1,581 research outputs found
The effect of symmetry breaking on the dynamics near a structurally stable heteroclinic cycle between equilibria and a periodic orbit
The effect of small forced symmetry breaking on the dynamics near a structurally stable heteroclinic
cycle connecting two equilibria and a periodic orbit is investigated. This type of system is known
to exhibit complicated, possibly chaotic dynamics including irregular switching of sign of various
phase space variables, but details of the mechanisms underlying the complicated dynamics have
not previously been investigated. We identify global bifurcations that induce the onset of chaotic
dynamics and switching near a heteroclinic cycle of this type, and by construction and analysis
of approximate return maps, locate the global bifurcations in parameter space. We find there is a
threshold in the size of certain symmetry-breaking terms below which there can be no persistent
switching. Our results are illustrated by a numerical example
On the feasibility of studying vortex noise in 2D superconductors with cold atoms
We investigate the feasibility of using ultracold neutral atoms trapped near
a thin superconductor to study vortex noise close to the
Kosterlitz-Thouless-Berezinskii transition temperature. Alkali atoms such as
rubidium probe the magnetic field produced by the vortices. We show that the
relaxation time of the Zeeman sublevel populations can be conveniently
adjusted to provide long observation times. We also show that the transverse
relaxation times for Zeeman coherences are ideal for studying the vortex
noise. We briefly consider the motion of atom clouds held close to the surface
as a method for monitoring the vortex motion.Comment: 4 pages, 1 figur
Upper bounds on success probabilities in linear optics
We develop an abstract way of defining linear-optics networks designed to
perform quantum information tasks such as quantum gates. We will be mainly
concerned with the nonlinear sign shift gate, but it will become obvious that
all other gates can be treated in a similar manner. The abstract scheme is
extremely well suited for analytical as well as numerical investigations since
it reduces the number of parameters for a general setting. With that we show
numerically and partially analytically for a wide class of states that the
success probability of generating a nonlinear sign shift gate does not exceed
1/4 which to our knowledge is the strongest bound to date.Comment: 8 pages, typeset using RevTex4, 5 EPS figure
On Toroidal Horizons in Binary Black Hole Inspirals
We examine the structure of the event horizon for numerical simulations of
two black holes that begin in a quasicircular orbit, inspiral, and finally
merge. We find that the spatial cross section of the merged event horizon has
spherical topology (to the limit of our resolution), despite the expectation
that generic binary black hole mergers in the absence of symmetries should
result in an event horizon that briefly has a toroidal cross section. Using
insight gained from our numerical simulations, we investigate how the choice of
time slicing affects both the spatial cross section of the event horizon and
the locus of points at which generators of the event horizon cross. To ensure
the robustness of our conclusions, our results are checked at multiple
numerical resolutions. 3D visualization data for these resolutions are
available for public access online. We find that the structure of the horizon
generators in our simulations is consistent with expectations, and the lack of
toroidal horizons in our simulations is due to our choice of time slicing.Comment: Submitted to Phys. Rev.
