16,039 research outputs found
Numerical method for determination of the NMR frequency of the single-qubit operation in a silicon-based solid-state quantum computer
A numerical method is introduced to determine the nuclear magnetic resonance frequency of a donor (P-31) doped inside a silicon substrate under the influence of an applied electric field. This phosphorus donor has been suggested for operation as a qubit for the realization of a solid-state scalable quantum computer. The operation of the qubit is achieved by a combination of the rotation of the phosphorus nuclear spin through a globally applied magnetic field and the selection of the phosphorus nucleus through a locally applied electric field. To realize the selection function, it is required to know the relationship between the applied electric field and the change of the nuclear magnetic resonance frequency of phosphorus. In this study, based on the wave functions obtained by the effective-mass theory, we introduce an empirical correction factor to the wave functions at the donor nucleus. Using the corrected wave functions, we formulate a first-order perturbation theory for the perturbed system under the influence of an electric field. In order to calculate the potential distributions inside the silicon and the silicon dioxide layers due to the applied electric field, we use the multilayered Green's functions and solve an integral equation by the moment method. This enables us to consider more realistic, arbitrary shape, and three-dimensional qubit structures. With the calculation of the potential distributions, we have investigated the effects of the thicknesses of silicon and silicon dioxide layers, the relative position of the donor, and the applied electric field on the nuclear magnetic resonance frequency of the donor
Optimal rates of convergence for covariance matrix estimation
Covariance matrix plays a central role in multivariate statistical analysis.
Significant advances have been made recently on developing both theory and
methodology for estimating large covariance matrices. However, a minimax theory
has yet been developed. In this paper we establish the optimal rates of
convergence for estimating the covariance matrix under both the operator norm
and Frobenius norm. It is shown that optimal procedures under the two norms are
different and consequently matrix estimation under the operator norm is
fundamentally different from vector estimation. The minimax upper bound is
obtained by constructing a special class of tapering estimators and by studying
their risk properties. A key step in obtaining the optimal rate of convergence
is the derivation of the minimax lower bound. The technical analysis requires
new ideas that are quite different from those used in the more conventional
function/sequence estimation problems.Comment: Published in at http://dx.doi.org/10.1214/09-AOS752 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
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
Gamma-ray emission from the globular clusters Liller 1, M80, NGC 6139, NGC 6541, NGC 6624, and NGC 6752
Globular clusters (GCs) are emerging as a new class of gamma-ray emitters,
thanks to the data obtained from the Fermi Gamma-ray Space Telescope. By now,
eight GCs are known to emit gamma-rays at energies >100~MeV. Based on the
stellar encounter rate of the GCs, we identify potential gamma-ray emitting GCs
out of all known GCs that have not been studied in details before. In this
paper, we report the discovery of a number of new gamma-ray GCs: Liller 1, NGC
6624, and NGC 6752, and evidence for gamma-ray emission from M80, NGC 6139, and
NGC 6541, in which gamma-rays were found within the GC tidal radius. With one
of the highest metallicity among all GCs in the Milky Way, the gamma-ray
luminosity of Liller 1 is found to be the highest of all known gamma-ray GCs.
In addition, we confirm a previous report of significant gamma-ray emitting
region next to NGC 6441. We briefly discuss the observed offset of gamma-rays
from some GC cores. The increasing number of known gamma-ray GCs at distances
out to ~10 kpc is important for us to understand the gamma-ray emitting
mechanism and provides an alternative probe to the underlying millisecond
pulsar populations of the GCs.Comment: 22 pages, 7 figures, 2 tables; ApJ, in pres
Discovery of gamma-ray emission from the supernova remnant Kes 17 with Fermi Large Area Telescope
We report the discovery of GeV emission at the position of supernova remnant
Kes 17 by using the data from the Large Area Telescope on board the Fermi
Gamma-ray Space Telescope. Kes 17 can be clearly detected with a significance
of ~12 sigma in the 1 - 20 GeV range. Moreover, a number of gamma-ray sources
were detected in its vicinity. The gamma-ray spectrum of Kes 17 can be well
described by a simple power-law with a photon index of ~ 2.4. Together with the
multi-wavelength evidence for its interactions with the nearby molecular cloud,
the gamma-ray detection suggests that Kes 17 is a candidate acceleration site
for cosmic-rays.Comment: 13 pages, 3 figures, 1 table, accepted for publication in ApJ Lette
A NuSTAR Observation of the Gamma-ray Emitting Millisecond Pulsar PSR J1723-2837
We report on the first NuSTAR observation of the gamma-ray emitting
millisecond pulsar binary PSR J1723-2837. X-ray radiation up to 79 keV is
clearly detected and the simultaneous NuSTAR and Swift spectrum is well
described by an absorbed power-law with a photon index of ~1.3. We also find
X-ray modulations in the 3-10 keV, 10-20 keV, 20-79 keV, and 3-79 keV bands at
the 14.8-hr binary orbital period. All these are entirely consistent with
previous X-ray observations below 10 keV. This new hard X-ray observation of
PSR J1723-2837 provides strong evidence that the X-rays are from the
intrabinary shock via an interaction between the pulsar wind and the outflow
from the companion star. We discuss how the NuSTAR observation constrains the
physical parameters of the intrabinary shock model.Comment: Accepted for publication in ApJ. 5 pages, 3 figure
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