709,224 research outputs found
Modelling of Multi-Agent Systems: Experiences with Membrane Computing and Future Challenges
Formal modelling of Multi-Agent Systems (MAS) is a challenging task due to
high complexity, interaction, parallelism and continuous change of roles and
organisation between agents. In this paper we record our research experience on
formal modelling of MAS. We review our research throughout the last decade, by
describing the problems we have encountered and the decisions we have made
towards resolving them and providing solutions. Much of this work involved
membrane computing and classes of P Systems, such as Tissue and Population P
Systems, targeted to the modelling of MAS whose dynamic structure is a
prominent characteristic. More particularly, social insects (such as colonies
of ants, bees, etc.), biology inspired swarms and systems with emergent
behaviour are indicative examples for which we developed formal MAS models.
Here, we aim to review our work and disseminate our findings to fellow
researchers who might face similar challenges and, furthermore, to discuss
important issues for advancing research on the application of membrane
computing in MAS modelling.Comment: In Proceedings AMCA-POP 2010, arXiv:1008.314
Charge response of the Majorana toric code
At zero temperature, a two dimensional lattice of Majorana zero modes on
mesoscopic superconducting islands has a topologically ordered toric code
phase. Recently, a Landau field theory has been proposed for the system that
captures its different phases and the associated phase-transitions. It was
shown that with the increase of Josephson tunneling between the islands, a
continuous symmetry-breaking 3D-XY transition gets transformed into a discrete
symmetry-breaking 3D-Ising transition through a couple of tricritical points
and first order transitions. Using the proposed field theory, we analyze the
charge-response of the system at the different continuous phase-transitions. We
calculate the universal conductivity at the 3D-XY transitions and the change in
the superconducting density at the Ising transition using 1/N expansion.
Furthermore, by computing a one-loop correction to the field theory, we show
that an additional tricritical point is likely to be present in the
phase-diagram. Finally, we provide a mean-field calculation that supports the
earlier proposed field theory.Comment: Published versio
Topological Quantum Computing with p-Wave Superfluid Vortices
It is shown that Majorana fermions trapped in three vortices in a p-wave
superfluid form a qubit in a topological quantum computing (TQC). Several
similar ideas have already been proposed: Ivanov [Phys. Rev. Lett. {\bf 86},
268 (2001)] and Zhang {\it et al.} [Phys. Rev. Lett. {\bf 99}, 220502 (2007)]
have proposed schemes in which a qubit is implemented with two and four
Majorana fermions, respectively, where a qubit operation is performed by
exchanging the positions of Majorana fermions. The set of gates thus obtained
is a discrete subset of the relevant unitary group. We propose, in this paper,
a new scheme, where three Majorana fermions form a qubit. We show that
continuous 1-qubit gate operations are possible by exchanging the positions of
Majorana fermions complemented with dynamical phase change. 2-qubit gates are
realized through the use of the coupling between Majorana fermions of different
qubits.Comment: 5 pages, 2 figures. Two-qubit gate implementation is added
Developing High Performance Computing Resources for Teaching Cluster and Grid Computing courses
High-Performance Computing (HPC) and the ability to process large amounts of data are of
paramount importance for UK business and economy as outlined by Rt Hon David Willetts
MP at the HPC and Big Data conference in February 2014. However there is a shortage of
skills and available training in HPC to prepare and expand the workforce for the HPC and
Big Data research and development. Currently, HPC skills are acquired mainly by students
and staff taking part in HPC-related research projects, MSc courses, and at the dedicated
training centres such as Edinburgh University’s EPCC. There are few UK universities teaching
the HPC, Clusters and Grid Computing courses at the undergraduate level. To address the
issue of skills shortages in the HPC it is essential to provide teaching and training as part of
both postgraduate and undergraduate courses. The design and development of such courses is
challenging since the technologies and software in the fields of large scale distributed systems
such as Cluster, Cloud and Grid computing are undergoing continuous change. The students
completing the HPC courses should be proficient in these evolving technologies and equipped
with practical and theoretical skills for future jobs in this fast developing area.
In this paper we present our experience in developing the HPC, Cluster and Grid modules
including a review of existing HPC courses offered at the UK universities. The topics covered in
the modules are described, as well as the coursework projects based on practical laboratory work.
We conclude with an evaluation based on our experience over the last ten years in developing
and delivering the HPC modules on the undergraduate courses, with suggestions for future work
Optical computing by injection-locked lasers
A programmable optical computer has remained an elusive concept. To construct
a practical computing primitive equivalent to an electronic Boolean logic, one
should find a nonlinear phenomenon that overcomes weaknesses present in many
optical processing schemes. Ideally, the nonlinearity should provide a
functionally complete set of logic operations, enable ultrafast all-optical
programmability, and allow cascaded operations without a change in the
operating wavelength or in the signal encoding format. Here we demonstrate a
programmable logic gate using an injection-locked Vertical-Cavity
Surface-Emitting Laser (VCSEL). The gate program is switched between the AND
and the OR operations at the rate of 1 GHz with Bit Error Ratio (BER) of 10e-6
without changes in the wavelength or in the signal encoding format. The scheme
is based on nonlinearity of normalization operations, which can be used to
construct any continuous complex function or operation, Boolean or otherwise.Comment: 47 pages, 7 figures in total, 2 tables. Intended for submission to
Nature Physics within the next two week
Quantum theory of curvature and synchro-curvature radiation in a strong and curved magnetic field, and applications to neutron star magnetospheres
In a previous paper, we derived the quantum states of a Dirac particle in a
circular, intense magnetic field in the limit of low momentum perpendicular to
the field with the purpose of giving a quantum description of the trajectory of
an electron, or a positron, in a typical pulsar or magnetar magnetosphere. Here
we continue this work by computing the radiation resulting from transitions
between these states. This leads to derive from first principles a quantum
theory of the so-called curvature and synchro-curvature radiations relevant for
rotating neutron-star magnetospheres. We find that, within the approximation of
an infinitely confined wave-function around the magnetic field and in the
continuous energy-level limit, classical curvature radiation can be recovered
in a fully consistent way. Further we introduce discrete transitions to account
for the change of momentum perpendicular to the field and derive expressions
for what we call quantum synchro-curvature radiation. Additionally, we express
deconfinement and quantum recoil corrections
Exact Computation of a Manifold Metric, via Lipschitz Embeddings and Shortest Paths on a Graph
Data-sensitive metrics adapt distances locally based the density of data
points with the goal of aligning distances and some notion of similarity. In
this paper, we give the first exact algorithm for computing a data-sensitive
metric called the nearest neighbor metric. In fact, we prove the surprising
result that a previously published -approximation is an exact algorithm.
The nearest neighbor metric can be viewed as a special case of a
density-based distance used in machine learning, or it can be seen as an
example of a manifold metric. Previous computational research on such metrics
despaired of computing exact distances on account of the apparent difficulty of
minimizing over all continuous paths between a pair of points. We leverage the
exact computation of the nearest neighbor metric to compute sparse spanners and
persistent homology. We also explore the behavior of the metric built from
point sets drawn from an underlying distribution and consider the more general
case of inputs that are finite collections of path-connected compact sets.
The main results connect several classical theories such as the conformal
change of Riemannian metrics, the theory of positive definite functions of
Schoenberg, and screw function theory of Schoenberg and Von Neumann. We develop
novel proof techniques based on the combination of screw functions and
Lipschitz extensions that may be of independent interest.Comment: 15 page
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