8,182 research outputs found
Optimal control theory based design of elasto-magnetic metamaterial
A method to design a new type of metamaterial is presented. A two-step strategy to define an optimal long-range force distribution embedded in an elastic support to control wave propagation is considered.
The first step uses a linear quadratic regulator (LQR) to produce an optimal set of long-range interactions. In the second step, a least square passive approximation of the LQR optimal gains is determined. The paper investigates numerical solutions obtained by the previously described procedure. Finally, we discuss physical and engineering implications and practical use of the present study
A new wavelength calibration for echelle spectrographs using Fabry-Perot etalons
The study of Earth-mass extrasolar planets via the radial-velocity technique
and the measurement of the potential cosmological variability of fundamental
constants call for very-high-precision spectroscopy at the level of
\updelta\lambda/\lambda<10^{-9}. Wavelength accuracy is obtained by providing
two fundamental ingredients: 1) an absolute and information-rich wavelength
source and 2) the ability of the spectrograph and its data reduction of
transferring the reference scale (wavelengths) to a measurement scale (detector
pixels) in a repeatable manner. The goal of this work is to improve the
wavelength calibration accuracy of the HARPS spectrograph by combining the
absolute spectral reference provided by the emission lines of a thorium-argon
hollow-cathode lamp (HCL) with the spectrally rich and precise spectral
information of a Fabry-P\'erot-based calibration source. On the basis of
calibration frames acquired each night since the Fabry-P\'erot etalon was
installed on HARPS in 2011, we construct a combined wavelength solution which
fits simultaneously the thorium emission lines and the Fabry-P\'erot lines. The
combined fit is anchored to the absolute thorium wavelengths, which provide the
`zero-point' of the spectrograph, while the Fabry-P\'erot lines are used to
improve the (spectrally) local precision. The obtained wavelength solution is
verified for auto-consistency and tested against a solution obtained using the
HARPS Laser-Frequency Comb (LFC). The combined thorium+Fabry-P\'erot wavelength
solution shows significantly better performances compared to the thorium-only
calibration. The presented techniques will therefore be used in the new HARPS
and HARPS-N pipeline, and will be exported to the ESPRESSO spectrograph.Comment: 15 pages, 8 figure
Local Hamiltonians for Maximally Multipartite Entangled States
We study the conditions for obtaining maximally multipartite entangled states
(MMES) as non-degenerate eigenstates of Hamiltonians that involve only
short-range interactions. We investigate small-size systems (with a number of
qubits ranging from 3 to 5) and show some example Hamiltonians with MMES as
eigenstates.Comment: 6 pages, 3 figures, published versio
Performing new identities: the community language of post-crisis Italian migrants in London
After the 2008 global crisis, Italy has experienced a relevant resumption of emigration. Tens of thousands of young Italians have chosen London as their favourite destination, giving rise to a new Italian community in the city. This article focuses on the transformation of migrants’ national identity and on a distinctive device of identity expression: language. Sample cases, extracted from a dataset collected for an original doctoral project, are used to explain how the insertion of English elements in speakers’ native language become the expression of the loss of pure national identity and of the renegotiation of transnational and migratory identities
Deriving High-Precision Radial Velocities
This chapter describes briefly the key aspects behind the derivation of
precise radial velocities. I start by defining radial velocity precision in the
context of astrophysics in general and exoplanet searches in particular. Next I
discuss the different basic elements that constitute a spectrograph, and how
these elements and overall technical choices impact on the derived radial
velocity precision. Then I go on to discuss the different wavelength
calibration and radial velocity calculation techniques, and how these are
intimately related to the spectrograph's properties. I conclude by presenting
some interesting examples of planets detected through radial velocity, and some
of the new-generation instruments that will push the precision limit further.Comment: Lecture presented at the IVth Azores International Advanced School in
Space Sciences on "Asteroseismology and Exoplanets: Listening to the Stars
