8,446 research outputs found
Fractal properties of quantum spacetime
We show that in general a spacetime having a quantum group symmetry has also
a scale dependent fractal dimension which deviates from its classical value at
short scales, a phenomenon that resembles what observed in some approaches to
quantum gravity. In particular we analyze the cases of a quantum sphere and of
\k-Minkowski, the latter being relevant in the context of quantum gravity.Comment: 4 pages, 2 figures; some minor corrections; reference adde
Characterization of qubit chains by Feynman probes
We address the characterization of qubit chains and assess the performances
of local measurements compared to those provided by Feynman probes, i.e.
nonlocal measurements realized by coupling a single qubit regis- ter to the
chain. We show that local measurements are suitable to estimate small values of
the coupling and that a Bayesian strategy may be successfully exploited to
achieve optimal precision. For larger values of the coupling Bayesian local
strategies do not lead to a consistent estimate. In this regime, Feynman probes
may be exploited to build a consistent Bayesian estimator that saturates the
Cram\'er-Rao bound, thus providing an effective characterization of the chain.
Finally, we show that ultimate bounds to precision, i.e. saturation of the
quantum Cram\'er-Rao bound, may be achieved by a two-step scheme employing
Feynman probes followed by local measurements.Comment: 8 pages, 5 figure
Spectral geometry as a probe of quantum spacetime
Employing standard results from spectral geometry, we provide strong evidence
that in the classical limit the ground state of three-dimensional causal
dynamical triangulations is de Sitter spacetime. This result is obtained by
measuring the expectation value of the spectral dimension on the ensemble of
geometries defined by these models, and comparing its large scale behaviour to
that of a sphere (Euclidean de Sitter). From the same measurement we are also
able to confirm the phenomenon of dynamical dimensional reduction observed in
this and other approaches to quantum gravity -- the first time this has been
done for three-dimensional causal dynamical triangulations. In this case, the
value for the short-scale limit of the spectral dimension that we find is
approximately 2. We comment on the relevance of these results for the
comparison to asymptotic safety and Horava-Lifshitz gravity, among other
approaches to quantum gravity.Comment: 25 pages, 6 figures. Version 2: references to figures added,
acknowledgment added
On QBF Proofs and Preprocessing
QBFs (quantified boolean formulas), which are a superset of propositional
formulas, provide a canonical representation for PSPACE problems. To overcome
the inherent complexity of QBF, significant effort has been invested in
developing QBF solvers as well as the underlying proof systems. At the same
time, formula preprocessing is crucial for the application of QBF solvers. This
paper focuses on a missing link in currently-available technology: How to
obtain a certificate (e.g. proof) for a formula that had been preprocessed
before it was given to a solver? The paper targets a suite of commonly-used
preprocessing techniques and shows how to reconstruct certificates for them. On
the negative side, the paper discusses certain limitations of the
currently-used proof systems in the light of preprocessing. The presented
techniques were implemented and evaluated in the state-of-the-art QBF
preprocessor bloqqer.Comment: LPAR 201
Lattice Glass Models
Motivated by the concept of geometrical frustration, we introduce a class of
statistical mechanics lattice models for the glass transition. Monte Carlo
simulations in three dimensions show that they display a dynamical glass
transition which is very similar to that observed in other off-lattice systems
and which does not depend on a specific dynamical rule. Whereas their analytic
solution within the Bethe approximation shows that they do have a discontinuous
glass transition compatible with the numerical observations.Comment: 4 pages, 2 figures; minor change
Biomass from microalgae: The potential of domestication towards sustainable biofactories
Interest in bulk biomass from microalgae, for the extraction of high-value nutraceuticals, bio-products, animal feed and as a source of renewable fuels, is high. Advantages of microalgal vs. plant biomass production include higher yield, use of non-arable land, recovery of nutrients from wastewater, efficient carbon capture and faster development of new domesticated strains. Moreover, adaptation to a wide range of environmental conditions evolved a great genetic diversity within this polyphyletic group, making microalgae a rich source of interesting and useful metabolites. Microalgae have the potential to satisfy many global demands; however, realization of this potential requires a decrease of the current production costs. Average productivity of the most common industrial strains is far lower than maximal theoretical estimations, suggesting