8,951 research outputs found
A first study of the galaxy HRG 2304 and its companion AM 1646-795 (NED01)
Aims. We report the first study of the peculiar ring-like galaxy HRG 2304
(NED02),which was previously classified as a ring galaxy with an elliptical
smooth ring. This object was selected to prove that it is a candidate for the
Solitaire-type ring galaxies in an early stage of ring formation. The main goal
of this work is to provide the spectral characteristics of the current object
and its companion AM 1646-795 (NED01). Methods. The study is based on
spectroscopic observations in the optical band to highlight the characteristics
of this interacting galaxy. To investigate the star formation history of HRG
2304 we used the stellar population synthesis code STARLIGHT. The direct V and
B broad band images were used to enhance some fine structures. Results. Along
the entire long-slit signal, the spectra of HRG 2304 and its companion resemble
that of an early-type galaxy. We estimated a heliocentric systemic redshift of
z = 0.0415, corresponding to heliocentric velocities of 12449 km s-1 for HRG
2304 (NED02) and 12430 km s-1 for AM1646-795 (NED01). The spatial variation in
the contribution of the stellar population components for both objects are
dominated by an old stellar population 2x10^9 < t < 13x10^9 yr. The observed
radial-velocity distribution and the fine structures around HRG 2304 suggest an
ongoing tidal interaction of both galaxies. Conclusions.The spectroscopic
results and the morphological peculiarities of HRG 2304 can be adequately
interpreted as an ongoing interaction with the companion galaxy. Both galaxies
are early-type, the companion is elliptical, and the smooth distribution of the
material around HRG 2304 and its off-center nucleus in the direction of
AM1646-795 (NED01) characterize HRG 2304 as a Solitaire-type galaxy candidate
in an early stage of ring formation.Comment: Accepted for publication in Astronomy and Astrophysics, 9 pages, 10
figures and 3 table
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
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
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
Estimación de la superficie quemada en los incendios forestales de Canarias en 2007 utilizando sinérgicamente imágenes MODIS y anomalÃas térmicas
El presente trabajo plantea una metodologÃa rápida, precisa y económica para la estimación del área quemada en los incendios forestales utilizando técnicas de teledetección. El algoritmo utilizado se basa en la determinación o establecimiento de umbrales a partir de la banda del infrarrojo cercano. Se han utilizados los productos de reflectancia MODIS y las series temporales de anomalÃas térmicas proporcionadas por la NASA. Dicho umbral que separa área quemada de noquemada se ha basado en la búsqueda de la máxima coincidencia entre área quemada definida por la banda del infrarrojo cercano y zona de influencia de las anomalÃas térmicas. Los resultados muestran la utilidad de la metodologÃa propuesta para la estimación de áreas quemadas y su importancia a la hora de localizar de una forma rápida las zonas de intervención prioritaria, asà como para la planificación de futuros trabajos de restauración de la zona afectada
Crashing with disorder: Reaching the precision limit with tensor-based wavefront shaping
Perturbations in complex media, due to their own dynamical evolution or to
external effects, are often seen as detrimental. Therefore, a common strategy,
especially for telecommunication and imaging applications, is to limit the
sensitivity to those perturbations in order to avoid them. Here, we instead
consider crashing straight into them in order to maximize the interaction
between light and the perturbations and thus produce the largest change in
output intensity. Our work hinges on the innovative use of tensor-based
techniques, presently at the forefront of machine learning explorations, to
study intensity-based measurements where its quadratic relationship to the
field prevents the use of standard matrix methods. With this tensor-based
framework, we are able to identify the optimal crashing channel which maximizes
the change in its output intensity distribution and the Fisher information
encoded in it about a given perturbation. We further demonstrate experimentally
its superiority for robust and precise sensing applications. Additionally, we
derive the appropriate strategy to reach the precision limit for
intensity-based measurements leading to an increase in Fisher information by
more than four orders of magnitude with respect to the mean for random
wavefronts when measured with the pixels of a camera
Single-channel transmission in gold one-atom contacts and chains
We induce superconductivity by proximity effect in thin layers of gold and
study the number of conduction channels which contribute to the current in
one-atom contacts and atomic wires. The atomic contacts and wires are
fabricated with a Scanning Tunneling Microscope. The set of transmission
probabilities of the conduction channels is obtained from the analysis of the
characteristic curve which is highly non-linear due to multiple Andreev
reflections. In agreement with theoretical calculations we find that there is
only one channel which is almost completely open.Comment: 4 pages, 2 figures. To be published in Phys. Rev. B, Rapid
Communications (2003
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