562 research outputs found
Sealed Computation: Abstract Requirements for Mechanisms to Support Trustworthy Cloud Computing
In cloud computing, data processing is delegated to a remote party for
efficiency and flexibility reasons. A practical user requirement usually is
that the confidentiality and integrity of data processing needs to be
protected. In the common scenarios of cloud computing today, this can only be
achieved by assuming that the remote party does not in any form act
maliciously. In this paper, we propose an approach that avoids having to trust
a single entity. Our approach is based on two concepts: (1) the technical
abstraction of sealed computation, i.e., a technical mechanism to confine the
processing of data within a tamper-proof hardware container, and (2) the
additional role of an auditing party that itself cannot add functionality to
the system but is able to check whether the system (including the mechanism for
sealed computation) works as expected. We discuss the abstract technical and
procedural requirements of these concepts and explain how they can be applied
in practice
Transit Timing and Duration Variations for the Discovery and Characterization of Exoplanets
Transiting exoplanets in multi-planet systems have non-Keplerian orbits which
can cause the times and durations of transits to vary. The theory and
observations of transit timing variations (TTV) and transit duration variations
(TDV) are reviewed. Since the last review, the Kepler spacecraft has detected
several hundred perturbed planets. In a few cases, these data have been used to
discover additional planets, similar to the historical discovery of Neptune in
our own Solar System. However, the more impactful aspect of TTV and TDV studies
has been characterization of planetary systems in which multiple planets
transit. After addressing the equations of motion and parameter scalings, the
main dynamical mechanisms for TTV and TDV are described, with citations to the
observational literature for real examples. We describe parameter constraints,
particularly the origin of the mass/eccentricity degeneracy and how it is
overcome by the high-frequency component of the signal. On the observational
side, derivation of timing precision and introduction to the timing diagram are
given. Science results are reviewed, with an emphasis on mass measurements of
transiting sub-Neptunes and super-Earths, from which bulk compositions may be
inferred.Comment: Revised version. Invited review submitted to 'Handbook of
Exoplanets,' Exoplanet Discovery Methods section, Springer Reference Works,
Juan Antonio Belmonte and Hans Deeg, Eds. TeX and figures may be found at
https://github.com/ericagol/TTV_revie
Adding control to arbitrary unknown quantum operations
While quantum computers promise significant advantages, the complexity of
quantum algorithms remains a major technological obstacle. We have developed
and demonstrated an architecture-independent technique that simplifies adding
control qubits to arbitrary quantum operations-a requirement in many quantum
algorithms, simulations and metrology. The technique is independent of how the
operation is done, does not require knowledge of what the operation is, and
largely separates the problems of how to implement a quantum operation in the
laboratory and how to add a control. We demonstrate an entanglement-based
version in a photonic system, realizing a range of different two-qubit gates
with high fidelity.Comment: 9 pages, 8 figure
Adherence to self-administered tuberculosis treatment in a high HIV-prevalence setting: a cross-sectional survey in Homa Bay, Kenya.
Good adherence to treatment is crucial to control tuberculosis (TB). Efficiency and feasibility of directly observed therapy (DOT) under routine program conditions have been questioned. As an alternative, Médecins sans Frontières introduced self-administered therapy (SAT) in several TB programs. We aimed to measure adherence to TB treatment among patients receiving TB chemotherapy with fixed dose combination (FDC) under SAT at the Homa Bay district hospital (Kenya). A second objective was to compare the adherence agreement between different assessment tools
Formation of Super-Earths
Super-Earths are the most abundant planets known to date and are
characterized by having sizes between that of Earth and Neptune, typical
orbital periods of less than 100 days and gaseous envelopes that are often
massive enough to significantly contribute to the planet's overall radius.
Furthermore, super-Earths regularly appear in tightly-packed multiple-planet
systems, but resonant configurations in such systems are rare. This chapters
summarizes current super-Earth formation theories. It starts from the formation
of rocky cores and subsequent accretion of gaseous envelopes. We follow the
thermal evolution of newly formed super-Earths and discuss their atmospheric
mass loss due to disk dispersal, photoevaporation, core-cooling and collisions.