that identification of factors limiting biomass yield and removing bottlenecks are pivotal in domestication strategies aimed to make algal-derived bio-products profitable on the industrial scale. In particular, the light-to-biomass conversion efficiency represents a major constraint to finally fill the gap between theoretical and industrial productivity. In this respect, recent results suggest that significant yield enhancement is feasible. Full realization of this potential requires further advances in cultivation techniques, together with genetic manipulation of both algal physiology and metabolic networks, to maximize the efficiency with which solar energy is converted into biomass and bio-products. In this review, we draft the molecular events of photosynthesis which regulate the conversion of light into biomass, and discuss how these can be targeted to enhance productivity through mutagenesis, strain selection or genetic engineering. We outline major successes reached, and promising strategies to achieving significant contributions to future microalgae-based biotechnology
Construction, Management and Visualization of 3D Models of Large Archeological and Architectural Sites for E-Heritage GIS Systems
In this paper we present an integrated system developed in order to record, construct, pre-process, manage,
visualize and visually navigate 3D models reality based of large archeological and architectural sites for
eHeritage GIS systems. The framework integrates structured geometrical and documentary information
resulting from multiple sources with the aim to enhance the knowledge of those sites within the frame of its
historical evolution and its institutional management in a 3D GIS/DB. The developed applications were
designed for different types of users, with a largely scalable interface, able to support different output
devices and to work at different levels of iconicity. The system allows a full comprehension of the buildings
in their own context, permitting to discover unknown relationships, to evaluate their architectural occupancy
and to quickly access a complex system of information. The framework has been tested in two different
systems - designed and developed to satisfy both internal (cataloguing, documentation, preservation,
management of archaeological heritage) and external (communication through the web portal) purposes:
the first, in Pompeii, developed in order to have a web-based system that uses Open Source software and
complies with national and international standards; the second one, a prototype designed to make available
on the Google Earth platform the complete Palladian corpus documentation implemented by the CISAAP
Cosmological measurements, time and observables in (2+1)-dimensional gravity
We investigate the relation between measurements and the physical observables
for vacuum spacetimes with compact spatial surfaces in (2+1)-gravity with
vanishing cosmological constant. By considering an observer who emits lightrays
that return to him at a later time, we obtain explicit expressions for several
measurable quantities as functions on the physical phase space of the theory:
the eigentime elapsed between the emission of a lightray and its return to the
observer, the angles between the directions into which the light has to be
emitted to return to the observer and the relative frequencies of the lightrays
at their emission and return. This provides a framework in which conceptual
questions about time, observables and measurements can be addressed. We analyse
the properties of these measurements and their geometrical interpretation and
show how they allow an observer to determine the values of the Wilson loop
observables that parametrise the physical phase space of (2+1)-gravity. We
discuss the role of time in the theory and demonstrate that the specification
of an observer with respect to the spacetime's geometry amounts to a gauge
fixing procedure yielding Dirac observables.Comment: 38 pages, 11 eps figures, typos corrected, references update
Caracterización reológica del suero de tofu a temperaturas próximas a la congelación
El suero de tofu es un subproducto obtenido en la producciónindustrial a partir de la leche de soja. Este suero tiene componentes beneficiosos para la salud, como su contenidoen isoflavonas. Caracterizar reológicamente el suero de tofu a bajas temperaturas tiene interés con el fin de facilitar la recuperación de este efluente y sus compuestos activos sin que se deterioren. Además puede suponer una importante reducción del impacto ambiental debido a la elevada carga orgánica.En el presente trabajo se ha determinado un comportamientoreológico newtoniano en las muestras de suero fresco a 1,9 ºBrix inicial, crioconcentrado hasta 18,5º Brix y posteriormente evaporado hasta 60,2ºBrix. El rango de temperaturas ensayadas es próximo a la congelación (de -4ºC a 4ºC) debido al interés de ser procesadas bajo las condiciones que puedan preservar los compuestos activos
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