40 research outputs found
Perturbations and chaos in quantum maps
The local density of states (LDOS) is a distribution that characterizes the
effect of perturbations on quantum systems. Recently, it was proposed a
semiclassical theory for the LDOS of chaotic billiards and maps. This theory
predicts that the LDOS is a Breit-Wigner distribution independent of the
perturbation strength and also gives a semiclassical expression for the LDOS
witdth. Here, we test the validity of such an approximation in quantum maps
varying the degree of chaoticity, the region in phase space where the
perturbation is applying and the intensity of the perturbation. We show that
for highly chaotic maps or strong perturbations the semiclassical theory of the
LDOS is accurate to describe the quantum distribution. Moreover, the width of
the LDOS is also well represented for its semiclassical expression in the case
of mixed classical dynamics.Comment: 9 pages, 11 figures. Accepted for publication in Phys. Rev.
Semiclassical approach to fidelity amplitude
The fidelity amplitude is a quantity of paramount importance in echo type
experiments. We use semiclassical theory to study the average fidelity
amplitude for quantum chaotic systems under external perturbation. We explain
analytically two extreme cases: the random dynamics limit --attained
approximately by strongly chaotic systems-- and the random perturbation limit,
which shows a Lyapunov decay. Numerical simulations help us bridge the gap
between both extreme cases.Comment: 10 pages, 9 figures. Version closest to published versio
Wave packet autocorrelation functions for quantum hard-disk and hard-sphere billiards in the high-energy, diffraction regime
We consider the time evolution of a wave packet representing a quantum
particle moving in a geometrically open billiard that consists of a number of
fixed hard-disk or hard-sphere scatterers. Using the technique of multiple
collision expansions we provide a first-principle analytical calculation of the
time-dependent autocorrelation function for the wave packet in the high-energy
diffraction regime, in which the particle's de Broglie wave length, while being
small compared to the size of the scatterers, is large enough to prevent the
formation of geometric shadow over distances of the order of the particle's
free flight path. The hard-disk or hard-sphere scattering system must be
sufficiently dilute in order for this high-energy diffraction regime to be
achievable. Apart from the overall exponential decay, the autocorrelation
function exhibits a generally complicated sequence of relatively strong peaks
corresponding to partial revivals of the wave packet. Both the exponential
decay (or escape) rate and the revival peak structure are predominantly
determined by the underlying classical dynamics. A relation between the escape
rate, and the Lyapunov exponents and Kolmogorov-Sinai entropy of the
counterpart classical system, previously known for hard-disk billiards, is
strengthened by generalization to three spatial dimensions. The results of the
quantum mechanical calculation of the time-dependent autocorrelation function
agree with predictions of the semiclassical periodic orbit theory.Comment: 24 pages, 13 figure
Vector and Tensor Analyzing Powers of the H(d,gamma)He-3 capture reaction
Precise measurements of the deuteron vector analyzing power Ayd and the
tensor analyzing power Ayy of the H(d,gamma)He-3 capture reaction have been
performed at deuteron energies of 29MeV and 45MeV. The data have been compared
to theoretical state-of-the-art calculations available today. Due to the large
sensitivity of polarization observables and the precision of the data light
could be shed on small effects present in the dynamics of the reaction.Comment: 11 pages, 24 figures, submitted for publication to PRC, revised after
referee proces
Measurement of the Electric Form Factor of the Neutron at Q^2=0.5 and 1.0 (GeV/c)^2
The electric form factor of the neutron was determined from measurements of
the \vec{d}(\vec{e},e' n)p reaction for quasielastic kinematics. Polarized
electrons were scattered off a polarized deuterated ammonia target in which the
deuteron polarization was perpendicular to the momentum transfer. The scattered
electrons were detected in a magnetic spectrometer in coincidence with neutrons
in a large solid angle detector. We find G_E^n = 0.0526 +/- 0.0033 (stat) +/-
0.0026 (sys) and 0.0454 +/- 0.0054 +/- 0.0037 at Q^2 = 0.5 and 1.0 (GeV/c)^2,
respectively.Comment: 5 pages, 2 figures, as publishe
Quantum circuits with many photons on a programmable nanophotonic chip
Growing interest in quantum computing for practical applications has led to a
surge in the availability of programmable machines for executing quantum
algorithms. Present day photonic quantum computers have been limited either to
non-deterministic operation, low photon numbers and rates, or fixed random gate
