114 research outputs found
Micro-scale surface-patterning influences biofilm formation
The formation of biofilms on indwelling/implanted medical devices is a
common problem. One of the approaches used to prevent biofilm formation
on medical devices is to inhibit bacterial attachment by modification
of the synthetic polymers used to fabricate the device. In this work,
we assessed how micro-scale features (patterns) imprinted onto the
surface of silicone elastomer similar to that used for medical
applications influenced biofilm formation by Staphylococcus aureus ,
Staphylococcus epidermidis , and Pseudomonas aeruginosa . Patterns
were transferred from a multi-patterned oxidized silicon-wafer
master-template to silicone elastomer. Features consisted of bars,
squares, and circles each extending 0.51 \ub5m above the surface.
Feature sizes ranged between 1.78 and 22.25 \ub5m. Distances
separating features ranged between 0.26 and 17.35 \ub5m. Bacterial
biofilm formation on discs cut from imprinted silicone elastomer was
assessed by direct microscopic observation and quantified as the
surface area covered by biofilm. Unpatterned silicone elastomer served
as a control. Several of the micro-scale patterns imprinted into the
silicone elastomer significantly reduced biofilm formation by each
bacterium and interrupted biofilm continuity. Although there were
differences in detail among strains, bacteria tended to attach in the
area between features more than to the surface of the feature itself
N=31, D=11
We show that eleven-dimensional supergravity backgrounds with thirty one
supersymmetries, N=31, admit an additional Killing spinor and so they are
locally isometric to maximally supersymmetric ones. This rules out the
existence of simply connected eleven-dimensional supergravity preons. We also
show that N=15 solutions of type I supergravities are locally isometric to
Minkowski spacetime.Comment: 17 page
On a class of 4D Kahler bases and AdS_5 supersymmetric Black Holes
We construct a class of toric Kahler manifolds, M_4, of real dimension four,
a subset of which corresponds to the Kahler bases of all known 5D
asymptotically AdS_5 supersymmetric black-holes. In a certain limit, these
Kahler spaces take the form of cones over Sasaki spaces, which, in turn, are
fibrations over toric manifolds of real dimension two. The metric on M_4 is
completely determined by a single function H(x), which is the conformal factor
of the two dimensional space. We study the solutions of minimal five
dimensional gauged supergravity having this class of Kahler spaces as base and
show that in order to generate a five dimensional solution H(x) must obey a
simple sixth order differential equation. We discuss the solutions in detail,
which include all known asymptotically AdS_5 black holes as well as other
spacetimes with non-compact horizons. Moreover we find an infinite number of
supersymmetric deformations of these spacetimes with less spatial isometries
than the base space. These deformations vanish at the horizon, but become
relevant asymptotically.Comment: 34 pages, 3 figures. v2: formula (8.35) and other minor typos
corrected; references added; accepted for publication in JHE
Predicting Student Failure in an Introductory Programming Course with Multiple Back-Propagation
One of the most challenging tasks in computer science and similar courses consists of both teaching and learning computer
programming. Usually this requires a great deal of work, dedication, and motivation from both teachers and students.
