275 research outputs found
Setting school-level outcome standards
To establish international standards for medical schools, an appropriate panel of experts must decide on performance standards. A pilot test of such standards was set in the context of a multidimensional (multiple-choice question examination, objective structured clinical examination, faculty observation) examination at 8 leading schools in China. Methods A group of 16 medical education leaders from a broad array of countries met over a 3-day period. These individuals considered competency domains, examination items, and the percentage of students who could fall below a cut-off score if the school was still to be considered as meeting competencies. This 2-step process started with a discussion of the borderline school and the relative difficulty of a borderline school in achieving acceptable standards in a given competency domain. Committee members then estimated the percentage of students falling below the standard that is tolerable at a borderline school and were allowed to revise their ratings after viewing pilot data. Results Tolerable failure rates ranged from 10% to 26% across competency domains and examination types. As with other standard-setting exercises, standard deviations from initial to final estimates of the tolerable failure rates fell, but the cut-off scores did not change significantly. Final, but not initial cut-off scores were correlated with student failure rates ( r = 0.59, P = 0.03). Discussion This paper describes a method to set school-level outcome standards at an international level based on prior established standard-setting methods. Further refinement of this process and validation using other examinations in other countries will be needed to achieve accurate international standards.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71572/1/j.1365-2929.2005.02374.x.pd
On the Round Complexity of Asynchronous Crusader Agreement
We present new lower and upper bounds on the number of communication rounds required for asynchronous Crusader Agreement (CA) and Binding Crusader Agreement (BCA), two primitives that are used for solving binary consensus. We show results for the information theoretic and authenticated settings. In doing so, we present a generic model for proving round complexity lower bounds in the asynchronous setting.
In some settings, our attempts to prove lower bounds on round complexity fail. Instead, we show new, tight, rather surprising round complexity upper bounds for Byzantine fault tolerant BCA with and without a PKI setup
Visualizing Poiseuille flow of hydrodynamic electrons
Hydrodynamics is a general description for the flow of a fluid, and is
expected to hold even for fundamental particles such as electrons when
inter-particle interactions dominate. While various aspects of electron
hydrodynamics were revealed in recent experiments, the fundamental spatial
structure of hydrodynamic electrons, the Poiseuille flow profile, has remained
elusive. In this work, we provide the first real-space imaging of Poiseuille
flow of an electronic fluid, as well as visualization of its evolution from
ballistic flow. Utilizing a scanning nanotube single electron transistor, we
image the Hall voltage of electronic flow through channels of high-mobility
graphene. We find that the profile of the Hall field across the channel is a
key physical quantity for distinguishing ballistic from hydrodynamic flow. We
image the transition from flat, ballistic field profiles at low temperature
into parabolic field profiles at elevated temperatures, which is the hallmark
of Poiseuille flow. The curvature of the imaged profiles is qualitatively
reproduced by Boltzmann calculations, which allow us to create a 'phase
diagram' that characterizes the electron flow regimes. Our results provide
long-sought, direct confirmation of Poiseuille flow in the solid state, and
enable a new approach for exploring the rich physics of interacting electrons
in real space
Model theory of operator algebras III: Elementary equivalence and II_1 factors
We use continuous model theory to obtain several results concerning
isomorphisms and embeddings between II_1 factors and their ultrapowers. Among
other things, we show that for any II_1 factor M, there are continuum many
nonisomorphic separable II_1 factors that have an ultrapower isomorphic to an
ultrapower of M. We also give a poor man's resolution of the Connes Embedding
Problem: there exists a separable II_1 factor such that all II_1 factors embed
into one of its ultrapowers.Comment: 16 page
Scalable architecture for trapped-ion quantum computing using RF traps and dynamic optical potentials
Qubits based on ions trapped in linear radio-frequency traps form a
successful platform for quantum computing, due to their high fidelity of
operations, all-to-all connectivity and degree of local control. In principle
there is no fundamental limit to the number of ion-based qubits that can be
confined in a single 1D register. However, in practice there are two main
issues associated with long trapped-ion crystals, that stem from the
'softening' of their modes of motion, upon scaling up: high heating rates of
the ions' motion, and a dense motional spectrum; both impede the performance of
high-fidelity qubit operations. Here we propose a holistic, scalable
architecture for quantum computing with large ion-crystals that overcomes these
issues. Our method relies on dynamically-operated optical potentials, that
instantaneously segment the ion-crystal into cells of a manageable size. We
show that these cells behave as nearly independent quantum registers, allowing
for parallel entangling gates on all cells. The ability to reconfigure the
optical potentials guarantees connectivity across the full ion-crystal, and
also enables efficient mid-circuit measurements. We study the implementation of
large-scale parallel multi-qubit entangling gates that operate simultaneously
on all cells, and present a protocol to compensate for crosstalk errors,
enabling full-scale usage of an extensively large register. We illustrate that
this architecture is advantageous both for fault-tolerant digital quantum
computation and for analog quantum simulations
Phylogenetic evidence from freshwater crayfishes that cave adaptation is not an evolutionary dead-end.
