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 $T_1$ of the Zeeman sublevel populations can be conveniently adjusted to provide long observation times. We also show that the transverse relaxation times $T_2$ 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 $\chi^{(2)}$ 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