and Searching for New Worlds" (arXiv:1709.00645), which took place in Horta,
Azores Islands, Portugal in July 201
MIDAS: Automated Approach to Design Microwave Integrated Inductors and Transformers on Silicon
The design of modern radiofrequency integrated circuits on silicon operating at microwave and millimeter-waves requires the integration of several spiral inductors and transformers that are not commonly available in the process design-kits of the technologies. In this work we present an auxiliary CAD tool for Microwave Inductor (and transformer) Design Automation on Silicon (MIDAS) that exploits commercial simulators and allows the implementation of an automatic design flow, including three-dimensional layout editing and electromagnetic simulations. In detail, MIDAS allows the designer to derive a preliminary sizing of the inductor (transformer) on the bases of the design entries (specifications). It draws the inductor (transformer) layers for the specific process design kit, including vias and underpasses, with or without patterned ground shield, and launches the electromagnetic simulations, achieving effective design automation with respect to the traditional design flow for RFICs. With the present software suite the complete design time is reduced significantly (typically 1 hour on a PC based on Intel® Pentium® Dual 1.80GHz CPU with 2-GB RAM). Afterwards both the device equivalent circuit and the layout are ready to be imported in the Cadence environment
Precision multi-epoch astrometry with VLT cameras FORS1/2
We investigate the astrometric performance of the FORS1 and FORS2 cameras of
the VLT at long time scales with emphasis on systematic errors which normally
prevent attainning a precision better than 1mas. The study is based on multi-
epoch time series of observations of a single sky region imaged with a time
spacing of 2-6 years at FORS1 and 1-5 months at FORS2. We performed a detailed
analysis of a random error of positions that was shown to be dominated by the
uncertainty of the star photocenter determination. The component of the random
error corresponding to image motion was found to be caused primarily by optical
aberrations and variations of atmospheric PSF size but not by the effect of
atmospheric image motion. Comparison of observed and model annual/monthly epoch
average positions yielded estimates of systematic errors for which temporal
properties and distribution in the CCD plane are given. At frame center, the
systematic component is about 25 mu-as. Systematic errors are shown to be
caused mainly by a combined effect of the image asymmetry and seeing variations
which therefore should be strongly limited to avoid generating random and
systematic errors. For a series of 30 images, we demonstrated presicion of
about 50 mu-as stable on daily, monthly, and annual time scales. Relative
proper motion and trigonometric parallaxes of stars in the center of the test
field were derived with a precision of 20 mu-as/yr and 40 mu-as for 17-19 mag
stars.Comment: 16 pages, 16 figures, 4 tables, accepted in A&A; typos and language
corrections; version sent to the printe
Consequences of spectrograph illumination for the accuracy of radial-velocimetry
For fiber-fed spectrographs with a stable external wavelength source,
scrambling properties of optical fibers and, homogeneity and stability of the
instrument illumination are important for the accuracy of radial-velocimetry.
Optical cylindric fibers are known to have good azimuthal scrambling. In
contrast, the radial one is not perfect. In order to improve the scrambling
ability of the fiber and to stabilize the illumination, optical double
scrambler are usually coupled to the fibers. Despite that, our experience on
SOPHIE and HARPS has lead to identified remaining radial-velocity limitations
due to the non-uniform illumination of the spectrograph. We conducted tests on
SOPHIE with telescope vignetting, seeing variation and centering errors on the
fiber entrance. We simulated the light path through the instrument in order to
explain the radial velocity variation obtained with our tests. We then
identified the illumination stability and uniformity has a critical point for
the extremely high-precision radial velocity instruments (ESPRESSO@VLT,
CODEX@E-ELT). Tests on square and octagonal section fibers are now under
development and SOPHIE will be used as a bench test to validate these new feed
optics.Comment: to appear in the Proceedings conference "New Technologies for Probing
the Diversity of Brown Dwarfs and Exoplanets", Shanghai, 200
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