6,821 research outputs found
Multiparticle Interference, GHZ Entanglement, and Full Counting Statistics
We investigate the quantum transport in a generalized N-particle Hanbury
Brown--Twiss setup enclosing magnetic flux, and demonstrate that the Nth-order
cumulant of current cross correlations exhibits Aharonov-Bohm oscillations,
while there is no such oscillation in all the lower-order cumulants. The
multiparticle interference results from the orbital Greenberger-Horne-Zeilinger
entanglement of N indistinguishable particles. For sufficiently strong
Aharonov-Bohm oscillations the generalized Bell inequalities may be violated,
proving the N-particle quantum nonlocality.Comment: 4 pages, 1 figure, published versio
Construction of optimal witness for unknown two-qubit entanglement
Whether entanglement in a state can be detected, distilled, and quantified
without full state reconstruction is a fundamental open problem. We demonstrate
a new scheme encompassing these three tasks for arbitrary two-qubit
entanglement, by constructing the optimal entanglement witness for
polarization-entangled mixed-state photon pairs without full state
reconstruction. With better efficiency than quantum state tomography, the
entanglement is maximally distilled by newly developed tunable polarization
filters, and quantified by the expectation value of the witness, which equals
the concurrence. This scheme is extendible to multiqubit
Greenberger-Horne-Zeilinger entanglement.Comment: Phys. Rev. Lett. 105, 230404 (2010); supplementary information
(OWitness_sup.pdf) is included in source zip fil
Ions in solution: Density Corrected Density Functional Theory (DC-DFT)
Standard density functional approximations often give questionable results
for odd-electron radical complexes, with the error typically attributed to
self-interaction. In density corrected density functional theory (DC-DFT),
certain classes of density functional theory calculations are significantly
improved by using densities more accurate than the self-consistent densities.
We discuss how to identify such cases, and how DC-DFT applies more generally.
To illustrate, we calculate potential energy surfaces of HOCl and
HOHO complexes using various common approximate functionals, with
and without this density correction. Commonly used approximations yield wrongly
shaped surfaces and/or incorrect minima when calculated self consistently,
while yielding almost identical shapes and minima when density corrected. This
improvement is retained even in the presence of implicit solvent
Attomolar detection of protein biomarkers using biofunctionalized gold nanorods with surface plasmon resonance
This paper describes an ultrasensitive surface plasmon resonance (SPR) detection method using biofunctionalized gold nanorods for the direct detection of protein biomarkers. Immunoglobulin E (IgE), which has separate antibody and DNA aptamer binding sites, was chosen as a model protein for which a sandwich assay platform was designed. Detection was achieved via the specific adsorption of unlabelled IgE proteins onto the surface immobilized aptamer followed by the specific adsorption of anti-IgE coated gold nanorods (Au-NRs). Using the biofunctionalized nanorods in conjunction with SPR, we were able to directly measure IgE proteins at attomolar concentrations. This is a remarkable 108 enhancement compared to conventional SPR measurements of the same surface sandwich assay format ‘anti-IgE/IgE/surface bound IgE-aptamer’ in the absence of gold nanorod signal amplification
The reverberation signatures of rotating disc winds in active galactic nuclei
The broad emission lines (BELs) in active galactic nuclei (AGN) respond to
ionizing continuum variations. The time and velocity dependence of their
response depends on the structure of the broad-line region: its geometry,
kinematics and ionization state. Here, we predict the reverberation signatures
of BELs formed in rotating accretion disc winds. We use a Monte Carlo radiative
transfer and ionization code to predict velocity-delay maps for representative
high- (C) and low-ionization (H) emission lines in both high- and
moderate-luminosity AGN. Self-shielding, multiple scattering and the ionization
structure of the outflows are all self-consistently taken into account, while
small-scale structure in the outflow is modelled in the micro-clumping
approximation. Our main findings are: (1) The velocity-delay maps of
smooth/micro-clumped outflows often contain significant negative responses.
(2)~The reverberation signatures of disc wind models tend to be rotation
dominated and can even resemble the classic "red-leads-blue" inflow signature.
(3) Traditional "blue-leads-red" outflow signatures can usually only be
observed in the long-delay limit. (4) Our models predict lag-luminosity
relationships similar to those inferred from observations, but systematically
underpredict the observed centroid delays. (5) The ratio between "virial
product" and black hole mass predicted by our models depends on viewing angle.