Accordingly, ever since the first programming languages emerged, the problems inherent to programming teaching and
learning have been studied and investigated. The theme is very serious, not only for the important concepts underlying
computer science courses but also for reducing the lack of motivation, failure, and abandonment that result from students
frustration. Therefore, early identification of potential problems and immediate response is a fundamental aspect to avoid
student’s failure and reduce dropout rates. In this paper, we propose a machine-learning (neural network) predictive model
of student failure based on the student profile, which is built throughout programming classes by continuously monitoring
and evaluating student activities. The resulting model allows teachers to early identify students that are more likely to fail,
allowing them to devote more time to those students and try novel strategies to improve their programming skills
All timelike supersymmetric solutions of N=2, D=4 gauged supergravity coupled to abelian vector multiplets
The timelike supersymmetric solutions of N=2, D=4 gauged supergravity coupled
to an arbitrary number of abelian vector multiplets are classified using
spinorial geometry techniques. We show that the generalized holonomy group for
vacua preserving N supersymmetries is GL((8-N)/2,C) N/2 C^((8-N)/2)
GL(8,R), where N=0,2,4,6,8. The spacetime turns out to be a
fibration over a three-dimensional base manifold with U(1) holonomy and
nontrivial torsion. Our results can be used to construct new supersymmetric AdS
black holes with nontrivial scalar fields turned on.Comment: 35 pages, uses JHEP3.cl
Quantum random walks in optical lattices
We propose an experimental realization of discrete quantum random walks using
neutral atoms trapped in optical lattices. The random walk is taking place in
position space and experimental implementation with present day technology
--even using existing set-ups-- seems feasible. We analyze the influence of
possible imperfections in the experiment and investigate the transition from a
quantum random walk to the classical random walk for increasing errors and
decoherence.Comment: 8 pages, 4 figure
Fast Non-Adiabatic Two Qubit Gates for the Kane Quantum Computer
In this paper we apply the canonical decomposition of two qubit unitaries to
find pulse schemes to control the proposed Kane quantum computer. We explicitly
find pulse sequences for the CNOT, swap, square root of swap and controlled Z
rotations. We analyze the speed and fidelity of these gates, both of which
compare favorably to existing schemes. The pulse sequences presented in this
paper are theoretically faster, higher fidelity, and simpler than existing
schemes. Any two qubit gate may be easily found and implemented using similar
pulse sequences. Numerical simulation is used to verify the accuracy of each
pulse scheme
Quantum computing with mixed states
We discuss a model for quantum computing with initially mixed states.
Although such a computer is known to be less powerful than a quantum computer
operating with pure (entangled) states, it may efficiently solve some problems
for which no efficient classical algorithms are known. We suggest a new
implementation of quantum computation with initially mixed states in which an
algorithm realization is achieved by means of optimal basis independent
transformations of qubits.Comment: 2 figures, 52 reference
Theory of Decoherence-Free Fault-Tolerant Universal Quantum Computation
Universal quantum computation on decoherence-free subspaces and subsystems
(DFSs) is examined with particular emphasis on using only physically relevant
interactions. A necessary and sufficient condition for the existence of
decoherence-free (noiseless) subsystems in the Markovian regime is derived here
for the first time. A stabilizer formalism for DFSs is then developed which
allows for the explicit understanding of these in their dual role as quantum
error correcting codes. Conditions for the existence of Hamiltonians whose
induced evolution always preserves a DFS are derived within this stabilizer
formalism. Two possible collective decoherence mechanisms arising from
permutation symmetries of the system-bath coupling are examined within this
framework. It is shown that in both cases universal quantum computation which
always preserves the DFS (*natural fault-tolerant computation*) can be
performed using only two-body interactions. This is in marked contrast to
standard error correcting codes, where all known constructions using one or
two-body interactions must leave the codespace during the on-time of the
fault-tolerant gates. A further consequence of our universality construction is
that a single exchange Hamiltonian can be used to perform universal quantum
computation on an encoded space whose asymptotic coding efficiency is unity.
The exchange Hamiltonian, which is naturally present in many quantum systems,
is thus *asymptotically universal*.Comment: 40 pages (body: 30, appendices: 3, figures: 5, references: 2). Fixed
problem with non-printing figures. New references added, minor typos
correcte
A First Search for coincident Gravitational Waves and High Energy Neutrinos using LIGO, Virgo and ANTARES data from 2007
We present the results of the first search for gravitational wave bursts
associated with high energy neutrinos. Together, these messengers could reveal
new, hidden sources that are not observed by conventional photon astronomy,
particularly at high energy. Our search uses neutrinos detected by the
underwater neutrino telescope ANTARES in its 5 line configuration during the
period January - September 2007, which coincided with the fifth and first
science runs of LIGO and Virgo, respectively. The LIGO-Virgo data were analysed
for candidate gravitational-wave signals coincident in time and direction with
the neutrino events. No significant coincident events were observed. We place
limits on the density of joint high energy neutrino - gravitational wave
emission events in the local universe, and compare them with densities of
merger and core-collapse events.Comment: 19 pages, 8 figures, science summary page at
http://www.ligo.org/science/Publication-S5LV_ANTARES/index.php. Public access
area to figures, tables at
https://dcc.ligo.org/cgi-bin/DocDB/ShowDocument?docid=p120000
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