sequences. Here we introduce a full-stack hardware-software system for
executing many-photon quantum circuits using integrated nanophotonics: a
programmable chip, operating at room temperature and interfaced with a fully
automated control system. It enables remote users to execute quantum algorithms
requiring up to eight modes of strongly squeezed vacuum initialized as two-mode
squeezed states in single temporal modes, a fully general and programmable
four-mode interferometer, and genuine photon number-resolving readout on all
outputs. Multi-photon detection events with photon numbers and rates exceeding
any previous quantum optical demonstration on a programmable device are made
possible by strong squeezing and high sampling rates. We verify the
non-classicality of the device output, and use the platform to carry out
proof-of-principle demonstrations of three quantum algorithms: Gaussian boson
sampling, molecular vibronic spectra, and graph similarity
Measurement of the asymmetries in 3(¯e, e′p)d and 3(¯e, e′p)np
Abstract.: The electron target asymmetries A || and A⊥ with target spin parallel and perpendicular to the momentum transfer \ensuremath{\boldsymbol{q}} were measured for both the two- and three-body breakup of 3He in the 3 (¯e, e'p)-reaction. Polarized electrons were scattered off polarized 3He in the quasielastic regime in parallel kinematics with the scattered electron and the knocked-out proton detected using the Three-Spectrometer Facility at MAMI. The results are compared to Faddeev calculations which take into account Final-State Interactions as well as Meson Exchange Currents. The experiment confirms the prediction of a large effect of Final-State Interactions in the asymmetry of the three-body breakup and of an almost negligible one for the two-body breaku
Inflammatory mechanisms in ischemic stroke: therapeutic approaches
Acute ischemic stroke is the third leading cause of death in industrialized countries and the most frequent cause of permanent disability in adults worldwide. Despite advances in the understanding of the pathophysiology of cerebral ischemia, therapeutic options remain limited. Only recombinant tissue-plasminogen activator (rt-PA) for thrombolysis is currently approved for use in the treatment of this devastating disease. However, its use is limited by its short therapeutic window (three hours), complications derived essentially from the risk of hemorrhage, and the potential damage from reperfusion/ischemic injury. Two important pathophysiological mechanisms involved during ischemic stroke are oxidative stress and inflammation. Brain tissue is not well equipped with antioxidant defenses, so reactive oxygen species and other free radicals/oxidants, released by inflammatory cells, threaten tissue viability in the vicinity of the ischemic core. This review will discuss the molecular aspects of oxidative stress and inflammation in ischemic stroke and potential therapeutic strategies that target neuroinflammation and the innate immune system. Currently, little is known about endogenous counterregulatory immune mechanisms. However, recent studies showing that regulatory T cells are major cerebroprotective immunomodulators after stroke suggest that targeting the endogenous adaptive immune response may offer novel promising neuroprotectant therapies
Role of Matrix Metalloproteinases and Therapeutic Benefits of Their Inhibition in Spinal Cord Injury
This review will focus on matrix metalloproteinases (MMPs) and their inhibitors in the context of spinal cord injury (SCI). MMPs have a specific cellular and temporal pattern of expression in the injured spinal cord. Here we consider their diverse functions in the acutely injured cord and during wound healing. Excessive activity of MMPs, and in particular gelatinase B (MMP-9), in the acutely injured cord contributes to disruption of the blood-spinal cord barrier, and the influx of leukocytes into the injured cord, as well as apoptosis. MMP-9 and MMP-2 regulate inflammation and neuropathic pain after peripheral nerve injury and may contribute to SCI-induced pain. Early pharmacologic inhibition of MMPs or the gelatinases (MMP-2 and MMP-9) results in an improvement in long-term neurological recovery and is associated with reduced glial scarring and neuropathic pain. During wound healing, gelatinase A (MMP-2) plays a critical role in limiting the formation of an inhibitory glial scar, and mice that are genetically deficient in this protease showed impaired recovery. Together, these findings illustrate complex, temporally distinct roles of MMPs in SCIs. As early gelatinase activity is detrimental, there is an emerging interest in developing gelatinase-targeted therapeutics that would be specifically tailored to the acute injured spinal cord. Thus, we focus this review on the development of selective gelatinase inhibitors