4,701 research outputs found
Learning Over Time: A Literature Review and Case Study
This is the publisher's version, also found at http://ehis.ebscohost.com/ehost/detail?sid=311dd0cd-6ea1-4d5c-8de0-704b9260d750%40sessionmgr10&vid=1&hid=17&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ%3d%3d#db=s3h&AN=2663392
Quantum squeezing of optical dissipative structures
We show that any optical dissipative structure supported by degenerate
optical parametric oscillators contains a special transverse mode that is free
from quantum fluctuations when measured in a balanced homodyne detection
experiment. The phenomenon is not critical as it is independent of the system
parameters and, in particular, of the existence of bifurcations. This result is
a consequence of the spatial symmetry breaking introduced by the dissipative
structure. Effects that could degrade the squeezing level are considered.Comment: 4 pages and a half, 1 fugure. Version to appear in Europhysics
Letter
TuRMoiL of Survival: A Unified Survival Criterion for Cloud-Wind Interactions
Cloud-wind interactions play an important role in long-lived multiphase flows
in many astrophysical contexts. When this interaction is primarily mediated by
hydrodynamics and radiative cooling, the survival of clouds can be phrased in
terms of the comparison between a timescale that dictates the evolution of the
cloud-wind interaction, (the dynamical time-scale ) and the
relevant cooling timescale . Previously proposed survival
criteria, which can disagree by large factors about the size of the smallest
surviving clouds, differ in both their choice of and (to a
lesser extent) . Here we present a new criterion which agrees
with a previously proposed empirical formulae but is based on simple physical
principles. The key insight is that clouds can grow if they are able to mix and
cool gas from the hot wind faster than it advects by the cloud. Whereas prior
criteria associate with the cloud crushing timescale, our new
criterion links it to the characteristic cloud-crossing timescale of a
hot-phase fluid element, making it more physically consistent with shear-layer
studies. We develop this insight into a predictive expression and validate it
with hydrodynamic ENZO-E simulations of ,
pressure-confined clouds in hot supersonic winds, exploring, in particular,
high wind/cloud density contrasts, where disagreements are most pronounced.
Finally, we illustrate how discrepancies among previous criteria primarily
emerged due to different choices of simulation conditions and cooling
properties, and discuss how they can be reconciled.Comment: 6.5 pages, 4 figures, submitted to ApJ
Spectral Analysis of a Four Mode Cluster State
We theoretically evaluate the squeezed joint operators produced in a single
optical parametric oscillator which generates quadripartite entangled outputs,
as demonstrated experimentally by Pysher et al. \cite{pysher}[Phys. Rev. Lett.
107, 030505 (2011)]. Using a linearized fluctuation analysis we calculate the
squeezing of the joint quadrature operators below threshold for a range of
local oscillator phases and frequencies. These results add to the existing
theoretical understanding of this potentially important system.Comment: 4 pages, 6 figure
Qubit phase space: SU(n) coherent-state P representations
We introduce a phase-space representation for qubits and spin models. The technique uses an SU(n) coherent-state basis and can equally be used for either static or dynamical simulations. We review previously known definitions and operator identities, and show how these can be used to define an off-diagonal, positive phase-space representation analogous to the positive-P function. As an illustration of the phase-space method, we use the example of the Ising model, which has exact solutions for the finite-temperature canonical ensemble in two dimensions. We show how a canonical ensemble for an Ising model of arbitrary structure can be efficiently simulated using SU(2) or atomic coherent states. The technique utilizes a transformation from a canonical (imaginary-time) weighted simulation to an equivalent unweighted real-time simulation. The results are compared to the exactly soluble two-dimensional case. We note that Ising models in one, two, or three dimensions are potentially achievable experimentally as a lattice gas of ultracold atoms in optical lattices. The technique is not restricted to canonical ensembles or to Ising-like couplings. It is also able to be used for real-time evolution and for systems whose time evolution follows a master equation describing decoherence and coupling to external reservoirs. The case of SU(n) phase space is used to describe n-level systems. In general, the requirement that time evolution be stochastic corresponds to a restriction to Hamiltonians and master equations that are quadratic in the group generators or generalized spin operators
Spectral Parameters for Scattering Amplitudes in N=4 Super Yang-Mills Theory
