approach the theoretical limit of the PND
monitor. Also, the passive scheme with the PNA works efficiently when the
signal-to-noise ratio () is not too low and so is highly applicable to
solve the untrusted source problem in the QKD system.Comment: 8 pages, 6 figures, published versio
A chi-squared time-frequency discriminator for gravitational wave detection
Searches for known waveforms in gravitational wave detector data are often
done using matched filtering. When used on real instrumental data, matched
filtering often does not perform as well as might be expected, because
non-stationary and non-Gaussian detector noise produces large spurious filter
outputs (events). This paper describes a chi-squared time-frequency test which
is one way to discriminate such spurious events from the events that would be
produced by genuine signals. The method works well only for broad-band signals.
The case where the filter template does not exactly match the signal waveform
is also considered, and upper bounds are found for the expected value of
chi-squared.Comment: 18 pages, five figures, RevTex
Estimate of the free energy difference in mechanical systems from work fluctuations: experiments and models
The work fluctuations of an oscillator in contact with a heat reservoir and
driven out of equilibrium by an external force are studied experimentally. The
oscillator dynamics is modeled by a Langevin equation. We find both
experimentally and theoretically that, if the driving force does not change the
equilibrium properties of the thermal fluctuations of this mechanical system,
the free energy difference between two equilibrium states can be
exactly computed using the Jarzynski equality (JE) and the Crooks relation (CR)
\cite{jarzynski1, crooks1, jarzynski2}, independently of the time scale and
amplitude of the driving force. The applicability limits for the JE and CR at
very large driving forces are discussed. Finally, when the work fluctuations
are Gaussian, we propose an alternative empirical method to compute
which can be safely applied, even in cases where the JE and CR might not hold.
The results of this paper are useful to compute in complex systems
such as the biological ones.Comment: submitted to Journal of Statistical Mechanics: Theory and experimen
Effects of low-frequency noise cross-correlations in coupled superconducting qubits
We study the effects of correlated low frequency noise sources acting on a
two qubit gate in a fixed coupling scheme. A phenomenological model for the
spatial and cross-talk correlations is introduced. The decoherence inside the
SWAP subspace is analysed by combining analytic results based on the adiabatic
approximation and numerical simulations. Results critically depend on amplitude
of the low frequency noise with respect to the qubits coupling strength.
Correlations between noise sources induce qualitative different behaviors
depending on the values of the above parameters. The possibility to reduce
dephasing due to correlated low frequency noise by a recalibration protocol is
discussed.Comment: 18 pages, 7 figure
Matching post-Newtonian and numerical relativity waveforms: systematic errors and a new phenomenological model for non-precessing black hole binaries
We present a new phenomenological gravitational waveform model for the
inspiral and coalescence of non-precessing spinning black hole binaries. Our
approach is based on a frequency domain matching of post-Newtonian inspiral
waveforms with numerical relativity based binary black hole coalescence
waveforms. We quantify the various possible sources of systematic errors that
arise in matching post-Newtonian and numerical relativity waveforms, and we use
a matching criteria based on minimizing these errors; we find that the dominant
source of errors are those in the post-Newtonian waveforms near the merger. An
analytical formula for the dominant mode of the gravitational radiation of
non-precessing black hole binaries is presented that captures the phenomenology
of the hybrid waveforms. Its implementation in the current searches for
gravitational waves should allow cross-checks of other inspiral-merger-ringdown
waveform families and improve the reach of gravitational wave searches.Comment: 22 pages, 11 figure
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