703 research outputs found
Fast Decoders for Topological Quantum Codes
We present a family of algorithms, combining real-space renormalization
methods and belief propagation, to estimate the free energy of a topologically
ordered system in the presence of defects. Such an algorithm is needed to
preserve the quantum information stored in the ground space of a topologically
ordered system and to decode topological error-correcting codes. For a system
of linear size L, our algorithm runs in time log L compared to L^6 needed for
the minimum-weight perfect matching algorithm previously used in this context
and achieves a higher depolarizing error threshold.Comment: 4 pages, 4 figure
The role of oxygen vacancies on the structure and the density of states of iron doped zirconia
In this paper we study, both with theoretical and experimental approach, the
effect of iron doping in zirconia. Combining density functional theory (DFT)
simulations with the experimental characterization of thin films, we show that
iron is in the Fe3+ oxidation state and accordingly that the films are rich in
oxygen vacancies (VO). VO favor the formation of the tetragonal phase in doped
zirconia (ZrO2:Fe) and affect the density of state at the Fermi level as well
as the local magnetization of Fe atoms. We also show that the Fe(2p) and Fe(3p)
energy levels can be used as a marker for the presence of vacancies in the
doped system. In particular the computed position of the Fe(3p) peak is
strongly sensitive to the VO to Fe atoms ratio. A comparison of the theoretical
and experimental Fe(3p) peak position suggests that in our films this ratio is
close to 0.5. Besides the interest in the material by itself, ZrO2:Fe
constitutes a test case for the application of DFT on transition metals
embedded in oxides. In ZrO2:Fe the inclusion of the Hubbard U correction
significantly changes the electronic properties of the system. However the
inclusion of this correction, at least for the value U = 3.3 eV chosen in the
present work, worsen the agreement with the measured photo-emission valence
band spectra.Comment: 24 pages, 8 figure
Taurus Tunable Filter -- seven years of observing
The Taurus Tunable Filter (TTF) has now been in regular use for seven years
on the Anglo-Australian Telescope. The instrument was also used for three years
(1996--1999) on the William Herschel Telescope. We present a brief review of
the different applications in order to illustrate the versatility of tunable
filters in optical/IR spectrophotometric imaging. Tunable filters are now
planned or are under development for 6-10m class telescopes which ensures their
use for years to come.Comment: PASA, accepted. 20 pages, 9 figure
Extensive counter-ion interactions seen at the surface of subtilisin in an aqueous medium
The extent of protein and counter-ion interactions in solution is still far from being fully described and understood. In low dielectric media there is documented evidence that counter-ions do bind and affect enzymatic activity. However, published crystal structures of macromolecules of biological interest in aqueous solution often do not report the presence of any counter-ions on the surface. The extent of counter-ion interactions within subtilisin in an aqueous medium has been investigated crystallographically using CsCl soak and X-ray wavelength optimised anomalous diffraction at the Cs K-edge. Ten Cs+, as well as six Cl- sites, have been clearly identified, revealing that in aqueous salt solutions ions can bind at defined points around the protein surface. The counter-ions do not generally interact with formal charges on the protein; formally neutral oxygens, mostly backbone carbonyls, mostly coordinate the Cs+ ions. The Cl- ion sites are also found likely to be near positive charges on the protein surface. The presence of counter-ions substantially changes the protein surface electrical charge. The surface charge distribution on a protein is commonly discussed in relation to enzyme function. The correct identification of counter-ions associated with a protein surface is necessary for a proper understanding of an enzyme's function
The Antisocial Effects of Social Media and How Colleges and Universities Can Manage Related Litigation Risks
Rapid advancements in information technology have transformed day-to-day university operations and, in doing so, have altered the landscape of risk management. Authors Gregory L. Demers, J. William Piereson, Mark A. Cianci, and Peter L. Welsh provide an overview of some of the most significant social-media-related risks faced by colleges and universities, before considering ways to mitigate these risks through a broad insurance coverage plan. The article explains how, given the relative novelty of this field, the coverage afforded by insurance policies inevitably will vary, often significantly, from insurer to insurer
Universal topological phase of 2D stabilizer codes
Two topological phases are equivalent if they are connected by a local
unitary transformation. In this sense, classifying topological phases amounts
to classifying long-range entanglement patterns. We show that all 2D
topological stabilizer codes are equivalent to several copies of one universal
phase: Kitaev's topological code. Error correction benefits from the
corresponding local mappings.Comment: 4 pages, 3 figure
Semi-Teleparallel Theories of Gravitation
A class of theories of gravitation that naturally incorporates preferred
frames of reference is presented. The underlying space-time geometry consists
of a partial parallelization of space-time and has properties of Riemann-Cartan
as well as teleparallel geometry. Within this geometry, the kinematic
quantities of preferred frames are associated with torsion fields. Using a
variational method, it is shown in which way action functionals for this
geometry can be constructed. For a special action the field equations are
derived and the coupling to spinor fields is discussed.Comment: 14 pages, LaTe
When does cyclic dominance lead to stable spiral waves?
Species diversity in ecosystems is often accompanied by characteristic spatio-temporal patterns. Here, we consider a generic two-dimensional population model and study the spiraling patterns arising from the combined effects of cyclic dominance of three species, mutation, pair-exchange and individual hopping. The dynamics is characterized by nonlinear mobility and a Hopf bifurcation around which the system's four-phase state diagram is inferred from a complex Ginzburg-Landau equation derived using a perturbative multiscale expansion. While the dynamics is generally characterized by spiraling patterns, we show that spiral waves are stable in only one of the four phases. Furthermore, we characterize a phase where nonlinearity leads to the annihilation of spirals and to the spatially uniform dominance of each species in turn. Away from the Hopf bifurcation, when the coexistence fixed point is unstable, the spiraling patterns are also affected by the nonlinear diffusion
Generalized parallel tempering on Bayesian inverse problems
Funder: Alexander von Humboldt-Stiftung; doi: http://dx.doi.org/10.13039/100005156In the current work we present two generalizations of the Parallel Tempering
algorithm, inspired by the so-called continuous-time Infinite Swapping
algorithm. Such a method, found its origins in the molecular dynamics
community, and can be understood as the limit case of the continuous-time
Parallel Tempering algorithm, where the (random) time between swaps of states
between two parallel chains goes to zero. Thus, swapping states between chains
occurs continuously. In the current work, we extend this idea to the context of
time-discrete Markov chains and present two Markov chain Monte Carlo algorithms
that follow the same paradigm as the continuous-time infinite swapping
procedure. We analyze the convergence properties of such discrete-time
algorithms in terms of their spectral gap, and implement them to sample from
different target distributions. Numerical results show that the proposed
methods significantly improve over more traditional sampling algorithms such as
Random Walk Metropolis and (traditional) Parallel Tempering
- …