49 pages, 20 figures; v2: typos fixedPlanar N=4 Super Yang-Mills theory appears to be a quantum integrable four-dimensional conformal theory. This has been used to find equations believed to describe its exact spectrum of anomalous dimensions. Integrability seemingly also extends to the planar space-time scattering amplitudes of the N=4 model, which show strong signs of Yangian invariance. However, in contradistinction to the spectral problem, this has not yet led to equations determining the exact amplitudes. We propose that the missing element is the spectral parameter, ubiquitous in integrable models. We show that it may indeed be included into recent on-shell approaches to scattering amplitude integrands, providing a natural deformation of the latter. Under some constraints, Yangian symmetry is preserved. Finally we speculate that the spectral parameter might also be the regulator of choice for controlling the infrared divergences appearing when integrating the integrands in exactly four dimensions.Peer reviewe
Quantum many-body simulations using Gaussian phase-space representations
Phase-space representations are of increasing importance as a viable and
successful means to study exponentially complex quantum many-body systems from
first principles. This review traces the background of these methods, starting
from the early work of Wigner, Glauber and Sudarshan. We focus on modern
phase-space approaches using non-classical phase-space representations. These
lead to the Gaussian representation, which unifies bosonic and fermionic
phase-space. Examples treated include quantum solitons in optical fibers,
colliding Bose-Einstein condensates, and strongly correlated fermions on
lattices.Comment: Short Review (10 pages); Corrected typo in eq (14); Added a few more
reference
Effective diffusion constant in a two dimensional medium of charged point scatterers
We obtain exact results for the effective diffusion constant of a two
dimensional Langevin tracer particle in the force field generated by charged
point scatterers with quenched positions. We show that if the point scatterers
have a screened Coulomb (Yukawa) potential and are uniformly and independently
distributed then the effective diffusion constant obeys the
Volgel-Fulcher-Tammann law where it vanishes. Exact results are also obtained
for pure Coulomb scatterers frozen in an equilibrium configuration of the same
temperature as that of the tracer.Comment: 9 pages IOP LaTex, no figure
Postoperative atrial fibrillation in patients on statins undergoing isolated cardiac valve surgery: a meta-analysis
Introduction: The efficacy of perioperative statin therapy in decreasing postoperative morbidity in patients undergoing valve replacements and repairs is unknown. The aim of our study was to determine whether or not the literature supports the hypothesis that statins decrease postoperative atrial fibrillation (AF), and hence improve short-term postoperative outcomes in patients undergoing isolated cardiac valve surgery.Method: We conducted a meta-analysis of studies on postoperative outcomes associated with statin therapy following isolated valve replacement or repair. The data was taken from published studies on valvular heart surgery patients. Participants were patients who underwent either isolated cardiac valve replacement or repair. Patients in the intervention group received statins prior to their surgery. Three databases were searched: Ovid Healthstar, 1966 to April 2012; Ovid Medline, 1946 to 31 May 2012; and Embase, 1974 to 30 May 2012. The meta-analysis was conducted using Review Manager® version 5.1.Results: Statins did not decrease the incidence of postoperative AF in patients undergoing isolated cardiac valve surgery [odds ratio (OR) 1.19, 95% confidence interval (CI): 0.80– 1.77)], although there was significant heterogeneity for the outcome of postoperative AF (I2 55%, 95% CI: 27–72). Statins were associated with a decrease in 30-day mortality (OR 0.43, 95% CI: 0.24–0.75).Conclusion: Although this meta-analysis suggests that chronic statin therapy did not prevent postoperative AF in unselected valvular heart surgical patients, the heterogeneity indicates that this outcome should be viewed with caution and further research is recommended.Keywords: atrial fibrillation, cardiac surgery, statin
Phase-space structures in quantum-plasma wave turbulence
The quasilinear theory of the Wigner-Poisson system in one spatial dimension
is examined. Conservation laws and properties of the stationary solutions are
determined. Quantum effects are shown to manifest themselves in transient
periodic oscillations of the averaged Wigner function in velocity space. The
quantum quasilinear theory is checked against numerical simulations of the
bump-on-tail and the two-stream instabilities. The predicted wavelength of the
oscillations in velocity space agrees well with the numerical results
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