385 research outputs found
Quantum Error Correction via Convex Optimization
We show that the problem of designing a quantum information error correcting
procedure can be cast as a bi-convex optimization problem, iterating between
encoding and recovery, each being a semidefinite program. For a given encoding
operator the problem is convex in the recovery operator. For a given method of
recovery, the problem is convex in the encoding scheme. This allows us to
derive new codes that are locally optimal. We present examples of such codes
that can handle errors which are too strong for codes derived by analogy to
classical error correction techniques.Comment: 16 page
Absence of large nanoscale electronic inhomogeneities in the Ba(Fe1-xCox)2As2 pnictide
75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal
for x=6% for various field H values and orientations. The sharpness of the
superconducting and magnetic transitions demonstrates a homogeneity of the Co
doping x better than +-0.25%. On the nanometer scale, the paramagnetic part of
the NMR spectra is found very anisotropic and very narrow for H//ab which
allows to rule out the interpretation of Ref.[6] in terms of strong Co induced
electronic inhomogeneities. We propose that a distribution of hyperfine
couplings and chemical shifts due to the Co effect on its nearest As explains
the observed linewidths and relaxations. All these measurements show that Co
substitution induces a very homogeneous electronic doping in BaFe2As2, from
nano to micrometer lengthscales, on the contrary to the K doping.Comment: 6 pages, 4 figure
Influence of AlN passivation on thermal performance of AlGaN/GaN high-electron mobility transistors on sapphire substrate: A simulation study
This work describes the self-heating effects on the behavior of AlGaN/GaN-based high-electron Mobility Transistors (HEMTs), which are grownon Sapphire substrate, using electro-thermal TCAD simulations. The proposed device, passivated with AlN/SiN, demonstrates more excellent thermal performance than the conventional one with SiN passivation due to the introduction of additional AlN on top of the device, which acts as a heat spreader. The electro-thermal simulations have carried out for different AlN thicknesses (0 µm to 25 µm), and the device with 5 µm AlN shows better performance compared to others. The proposed AlN/SiN stacked passivation HEMT shows a comparatively small lattice temperature of 418 K, whereas the conventional HEMT with SiN passivation shows 578 K. All the devices (gate length, LG = 1 µm) switch from OFF- to ON-states using the voltage, VGS from −10 V to 0 V with fixed bias, VDS = 5 V. The values of saturation drain current density (IDSS) and transconductance (gm) are 0.7 A/mm and 173 mS/mm for the proposed HEMT with 5 µm AlN considering the thermal simulation model. In contrast, the conventional device demonstrates those of 0.42 A/mm and 109 mS/mm, respectively. The ∼ 0.32 A/mm of drain current recover for the proposed device with 5 µm AlN from a conventional device because of the reduction of self-heating effects. Our study reveals that the AlN/SiN passivation HEMTs are a promising technology for high-power switching and microwave applications without significant reduction in device performance at high drain bias
Implementation of an Optimal First-Order Method for Strongly Convex Total Variation Regularization
We present a practical implementation of an optimal first-order method, due
to Nesterov, for large-scale total variation regularization in tomographic
reconstruction, image deblurring, etc. The algorithm applies to -strongly
convex objective functions with -Lipschitz continuous gradient. In the
framework of Nesterov both and are assumed known -- an assumption
that is seldom satisfied in practice. We propose to incorporate mechanisms to
estimate locally sufficient and during the iterations. The mechanisms
also allow for the application to non-strongly convex functions. We discuss the
iteration complexity of several first-order methods, including the proposed
algorithm, and we use a 3D tomography problem to compare the performance of
these methods. The results show that for ill-conditioned problems solved to
high accuracy, the proposed method significantly outperforms state-of-the-art
first-order methods, as also suggested by theoretical results.Comment: 23 pages, 4 figure
Interference, reduced action, and trajectories
Instead of investigating the interference between two stationary, rectilinear
wave functions in a trajectory representation by examining the two rectilinear
wave functions individually, we examine a dichromatic wave function that is
synthesized from the two interfering wave functions. The physics of
interference is contained in the reduced action for the dichromatic wave
function. As this reduced action is a generator of the motion for the
dichromatic wave function, it determines the dichromatic wave function's
trajectory. The quantum effective mass renders insight into the behavior of the
trajectory. The trajectory in turn renders insight into quantum nonlocality.Comment: 12 pages text, 5 figures. Typos corrected. Author's final submission.