Quantum theory of light and noise polarization in nonlinear optics
We present a consistent quantum theory of the electromagnetic field in
nonlinearly responding causal media, with special emphasis on
media. Starting from QED in linearly responding causal media, we develop a
method to construct the nonlinear Hamiltonian expressed in terms of the complex
nonlinear susceptibility in a quantum mechanically consistent way. In
particular we show that the method yields the nonlinear noise polarization,
which together with the linear one is responsible for intrinsic quantum
decoherence.Comment: 4 pages, no figure
Efficiency limits for linear optical processing of single photons and single-rail qubits
We analyze the problem of increasing the efficiency of single-photon sources
or single-rail photonic qubits via linear optical processing and destructive
conditional measurements. In contrast to previous work we allow for the use of
coherent states and do not limit to photon-counting measurements. We conjecture
that it is not possible to increase the efficiency, prove this conjecture for
several important special cases, and provide extensive numerical results for
the general case.Comment: 10 pages, 4 figure
Energy as an Entanglement Witness for Quantum Many-Body Systems
We investigate quantum many-body systems where all low-energy states are
entangled. As a tool for quantifying such systems, we introduce the concept of
the entanglement gap, which is the difference in energy between the
ground-state energy and the minimum energy that a separable (unentangled) state
may attain. If the energy of the system lies within the entanglement gap, the
state of the system is guaranteed to be entangled. We find Hamiltonians that
have the largest possible entanglement gap; for a system consisting of two
interacting spin-1/2 subsystems, the Heisenberg antiferromagnet is one such
example. We also introduce a related concept, the entanglement-gap temperature:
the temperature below which the thermal state is certainly entangled, as
witnessed by its energy. We give an example of a bipartite Hamiltonian with an
arbitrarily high entanglement-gap temperature for fixed total energy range. For
bipartite spin lattices we prove a theorem demonstrating that the entanglement
gap necessarily decreases as the coordination number is increased. We
investigate frustrated lattices and quantum phase transitions as physical
phenomena that affect the entanglement gap.Comment: 16 pages, 3 figures, published versio
Surface-induced heating of cold polar molecules
We study the rotational and vibrational heating of diatomic molecules placed
near a surface at finite temperature on the basis of macroscopic quantum
electrodynamics. The internal molecular evolution is governed by transition
rates that depend on both temperature and position. Analytical and numerical
methods are used to investigate the heating of several relevant molecules near
various surfaces. We determine the critical distances at which the surface
itself becomes the dominant source of heating and we investigate the transition
between the long-range and short-range behaviour of the heating rates. A simple
formula is presented that can be used to estimate the surface-induced heating
rates of other molecules of interest. We also consider how the heating depends
on the thickness and composition of the surface.Comment: 17 pages, 7 figure
Development of the WHO-INTEGRATE evidence-to-decision framework: an overview of systematic reviews of decision criteria for health decision-making
Background Decision-making in public health and health policy is complex and requires careful deliberation of many and sometimes conflicting normative and technical criteria. Several approaches and tools, such as multi-criteria decision analysis, health technology assessments and evidence-to-decision (EtD) frameworks, have been proposed to guide decision-makers in selecting the criteria most relevant and appropriate for a transparent decision-making process. This study forms part of the development of the WHO-INTEGRATE EtD framework, a framework rooted in global health norms and values as reflected in key documents of the World Health Organization and the United Nations system. The objective of this study was to provide a comprehensive overview of criteria used in or proposed for real-world decision-making processes, including guideline development, health technology assessment, resource allocation and others. Methods We conducted an overview of systematic reviews through a combination of systematic literature searches and extensive reference searches. Systematic reviews reporting criteria used for real-world health decision-making by governmental or non-governmental organization on a supranational, national, or programme level were included and their quality assessed through a bespoke critical appraisal tool. The criteria reported in the reviews were extracted, de-duplicated and sorted into first-level (i.e. criteria), second-level (i.e. sub-criteria) and third-level (i.e. decision aspects) categories. First-level categories were developed a priori using a normative approach; second- and third-level categories were developed inductively. Results We included 36 systematic reviews providing criteria, of which one met all and another eleven met at least five of the items of our critical appraisal tool. The criteria were subsumed into 8 criteria, 45 sub-criteria and 200 decision aspects. The first-level of the category system comprised the following seven substantive criteria: \textquotedblHealth-related balance of benefits and harms\textquotedbl; \textquotedblHuman and individual rights\textquotedbl; \textquotedblAcceptability considerations\textquotedbl; \textquotedblSocietal considerations\textquotedbl; \textquotedblConsiderations of equity, equality and fairness\textquotedbl; \textquotedblCost and financial considerations\textquotedbl; and \textquotedblFeasibility and health system considerations\textquotedbl. In addition, we identified an eight criterion \textquotedblEvidence\textquotedbl. Conclusion This overview of systematic reviews provides a comprehensive overview of criteria used or suggested for real-world health decision-making. It also discusses key challenges in the selection of the most appropriate criteria and in seeking to implement a fair decision-making process
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