15,685 research outputs found
Localization and its consequences for quantum walk algorithms and quantum communication
The exponential speed-up of quantum walks on certain graphs, relative to
classical particles diffusing on the same graph, is a striking observation. It
has suggested the possibility of new fast quantum algorithms. We point out here
that quantum mechanics can also lead, through the phenomenon of localization,
to exponential suppression of motion on these graphs (even in the absence of
decoherence). In fact, for physical embodiments of graphs, this will be the
generic behaviour. It also has implications for proposals for using spin
networks, including spin chains, as quantum communication channels.Comment: 4 pages, 1 eps figure. Updated references and cosmetic changes for v
Magnetic Fields in Star-Forming Molecular Clouds I. The First Polarimetry of OMC-3 in Orion A
The first polarimetric images of the OMC-3 region of the Orion A filamentary
molecular cloud are presented. Using the JCMT, we have detected polarized
thermal emission at 850 microns from dust along a 6' length of the dense
filament. The polarization pattern is highly ordered and is aligned with the
filament throughout most of the region. The plane-of-sky magnetic field
direction is perpendicular to the measured polarization. The mean percentage
polarization is 4.2% with a 1 sigma dispersion of 1%. This region is part of
the integral-shaped filament, and active star formation is ongoing along its
length. The protostellar outflow directions do not appear to be consistently
correlated with the direction of the plane-of-sky field or the filament
structure itself. Depolarization toward the filament center, previously
detected in many other star-forming cores and protostars, is also evident in
our data. (abstract abridged)Comment: 9 pages plus 2 figures (1 colour); accepted for publication in the
March 10, 2000 issue (vol. 531 #2) of The Astrophysical Journa
Complex responses to movement-based disease control: when livestock trading helps
Livestock disease controls are often linked to movements between farms, for example, via quarantine and pre- or post-movement testing. Designing effective controls, therefore, benefits from accurate assessment of herd-to-herd transmission. Household models of human infections make use of R*, the number of groups infected by an initial infected group, which is a metapopulation level analogue of the basic reproduction number R0 that provides a better characterization of disease spread in a metapopulation. However, existing approaches to calculate R* do not account for individual movements between locations which means we lack suitable tools for livestock systems. We address this gap using next-generation matrix approaches to capture movements explicitly and introduce novel tools to calculate R* in any populations coupled by individual movements. We show that depletion of infectives in the source group, which hastens its recovery, is a phenomenon with important implications for design and efficacy of movement-based controls. Underpinning our results is the observation that R* peaks at intermediate livestock movement rates. Consequently, under movement-based controls, infection could be controlled at high movement rates but persist at intermediate rates. Thus, once control schemes are present in a livestock system, a reduction in movements can counterintuitively lead to increased disease prevalence. We illustrate our results using four important livestock diseases (bovine viral diarrhoea, bovine herpes virus, Johne's disease and Escherichia coli O157) that each persist across different movement rate ranges with the consequence that a change in livestock movements could help control one disease, but exacerbate another
The Nikolaevskiy equation with dispersion
The Nikolaevskiy equation was originally proposed as a model for seismic
waves and is also a model for a wide variety of systems incorporating a
neutral, Goldstone mode, including electroconvection and reaction-diffusion
systems. It is known to exhibit chaotic dynamics at the onset of pattern
formation, at least when the dispersive terms in the equation are suppressed,
as is commonly the practice in previous analyses. In this paper, the effects of
reinstating the dispersive terms are examined. It is shown that such terms can
stabilise some of the spatially periodic traveling waves; this allows us to
study the loss of stability and transition to chaos of the waves. The secondary
stability diagram (Busse balloon) for the traveling waves can be remarkably
complicated.Comment: 24 pages; accepted for publication in Phys. Rev.