Caves are perceived as isolated, extreme habitats with a uniquely specialized biota, which long ago led to the idea that caves are evolutionary dead-ends. This implies that cave-adapted taxa may be doomed for extinction before they can diversify or transition to a more stable state. However, this hypothesis has not been explicitly tested in a phylogenetic framework with multiple independently evolved cave-dwelling groups. Here, we use the freshwater crayfish, a group with dozens of cave-dwelling species in multiple lineages, as a system to test this hypothesis. We consider historical patterns of lineage diversification and habitat transition as well as current patterns of geographic range size. We find that while cave-dwelling lineages have small relative range sizes and rarely transition back to the surface, they exhibit remarkably similar diversification patterns to those of other habitat types and appear to be able to maintain a diversity of lineages through time. This suggests that cave adaptation is not a dead-end for freshwater crayfish, which has positive implications for our understanding of biodiversity and conservation in cave habitats
Ions in mixed dielectric solvents: density profiles and osmotic pressure between charged interfaces
The forces between charged macromolecules, usually given in terms of osmotic
pressure, are highly affected by the intervening ionic solution. While in most
theoretical studies the solution is treated as a homogeneous structureless
dielectric medium, recent experimental studies concluded that, for a bathing
solution composed of two solvents (binary mixture), the osmotic pressure
between charged macromolecules is affected by the binary solvent composition.
By adding local solvent composition terms to the free energy, we obtain a
general expression for the osmotic pressure, in planar geometry and within the
mean-field framework. The added effect is due to the permeability inhomogeneity
and nonelectrostatic short-range interactions between the ions and solvents
(preferential solvation). This effect is mostly pronounced at small distances
and leads to a reduction in the osmotic pressure for macromolecular separations
of the order 1--2 nm. Furthermore, it leads to a depletion of one of the two
solvents from the charged macromolecules (modeled as planar interfaces).
Lastly, by comparing the theoretical results with experimental ones, an
explanation based on preferential solvation is offered for recent experiments
on the osmotic pressure of DNA solutions.Comment: 13 pages, 8 figure
A New Class of Changing-Look LINERs
We report the discovery of six active galactic nuclei (AGN) caught "turning
on" during the first nine months of the Zwicky Transient Facility (ZTF) survey.
The host galaxies were classified as LINERs by weak narrow forbidden line
emission in their archival SDSS spectra, and detected by ZTF as nuclear
transients. In five of the cases, we found via follow-up spectroscopy that they
had transformed into broad-line AGN, reminiscent of the changing-look LINER
iPTF 16bco. In one case, ZTF18aajupnt/AT2018dyk, follow-up HST UV and
ground-based optical spectra revealed the transformation into a narrow-line
Seyfert 1 (NLS1) with strong [Fe VII, X, XIV] and He II 4686 coronal lines.
Swift monitoring observations of this source reveal bright UV emission that
tracks the optical flare, accompanied by a luminous soft X-ray flare that peaks
~60 days later. Spitzer follow-up observations also detect a luminous
mid-infrared flare implying a large covering fraction of dust. Archival light
curves of the entire sample from CRTS, ATLAS, and ASAS-SN constrain the onset
of the optical nuclear flaring from a prolonged quiescent state. Here we
present the systematic selection and follow-up of this new class of
changing-look LINERs, compare their properties to previously reported
changing-look Seyfert galaxies, and conclude that they are a unique class of
transients well-suited to test the uncertain physical processes associated with
the LINER accretion state.Comment: Submitted to ApJ, 31 pages, 17 Figures (excluding Appendix due to
file size constraints but will be available in electronic version
The WiggleZ Dark Energy Survey: measuring the cosmic expansion history using the Alcock-Paczynski test and distant supernovae
Astronomical observations suggest that today's Universe is dominated by a
dark energy of unknown physical origin. One of the most notable consequences in
many models is that dark energy should cause the expansion of the Universe to
accelerate: but the expansion rate as a function of time has proven very
difficult to measure directly. We present a new determination of the cosmic
expansion history by combining distant supernovae observations with a
geometrical analysis of large-scale galaxy clustering within the WiggleZ Dark
Energy Survey, using the Alcock-Paczynski test to measure the distortion of
standard spheres. Our result constitutes a robust and non-parametric
measurement of the Hubble expansion rate as a function of time, which we
measure with 10-15% precision in four bins within the redshift range 0.1 < z <
0.9. We demonstrate that the cosmic expansion is accelerating, in a manner
independent of the parameterization of the cosmological model (although
assuming cosmic homogeneity in our data analysis). Furthermore, we find that
this expansion history is consistent with a cosmological-constant dark energy.Comment: 13 pages, 7 figures, accepted for publication by MNRA
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