We conclude with a comparison of observations and theoretical predictions,
highlighting that even super-Earths that appear as barren rocky cores today
likely formed with primordial hydrogen and helium envelopes and discuss some
paths forward for the future.Comment: Invited review accepted for publication in the 'Handbook of
Exoplanets,' Planet Formation section, Springer Reference Works, Juan Antonio
Belmonte and Hans Deeg, Ed
Axion-like-particle search with high-intensity lasers
We study ALP-photon-conversion within strong inhomogeneous electromagnetic
fields as provided by contemporary high-intensity laser systems. We observe
that probe photons traversing the focal spot of a superposition of Gaussian
beams of a single high-intensity laser at fundamental and frequency-doubled
mode can experience a frequency shift due to their intermittent propagation as
axion-like-particles. This process is strongly peaked for resonant masses on
the order of the involved laser frequencies. Purely laser-based experiments in
optical setups are sensitive to ALPs in the mass range and can
thus complement ALP searches at dipole magnets.Comment: 25 pages, 2 figure
Discontinuous Galerkin methods for nonlinear scalar hyperbolic conservation laws: divided difference estimates and accuracy enhancement
In this paper, an analysis of the accuracy-enhancement for the discontinuous Galerkin (DG) method applied to one-dimensional scalar nonlinear hyperbolic conservation laws is carried out. This requires analyzing the divided difference of the errors for the DG solution. We therefore first prove that the alpha-th order (1 <= \alpha <= k+1) divided difference of the DG error in the L2-norm is of order k+(3-alpha)/2 when upwind fluxes are used, under the condition that |f'(u)| possesses a uniform positive lower bound. By the duality argument, we then derive superconvergence results of order k+(3-alpha)/2 in the negative-order norm, demonstrating that it is possible to extend the Smoothness-Increasing Accuracy-Conserving filter to nonlinear conservation laws to obtain at least (3k/2+1)th order superconvergence for post-processed solutions. As a by-product, for variable coefficient hyperbolic equations, we provide an explicit proof for optimal convergence results of order k+1 in the L2-norm for the divided differences of DG errors and thus (2k+1)th order superconvergence in negative-order norm holds. Numerical experiments are given that confirm the theoretical results
Early results from ChanPLaNS: mystery of hard X-ray emitting CSPNe
We are presently using the Chandra X-ray Observatory to conduct the first systematic X-ray survey of planetary nebulae (PNe) in the solar neighborhood. The Chandra Planetary Nebula Survey (ChanPlaNS) is a 570 ks Chandra Cycle 12 Large Program targeting 21 high-excitation PNe within ∼1.5 kpc of Earth. When complete, this survey will provide a suite of new X-ray diagnostics that will inform the study of late stellar evolution, binary star astrophysics, and wind interactions. Among the early results of ChanPlaNS (when combined with archival Chandra data) is a surprisingly high detection rate of relatively hard X-ray emission from CSPNe. Specifically, X-ray point sources are clearly detected in roughly half of the ∼30 high-excitation PNe observed thus far by Chandra, and all but one of these X-ray-emitting CSPNe display evidence for a hard (few MK) component in their Chandra spectra. Only the central star of the Dumbbell appears to display 'pure' hot blackbody emission from a ∼200 kK hot white dwarf photosphere in the X-ray band. Potential explanations for the'excess' hard X-ray emission detected from the other CSPNe include late-type companions (heretofore undetected, in most cases) whose coronae have been rejuvenated by recent interactions with the mass-losing WD progenitor, non-LTE effects in hot white dwarf photospheres, self-shocking variable winds from the central star, and slow (re-)accretion of previously ejected red giant envelope mass. © 2012 International Astronomical Union.published_or_final_versio
Quantum Process Tomography of a Universal Entangling Gate Implemented with Josephson Phase Qubits
Quantum logic gates must perform properly when operating on their standard
input basis states, as well as when operating on complex superpositions of
these states. Experiments using superconducting qubits have validated the truth
table for particular implementations of e.g. the controlled-NOT gate [1,2], but
have not fully characterized gate operation for arbitrary superpositions of
input states. Here we demonstrate the use of quantum process tomography (QPT)
[3,4] to fully characterize the performance of a universal entangling gate
between two superconducting quantum bits. Process tomography permits complete
gate analysis, but requires precise preparation of arbitrary input states,
control over the subsequent qubit interaction, and simultaneous single-shot
measurement of the output states. We use QPT to measure the fidelity of the
entangling gate and to quantify the decoherence mechanisms affecting the gate
performance. In addition to demonstrating a promising fidelity, our entangling
gate has a on/off ratio of 300, a level of adjustable coupling that will become
a requirement for future high-fidelity devices. This is the first solid-state
demonstration of QPT in a two-qubit system, as solid-state process tomography
has previously only been demonstrated with single qubits [5,6]
Smoothness-Increasing Accuracy-Conserving (SIAC) filtering and quasi interpolation: A unified view
Filtering plays a crucial role in postprocessing and analyzing data in scientific and engineering applications. Various application-specific filtering schemes have been proposed based on particular design criteria. In this paper, we focus on establishing the theoretical connection between quasi-interpolation and a class of kernels (based on B-splines) that are specifically designed for the postprocessing of the discontinuous Galerkin (DG) method called Smoothness-Increasing Accuracy-Conserving (SIAC) filtering. SIAC filtering, as the name suggests, aims to increase the smoothness of the DG approximation while conserving the inherent accuracy of the DG solution (superconvergence). Superconvergence properties of SIAC filtering has been studied in the literature. In this paper, we present the theoretical results that establish the connection between SIAC filtering to long-standing concepts in approximation theory such as quasi-interpolation and polynomial reproduction. This connection bridges the gap between the two related disciplines and provides a decisive advancement in designing new filters and mathematical analysis of their properties. In particular, we derive a closed formulation for convolution of SIAC kernels with polynomials. We also compare and contrast cardinal spline functions as an example of filters designed for image processing applications with SIAC filters of the same order, and study their properties
- …