A companion paper to "Welcher Weg? A trajectory representation of a quantum
Young's diffraction experiment", quant-ph/0605121. Keywords: interference,
nonlocality, trajectory representation, entanglement, dwell time, determinis
Quantum Oscillation Studies of the Fermi Surface of LaFePO
We review recent experimental measurements of the Fermi surface of the
iron-pnictide superconductor LaFePO using quantum oscillation techniques. These
studies show that the Fermi surface topology is close to that predicted by
first principles density functional theory calculations, consisting of
quasi-two-dimensional electron-like and hole-like sheets. The total volume of
the two hole sheets is almost equal to that of the two electron sheets, and the
hole and electron Fermi surface sheets are close to a nesting condition. No
evidence for the predicted three dimensional pocket arising from the Fe
band is found. Measurements of the effective mass suggest a
renormalisation of around two, close to the value for the overall band
renormalisation found in recent angle resolved photoemission measurements.Comment: Submitted to Physica C special issue on iron-pnictide superconductor
A clinical and cost effectiveness trial of a parent group intervention to manage challenging restricted and repetitive behaviours in young children with autism spectrum disorder: study protocol for a randomized controlled trial
Background Restricted and repetitive behaviours vary greatly across the autism spectrum, and although not all are problematic some can cause distress and interfere with learning and social opportunities. We have, alongside parents, developed a parent group based intervention for families of young children with autism, which aims to offer support to parents and carers; helping them to recognise, understand and learn how to respond to their child’s challenging restricted repetitive behaviours. Methods The study is a clinical and cost-effectiveness, multi-site randomised controlled trial of the Managing Repetitive Behaviours (MRB) parent group intervention versus a psychoeducation parent group Learning About Autism (LAA) (n = 250; 125 intervention/125 psychoeducation; ~ 83/site) for parents of young children aged 3–9 years 11 months with a diagnosis of autism. All analyses will be done under intention-to-treat principle. The primary outcome at 24 weeks will use generalised estimating equation (GEE) to compare proportion of children with improved RRB between the MRB group and the LAA group. The GEE model will account for the clustering of children by parent groups using exchangeable working correlation. All secondary outcomes will be analysed in a similar way using appropriate distribution and link function. The economic evaluation will be conducted from the perspective of both NHS costs and family access to local community services. A ‘within trial’ cost-effectiveness analysis with results reported as the incremental cost per additional child achieving at least the target improvement in CGI-I scale at 24 weeks. Discussion This is an efficacy trial to investigate the clinical and cost-effectiveness of a parent group based intervention designed to help parents understand and manage their child’s challenging RRB. If found to be effective, this intervention has the potential to improve the well-being of children and their families, reduce parental stress, greatly enhance community participation and potential for learning, and improve longer-term outcomes. Trial registration Trial ID: ISRCTN15550611 Date registered: 07/08/2018. Sponsor and Monitor: Cumbria, Northumberland, Tyne and Wear NHS Foundation Trust R&D Manager Lyndsey Dixon, Address: St Nicholas Hospital, Jubliee Road, Gosforth, Newcastle upon Tyne NE3 3XT, [email protected], Tel: 0191 246 722
Measurement of the top pair production cross section in 8 TeV proton-proton collisions using kinematic information in the lepton plus jets final state with ATLAS
A measurement is presented of the inclusive production
cross-section in collisions at a center-of-mass energy of TeV
using data collected by the ATLAS detector at the CERN Large Hadron Collider.
The measurement was performed in the lepton+jets final state using a data set
corresponding to an integrated luminosity of 20.3 fb. The cross-section
was obtained using a likelihood discriminant fit and -jet identification was
used to improve the signal-to-background ratio. The inclusive
production cross-section was measured to be
pb assuming a top-quark mass of 172.5 GeV, in good agreement with the
theoretical prediction of pb. The production cross-section in the fiducial region
determined by the detector acceptance is also reported.Comment: Published version, 19 pages plus author list (35 pages total), 3
figures, 2 tables, all figures including auxiliary figures are available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2013-06
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