1,673 research outputs found
Fully representable and *-semisimple topological partial *-algebras
We continue our study of topological partial *-algebras, focusing our
attention to *-semisimple partial *-algebras, that is, those that possess a
{multiplication core} and sufficiently many *-representations. We discuss the
respective roles of invariant positive sesquilinear (ips) forms and
representable continuous linear functionals and focus on the case where the two
notions are completely interchangeable (fully representable partial *-algebras)
with the scope of characterizing a *-semisimple partial *-algebra. Finally we
describe various notions of bounded elements in such a partial *-algebra, in
particular, those defined in terms of a positive cone (order bounded elements).
The outcome is that, for an appropriate order relation, one recovers the
\M-bounded elements introduced in previous works.Comment: 26 pages, Studia Mathematica (2012) to appea
Partial inner product spaces: Some categorical aspects
We make explicit in terms of categories a number of statements from the
theory of partial inner product spaces (PIP spaces) and operators on them.
In particular, we construct sheaves and cosheaves of operators on certain PIP
spaces of practical interest.Comment: 21 page
Experimental observation of the X-shaped near field spatio-temporal correlation of ultra-broadband twin beams
In this work we present the experimental observation of the non factorable
near field spatio-temporal correlation of ultra-broadband twin beams generated
by parametric down conversion (PDC), in an interferometric-type experiment
using sum frequency generation, where both the temporal and spatial degrees of
freedom of PDC light are controlled with very high resolution. The revealed
X-structure of the correlation is in accordance with the predictions of the
theory.Comment: 5 pages, 3 figure
IL DESIGN DEI BENI CULTURALI.CRISI TERRITORIO, IDENTITĂ€
Sino a pochi anni fa accostare il termine Design a quello di Beni Culturali avrebbe avuto il sapore di una provocazione. Un evidente ossimoro, insomma, tra una parola -Design- per sua natura legata all’idea del nuovo e un’altra -Beni Culturali- evidentemente connessa alla conservazione dell’antico. Se il Design inoltre rimandava quasi automaticamente a una modalità del progetto fondata ancora largamente sul riferimento al paradigma industriale, i Beni Culturali evocavano una concezione “patrimoniale” del lascito storico, per la massima parte individuato nel paesaggio e nei “beni” ereditati dalla cultura delle età preindustriali. E’ dunque evidente che la nuova dizione di Design dei Beni Culturali è il frutto di una progressiva, doppia rivoluzione che ha modificato sia la nostra percezione del concetto di “bene” legato al passato, sia il campo di significati attribuiti alle pratiche del progetto industriale. Il design investe i Beni Culturali con tutto il peso delle nuove tecnologie -del digitale e del virtuale innanzitutto- facendole entrare prepotentemente nel territorio dei musei, dell’archeologia, dell’archivistica, ma anche nella valorizzazione in tempo di crisi delle risorse territoriali diffuse nei contesti urbani e territoriali.Up until just a few years ago, putting the terms “design” and “cultural heritage” together would have sounded like a provocation. An obvious oxymoron, really, between one word – design - by its nature tied to the idea of the new and another – cultural heritage – clearly connected to the conservation of the old. Furthermore, while “design” almost automatically turned to a design process still largely based on the industrial paradigm, “cultural heritage” evoked an interpretation of history’s legacy as “endowments”, for the most part identified in the landscape and in assets inherited from pre-industrial cultures. This new wording of the “design of cultural heritage is the result of a progressive double revolution, which has changed both our perception of the concept of “asset” tied to the past, as well as the range of meanings attributed to the industrial design process.
Today, design invests cultural heritage in particular with an armament of new technologies – spanning from digital to virtual,– makes them important protagonists in the fields of museums, archaeology, archiving, but also in the development, in times of crisis, of cultural recourses spread throughout the urban and regional context
Cross-spectral analysis of the X-ray variability of Mrk 421
Using the cross-spectral method, we confirm the existence of the X-ray hard
lags discovered with cross-correlation function technique during a large flare
of Mrk 421 observed with BeppoSAX . For the 0.1--2 versus 2--10keV light
curves, both methods suggest sub-hour hard lags. In the time domain, the degree
of hard lag, i.e., the amplitude of the 3.2--10 keV photons lagging the lower
energy ones, tends to increase with the decreasing energy. In the Fourier
frequency domain, by investigating the cross-spectra of the 0.1--2/2--10 keV
and the 2--3.2/3.2--10 keV pairs of light curves, the flare also shows hard
lags at the lowest frequencies. However, with the present data, it is
impossible to constrain the dependence of the lags on frequencies even though
the detailed simulations demonstrate that the hard lags at the lowest
frequencies probed by the flare are not an artifact of sparse sampling, Poisson
and red noise. As a possible interpretation, the implication of the hard lags
is discussed in the context of the interplay between the (diffusive)
acceleration and synchrotron cooling of relativistic electrons responsible for
the observed X-ray emission. The energy-dependent hard lags are in agreement
with the expectation of an energy-dependent acceleration timescale. The
inferred magnetic field (B ~ 0.11 Gauss) is consistent with the value inferred
from the Spectral Energy Distributions of the source. Future investigations
with higher quality data that whether or not the time lags are
energy-/frequency-dependent will provide a new constraint on the current models
of the TeV blazars.Comment: 11 pages, 6 figures, accepted by MNRA
Detection of the ultranarrow temporal correlation of twin beams via sum-frequency generation
We demonstrate the ultranarrow temporal correlation (6 fs full width half
maximum) of twin beams generated by parametric down-conversion, by using the
inverse process of sum-frequency generation. The result relies on an achromatic
imaging of a huge bandwith of twin beams and on a careful control of their
spatial degrees of freedom. The detrimental effects of spatial filtering and of
imperfect imaging are shown toghether with the theoretical model used to
describe the results
Quantum spatial correlations in high-gain parametric down-conversion measured by means of a CCD camera
We consider travelling-wave parametric down-conversion in the high-gain
regime and present the experimental demonstration of the quantum character of
the spatial fluctuations in the system. In addition to showing the presence of
sub-shot noise fluctuations in the intensity difference, we demonstrate that
the peak value of the normalized spatial correlations between signal and idler
lies well above the line marking the boundary between the classical and the
quantum domain. This effect is equivalent to the apparent violation of the
Cauchy-Schwartz inequality, predicted by some of us years ago, which represents
a spatial analogue of photon antibunching in time. Finally, we analyse
numerically the transition from the quantum to the classical regime when the
gain is increased and we emphasize the role of the inaccuracy in the
determination of the symmetry center of the signal/idler pattern in the
far-field plane.Comment: 21 pages, 11 figures, submitted to J. Mod. Opt. special issue on
Quantum Imagin
- …