Our results imply that considerable care needs to be taken in interpreting data
obtained by observational reverberation mapping campaigns. In particular, basic
signatures such as "red-leads-blue", "blue-leads-red" and "blue and red vary
jointly" are not always reliable indicators of inflow, outflow or rotation.
This may help to explain the perplexing diversity of such signatures seen in
observational campaigns to date.Comment: 15 pages, 17 figures, 2 tables. Accepted by MNRAS 20/7/201
B cells are capable of independently eliciting rapid reactivation of encephalitogenic CD4 T cells in a murine model of multiple sclerosis
<div><p>Recent success with B cell depletion therapies has revitalized efforts to understand the pathogenic role of B cells in Multiple Sclerosis (MS). Using the adoptive transfer system of experimental autoimmune encephalomyelitis (EAE), a murine model of MS, we have previously shown that mice in which B cells are the only MHCII-expressing antigen presenting cell (APC) are susceptible to EAE. However, a reproducible delay in the day of onset of disease driven by exclusive B cell antigen presentation suggests that B cells require optimal conditions to function as APCs in EAE. In this study, we utilize an <i>in vivo</i> genetic system to conditionally and temporally regulate expression of MHCII to test the hypothesis that B cell APCs mediate attenuated and delayed neuroinflammatory T cell responses during EAE. Remarkably, induction of MHCII on B cells following the transfer of encephalitogenic CD4 T cells induced a rapid and robust form of EAE, while no change in the time to disease onset occurred for recipient mice in which MHCII is induced on a normal complement of APC subsets. Changes in CD4 T cell activation over time did not account for more rapid onset of EAE symptoms in this new B cell-mediated EAE model. Our system represents a novel model to study how the timing of pathogenic cognate interactions between lymphocytes facilitates the development of autoimmune attacks within the CNS.</p></div
Minimax optimization of entanglement witness operator for the quantification of three-qubit mixed-state entanglement
We develop a numerical approach for quantifying entanglement in mixed quantum
states by convex-roof entanglement measures, based on the optimal entanglement
witness operator and the minimax optimization method. Our approach is
applicable to general entanglement measures and states and is an efficient
alternative to the conventional approach based on the optimal pure-state
decomposition. Compared with the conventional one, it has two important merits:
(i) that the global optimality of the solution is quantitatively verifiable,
and (ii) that the optimization is considerably simplified by exploiting the
common symmetry of the target state and measure. To demonstrate the merits, we
quantify Greenberger-Horne-Zeilinger (GHZ) entanglement in a class of
three-qubit full-rank mixed states composed of the GHZ state, the W state, and
the white noise, the simplest mixtures of states with different genuine
multipartite entanglement, which have not been quantified before this work. We
discuss some general properties of the form of the optimal witness operator and
of the convex structure of mixed states, which are related to the symmetry and
the rank of states
A Simulation Model for Electron Irradiation Induced Specimen Charging in a Scanning Electron Microscope
A numerical model has been formulated to simulate the dynamics of specimen charging in a scanning electron microscope. In this model, the electric field due to imposed boundary conditions and fixed charges is solved by the finite element method. The empirical electron yield data are stored in Universal Yield Curves (UYC) . These UYCs control the generation of secondary and backscattered electrons from various materials. The electrons emitted from electron-solid interactions are tracked using a leapfrog integration scheme. Excess charges generated on the surface of electrically floating solids are assigned to numerical grids using a linear charge redistribution scheme. The validity of the simulation model was verified by measurements in a special setup which consisted of several isolated electrodes in the SEM chamber. Excess currents generated inside each electrode due to electron irradiation were measured simultaneously. Measurements and simulation results are in broad agreement and show that electrically floating electrodes, not directly irradiated by the primary beam, can charge-up if they are irradiated by secondary electrons and backscattered electrons emitted from a nearby electrode. The polarity of charge generation on the electrically floating solid depends on its own material property, and also strongly on the potential distribution in the space surrounding the floating electrode
Towards unified understanding of conductance of stretched monatomic contacts
When monatomic contacts are stretched, their conductance behaves in
qualitatively different ways depending on their constituent atomic elements.
Under a single assumption of resonance formation, we show that various
conductance behavior can be understood in a unified way in terms of the
response of the resonance to stretching. This analysis clarifies the crucial
roles played by the number of valence electrons, charge neutrality, and orbital
shapes.Comment: 2 figure
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