61,277 research outputs found
Optimal dense coding with arbitrary pure entangled states
We examine dense coding with an arbitrary pure entangled state sharing
between the sender and the receiver. Upper bounds on the average success
probability in approximate dense coding and on the probability of conclusive
results in unambiguous dense coding are derived. We also construct the optimal
protocol which saturates the upper bound in each case.Comment: 5 pages, journal versio
Time-optimal synthesis of unitary transformations in coupled fast and slow qubit system
In this paper, we study time-optimal control problems related to system of
two coupled qubits where the time scales involved in performing unitary
transformations on each qubit are significantly different. In particular, we
address the case where unitary transformations produced by evolutions of the
coupling take much longer time as compared to the time required to produce
unitary transformations on the first qubit but much shorter time as compared to
the time to produce unitary transformations on the second qubit. We present a
canonical decomposition of SU(4) in terms of the subgroup SU(2)xSU(2)xU(1),
which is natural in understanding the time-optimal control problem of such a
coupled qubit system with significantly different time scales. A typical
setting involves dynamics of a coupled electron-nuclear spin system in pulsed
electron paramagnetic resonance experiments at high fields. Using the proposed
canonical decomposition, we give time-optimal control algorithms to synthesize
various unitary transformations of interest in coherent spectroscopy and
quantum information processing.Comment: 8 pages, 3 figure
ADN: An Information-Centric Networking Architecture for the Internet of Things
Forwarding data by name has been assumed to be a necessary aspect of an
information-centric redesign of the current Internet architecture that makes
content access, dissemination, and storage more efficient. The Named Data
Networking (NDN) and Content-Centric Networking (CCNx) architectures are the
leading examples of such an approach. However, forwarding data by name incurs
storage and communication complexities that are orders of magnitude larger than
solutions based on forwarding data using addresses. Furthermore, the specific
algorithms used in NDN and CCNx have been shown to have a number of
limitations. The Addressable Data Networking (ADN) architecture is introduced
as an alternative to NDN and CCNx. ADN is particularly attractive for
large-scale deployments of the Internet of Things (IoT), because it requires
far less storage and processing in relaying nodes than NDN. ADN allows things
and data to be denoted by names, just like NDN and CCNx do. However, instead of
replacing the waist of the Internet with named-data forwarding, ADN uses an
address-based forwarding plane and introduces an information plane that
seamlessly maps names to addresses without the involvement of end-user
applications. Simulation results illustrate the order of magnitude savings in
complexity that can be attained with ADN compared to NDN.Comment: 10 page
Origin of the X-ray Emission in the Nuclei of FR Is
We investigate the X-ray origin in FRIs using the multi-waveband high
resolution data of eight FR I sources, which have very low Eddington ratios. We
fit their multi-waveband spectrum using a coupled accretion-jet model. We find
that X-ray emission in the source with the highest L_X (~1.8*10^-4 L_Edd) is
from the advection-dominated accretion flow (ADAF). Four sources with moderate
L_X(~several*10^-6 L_Edd) are complicated. The X-ray emission of one FR I is
from the jet, and the other three is from the sum of the jet and ADAF. The
X-ray emission in the three least luminous sources (L_X<1.0*10^-6L_Edd) is
dominated by the jet. These results roughly support the predictions of Yuan and
Cui(2005) where they predict that when the X-ray luminosity of the system is
below a critical value, the X-radiation will not be dominated by the emission
from the ADAF any longer, but by the jet. We also find that the accretion rates
in four sources must be higher than the Bondi rates, which implies that other
fuel supply (e.g., stellar winds) inside the Bondi radius should be important.Comment: 6 pages. To published in Journal of Physics, in proceedings of "The
Universe under the Microscope - Astrophysics at High Angular Resolution" (Bad
Honnef, Germany, April 2008), eds. R. Schoedel, A. Eckart, S. Pfalzner, and
E. Ro
Quantum Decoherence at Finite Temperatures
We study measures of decoherence and thermalization of a quantum system
in the presence of a quantum environment (bath) . The whole system is
prepared in a canonical thermal state at a finite temperature. Applying
perturbation theory with respect to the system-environment coupling strength,
we find that under common Hamiltonian symmetries, up to first order in the
coupling strength it is sufficient to consider the uncoupled system to predict
decoherence and thermalization measures of . This decoupling allows closed
form expressions for perturbative expansions for the measures of decoherence
and thermalization in terms of the free energies of and of . Numerical
results for both coupled and decoupled systems with up to 40 quantum spins
validate these findings.Comment: 5 pages, 3 figure
Phonon-induced dephasing of chromium colour centres in diamond
We report on the coherence properties of single photons from chromium-based
colour centres in diamond. We use field-correlation and spectral lineshape
measurements to reveal the interplay between slow spectral wandering and fast
dephasing mechanisms as a function of temperature. We show that the zero-phonon
transition frequency and its linewidth follow a power-law dependence on
temperature indicating that the dominant fast dephasing mechanisms for these
centres are direct electron-phonon coupling and phonon-modulated Coulomb
coupling to nearby impurities. Further, the observed reduction in the quantum
yield for photon emission as a function of temperature is consistent with the
opening of additional nonradiative channels through thermal activation to
higher energy states predominantly and indicates a near-unity quantum
efficiency at 4 K
Reconstruction from Radon projections and orthogonal expansion on a ball
The relation between Radon transform and orthogonal expansions of a function
on the unit ball in \RR^d is exploited. A compact formula for the partial
sums of the expansion is given in terms of the Radon transform, which leads to
algorithms for image reconstruction from Radon data. The relation between
orthogonal expansion and the singular value decomposition of the Radon
transform is also exploited.Comment: 15 page
Exact Hybrid Covariance Thresholding for Joint Graphical Lasso
This paper considers the problem of estimating multiple related Gaussian
graphical models from a -dimensional dataset consisting of different
classes. Our work is based upon the formulation of this problem as group
graphical lasso. This paper proposes a novel hybrid covariance thresholding
algorithm that can effectively identify zero entries in the precision matrices
and split a large joint graphical lasso problem into small subproblems. Our
hybrid covariance thresholding method is superior to existing uniform
thresholding methods in that our method can split the precision matrix of each
individual class using different partition schemes and thus split group
graphical lasso into much smaller subproblems, each of which can be solved very
fast. In addition, this paper establishes necessary and sufficient conditions
for our hybrid covariance thresholding algorithm. The superior performance of
our thresholding method is thoroughly analyzed and illustrated by a few
experiments on simulated data and real gene expression data
Efimov states in asymmetric systems
The conditions for occurrence of the Efimov effect is briefly described using
hyperspherical coordinates. The strength of the effective hyperradial
potential appearing for two or three large scattering lengths is
computed and discussed as function of two independent mass ratios of the three
constituent particles. The effect is by far most pronounced for asymmetric
systems with three very different masses. One Efimov state may by chance appear
in nuclei. Many states could be present for systems with one electron and two
neutral atoms or molecules. Estimates of the number of states and their sizes
and energies are given.Comment: 7 pages, 3 figure
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