2,323 research outputs found
Back-reaction in a cylinder
A system is studied in which initially a strong classical electric field
exists within an infinitely-long cylinder and no charges are present.
Subsequently, within the cylinder, pairs of charged particles tunnel out from
the vacuum and the current produced through their acceleration by the field
acts back on the field, setting up plasma oscillations. This yields a rough
model of phenomena that may occur in the pre-equilibrium formation phase of a
quark-gluon plasma. In an infinite volume, this back-reaction has been studied
in a field-theory description, and it has been found that the results of a full
calculation of this sort are well represented in a much simpler transport
formalism. It is the purpose here to explore that comparison for a situation
involving a cylindrical volume of given radius.Comment: 19 pages plus 13 figure
Quantum algorithms for classical lattice models
We give efficient quantum algorithms to estimate the partition function of
(i) the six vertex model on a two-dimensional (2D) square lattice, (ii) the
Ising model with magnetic fields on a planar graph, (iii) the Potts model on a
quasi 2D square lattice, and (iv) the Z_2 lattice gauge theory on a
three-dimensional square lattice. Moreover, we prove that these problems are
BQP-complete, that is, that estimating these partition functions is as hard as
simulating arbitrary quantum computation. The results are proven for a complex
parameter regime of the models. The proofs are based on a mapping relating
partition functions to quantum circuits introduced in [Van den Nest et al.,
Phys. Rev. A 80, 052334 (2009)] and extended here.Comment: 21 pages, 12 figure
Testing equivalence of pure quantum states and graph states under SLOCC
A set of necessary and sufficient conditions are derived for the equivalence
of an arbitrary pure state and a graph state on n qubits under stochastic local
operations and classical communication (SLOCC), using the stabilizer formalism.
Because all stabilizer states are equivalent to a graph state by local unitary
transformations, these conditions constitute a classical algorithm for the
determination of SLOCC-equivalence of pure states and stabilizer states. This
algorithm provides a distinct advantage over the direct solution of the
SLOCC-equivalence condition for an unknown invertible local operator S, as it
usually allows for easy detection of states that are not SLOCC-equivalent to
graph states.Comment: 9 pages, to appear in International Journal of Quantum Information;
Minor typos corrected, updated references
Mapping all classical spin models to a lattice gauge theory
In our recent work [Phys. Rev. Lett. 102, 230502 (2009)] we showed that the
partition function of all classical spin models, including all discrete
standard statistical models and all Abelian discrete lattice gauge theories
(LGTs), can be expressed as a special instance of the partition function of a
4-dimensional pure LGT with gauge group Z_2 (4D Z_2 LGT). This provides a
unification of models with apparently very different features into a single
complete model. The result uses an equality between the Hamilton function of
any classical spin model and the Hamilton function of a model with all possible
k-body Ising-type interactions, for all k, which we also prove. Here, we
elaborate on the proof of the result, and we illustrate it by computing
quantities of a specific model as a function of the partition function of the
4D Z_2 LGT. The result also allows one to establish a new method to compute the
mean-field theory of Z_2 LGTs with d > 3, and to show that computing the
partition function of the 4D Z_2 LGT is computationally hard (#P hard). The
proof uses techniques from quantum information.Comment: 21 pages, 21 figures; published versio
Density-functional study of defects in two-dimensional circular nematic nanocavities
We use density--functional theory to study the structure of two-dimensional
defects inside a circular nematic nanocavity. The density, nematic order
parameter, and director fields, as well as the defect core energy and core
radius, are obtained in a thermodynamically consistent way for defects with
topological charge (with radial and tangential symmetries) and .
An independent calculation of the fluid elastic constants, within the same
theory, allows us to connect with the local free--energy density predicted by
elastic theory, which in turn provides a criterion to define a defect core
boundary and a defect core free energy for the two types of defects. The radial
and tangential defects turn out to have very different properties, a feature
that a previous Maier--Saupe theory could not account for due to the simplified
nature of the interactions --which caused all elastic constants to be equal. In
the case with two defects in the cavity, the elastic r\'egime cannot
be reached due to the small radii of the cavities considered, but some trends
can already be obtained.Comment: 9 figures. Accepted for publication in liquid crystal
Fundamental limitations in the purifications of tensor networks
We show a fundamental limitation in the description of quantum many-body
mixed states with tensor networks in purification form. Namely, we show that
there exist mixed states which can be represented as a translationally
invariant (TI) matrix product density operator (MPDO) valid for all system
sizes, but for which there does not exist a TI purification valid for all
system sizes. The proof is based on an undecidable problem and on the
uniqueness of canonical forms of matrix product states. The result also holds
for classical states.Comment: v1: 11 pages, 1 figure. v2: very minor changes. About to appear in
Journal of Mathematical Physic
SPATIALIZING OPEN DATA FOR THE ASSESSMENT AND THE IMPROVEMENT OF TERRITORIAL AND SOCIAL COHESION
An integrated place-based approach for the improvement of territorial and social cohesion is the new instance for planning disciplines, coming from EU New Cohesion Policies. This paper considers the territorial impact assessment of regional development policies as a precondition in order to develop balanced and effective operative programs at national and regional levels. The contribution of 'open data' appears to be mature in order to support this application and in this paper we present a spatial analysis technique for the evaluation of EU funds effects at territorial level, starting from open data by Open Cohesion. The application is focused on internal areas of Basilicata Region: Agri river Valley. A complex contest, where environmental and agricultural traditional vocations conflict with a recent development of oil extraction industries. Conclusions concern further applications and perspectives to improve and support regional development planning considering the exploitation of open data sources and spatial analysis
A Systematic Search for High Surface Brightness Giant Arcs in a Sloan Digital Sky Survey Cluster Sample
We present the results of a search for gravitationally-lensed giant arcs
conducted on a sample of 825 SDSS galaxy clusters. Both a visual inspection of
the images and an automated search were performed and no arcs were found. This
result is used to set an upper limit on the arc probability per cluster. We
present selection functions for our survey, in the form of arc detection
efficiency curves plotted as functions of arc parameters, both for the visual
inspection and the automated search. The selection function is such that we are
sensitive only to long, high surface brightness arcs with g-band surface
brightness mu_g 10. Our upper limits on
the arc probability are compatible with previous arc searches. Lastly, we
report on a serendipitous discovery of a giant arc in the SDSS data, known
inside the SDSS Collaboration as Hall's arc.Comment: 34 pages,8 Fig. Accepted ApJ:Jan-200
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