Protein-mediated DNA Loop Formation and Breakdown in a Fluctuating Environment
Living cells provide a fluctuating, out-of-equilibrium environment in which
genes must coordinate cellular function. DNA looping, which is a common means
of regulating transcription, is very much a stochastic process; the loops arise
from the thermal motion of the DNA and other fluctuations of the cellular
environment. We present single-molecule measurements of DNA loop formation and
breakdown when an artificial fluctuating force, applied to mimic a fluctuating
cellular environment, is imposed on the DNA. We show that loop formation is
greatly enhanced in the presence of noise of only a fraction of , yet
find that hypothetical regulatory schemes that employ mechanical tension in the
DNA--as a sensitive switch to control transcription--can be surprisingly robust
due to a fortuitous cancellation of noise effects
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
Coplanar Circumbinary Debris Disks
We present resolved Herschel images of circumbinary debris disks in the alpha
CrB (HD139006) and beta Tri (HD13161) systems. We find that both disks are
consistent with being aligned with the binary orbital planes. Though secular
perturbations from the binary can align the disk, in both cases the alignment
time at the distances at which the disk is resolved is greater than the stellar
age, so we conclude that the coplanarity was primordial. Neither disk can be
modelled as a narrow ring, requiring extended radial distributions. To satisfy
both the Herschel and mid-IR images of the alpha CrB disk, we construct a model
that extends from 1-300AU, whose radial profile is broadly consistent with a
picture where planetesimal collisions are excited by secular perturbations from
the binary. However, this model is also consistent with stirring by other
mechanisms, such as the formation of Pluto-sized objects. The beta Tri disk
model extends from 50-400AU. A model with depleted (rather than empty) inner
regions also reproduces the observations and is consistent with binary and
other stirring mechanisms. As part of the modelling process, we find that the
Herschel PACS beam varies by as much as 10% at 70um and a few % at 100um. The
70um variation can therefore hinder image interpretation, particularly for
poorly resolved objects. The number of systems in which circumbinary debris
disk orientations have been compared with the binary plane is now four. More
systems are needed, but a picture in which disks around very close binaries
(alpha CrB, beta Tri, and HD 98800, with periods of a few weeks to a year) are
aligned, and disks around wider binaries (99 Her, with a 50 yr period) are
misaligned, may be emerging. This picture is qualitatively consistent with the
expectation that the protoplanetary disks from which the debris emerged are
more likely to be aligned if their binaries have shorter periods.Comment: accepted to MNRA
Feeding of Conserved Forage - Implications to Grassland Management and Production
This review will focus on the use of conserved forages in pastoral dairy farm systems; especially on their conservation as an aid to good grazing management during periods of rapid pasture growth, and on their consumption as an aid to good feeding management and grazing management during periods of slow pasture growth. Conservation of pasture, in order to move it from one time to another later time, always involves “costs”, either financial expenditure or physical losses (of dry matter/quality) or both, which must be evaluated in the whole system.
Most of the data is drawn from New Zealand and Australia, with some from England and Ireland. The topic has been reviewed before (e.g. Rogers, 1984 and 1985; Leaver, 1985; Phillips, 1988; Mayne, 1991; Stockdale et al., 1997), but the present review will put greater emphasis on the effects on pasture management and on the whole pastoral system, and on recent results, because the cows’ ability to respond to supplementary feeds is now larger than it used to be, as a result of steady genetic improvement in milk producing capacities. In recent years there has been marked increase in supplement use on pasture based dairy farming systems in New Zealand often driven by perceived limitations to production increases in existing systems rather than profit (Attrill and Miller 1996). This review attempts to identify, and discuss, the factors involved in supplementary feeding (i.e. the responses of cows and farming systems to extra feed) to enable sustainable increases in productivity and profitability on pastoral based systems
Coherent Time Evolution and Boundary Conditions of Two-Photon Quantum Walks
Multi-photon quantum walks in integrated optics are an attractive controlled
quantum system, that can mimic less readily accessible quantum systems and
exhibit behavior that cannot in general be accurately replicated by classical
light without an exponential overhead in resources. The ability to observe time
evolution of such systems is important for characterising multi-particle
quantum dynamics---notably this includes the effects of boundary conditions for
walks in spaces of finite size. Here we demonstrate the coherent evolution of
quantum walks of two indistinguishable photons using planar arrays of 21
evanescently coupled waveguides fabricated in silicon oxynitride technology. We
compare three time evolutions, that follow closely a model assuming unitary
evolution, corresponding to three different lengths of the array---in each case
we observe quantum interference features that violate classical predictions.
The longest array includes reflecting boundary conditions.Comment: 7 pages,7 figure
Tunneling of a composite particle: Effects of intrinsic structure
We consider simple models of tunneling of an object with intrinsic degrees of
freedom. This important problem was not extensively studied until now, in spite
of numerous applications in various areas of physics and astrophysics. We show
possibilities of enhancement for the probability of tunneling due to the
presence of intrinsic degrees of freedom split by weak external fields or by
polarizability of the slow composite object.Comment: 6 pages, 3 figures, version to be published in Journal of Physics
- …