10,981 research outputs found
Rail roughness and rolling noise in tramways
Companies which manage railway networks have to cope continually with the problem of operating safety and maintenance intervention issues related to rail surface
irregularities. A lot of experience has been gained in recent years in railway applications but the case of tramways is quite different; in this field there are no specific criteria to define any intervention on rail surface restoration. This paper shows measurements carried out on some stretches of a tram network with the CAT equipment (Corrugation Analysis Trolley) for the principal purpose of detecting different states of degradation of the rails and identifying a level of deterioration to be associated with the need for maintenance through rail grinding. The measured roughness is used as an input parameter into prediction models for both rolling noise and ground vibration to show the potential effect that high levels of roughness can have in urban environment. Rolling noise predictions are also compared with noise measurements to illustrate the applicability of the modelling approach. Particular attention is given to the way the contact filter needs to be modelled in the specific case of trams that generally operate at low speed. Finally an empirical approach to assess vibration levels in buildings is presented
Sensitivity-bandwidth limit in a multi-mode opto-electro-mechanical transducer
An opto--electro--mechanical system formed by a nanomembrane capacitively
coupled to an LC resonator and to an optical interferometer has been recently
employed for the high--sensitive optical readout of radio frequency (RF)
signals [T. Bagci, \emph{et~al.}, Nature {\bf 507}, 81 (2013)]. Here we propose
and experimentally demonstrate how the bandwidth of such kind of transducer can
be increased by controlling the interference between two--electromechanical
interaction pathways of a two--mode mechanical system. With a
proof--of--principle device \new{operating at room temperature, we achieve a
sensitivity of 300 nV/Hz^(1/2) over a bandwidth of 15 kHz in the presence of
radiofrequency noise, and an optimal shot-noise limited sensitivity of 10
nV/Hz^(1/2) over a bandwidth of 5 kHz. We discuss strategies for improving the
performance of the device, showing that, for the same given sensitivity, a
mechanical multi--mode transducer can achieve a bandwidth} significantly larger
than that of a single-mode one
Exciton-phonon scattering and photo-excitation dynamics in J-aggregate microcavities
We have developed a model accounting for the photo-excitation dynamics and
the photoluminescence of strongly coupled J-aggregate microcavities. Our model
is based on a description of the J-aggregate film as a disordered Frenkel
exciton system in which relaxation occurs due to the presence of a thermal bath
of molecular vibrations. In a strongly coupled microcavity exciton-polaritons
are formed, mixing superradiant excitons and cavity photons. The calculation of
the microcavity steady-state photoluminescence, following a CW non resonant
pumping, is carried out. The experimental photoluminescence intensity ratio
between upper and lower polariton branches is accurately reproduced. In
particular both thermal activation of the photoluminescence intensity ratio and
its Rabi splitting dependence are a consequence of the bottleneck in the
relaxation, occurring at the bottom of the excitonic reservoir. The effects due
to radiative channels of decay of excitons and to the presence of a
paritticular set of discrete optical molecular vibrations active in relaxation
processes are investigared.Comment: 8 pages, 6 figure
Optimization by Quantum Annealing: Lessons from Simple Cases
This paper investigates the basic behavior and performance of simulated
quantum annealing (QA) in comparison with classical annealing (CA). Three
simple one dimensional case study systems are considered, namely a parabolic
well, a double well, and a curved washboard. The time dependent Schr\"odinger
evolution in either real or imaginary time describing QA is contrasted with the
Fokker Planck evolution of CA. The asymptotic decrease of excess energy with
annealing time is studied in each case, and the reasons for differences are
examined and discussed. The Huse-Fisher classical power law of double well CA
is replaced with a different power law in QA. The multi-well washboard problem
studied in CA by Shinomoto and Kabashima and leading classically to a
logarithmic annealing even in the absence of disorder, turns to a power law
behavior when annealed with QA. The crucial role of disorder and localization
is briefly discussed.Comment: 16 pages, 9 figure
Theoretical fits of the \delta Cephei light, radius and radial velocity curves
We present a theoretical investigation of the light, radius and radial
velocity variations of the prototype Cephei. We find that the best fit
model accounts for luminosity and velocity amplitudes with an accuracy better
than , and for the radius amplitude with an accuracy of .
The chemical composition of this model suggests a decrease in both helium (0.26
vs 0.28) and metal (0.01 vs 0.02) content in the solar neighborhood. Moreover,
distance determinations based on the fit of light curves agree at the
level with the trigonometric parallax measured by the Hubble Space
Telescope (HST). On the other hand, distance determinations based on angular
diameter variations, that are independent of interstellar extinction and of the
-factor value, indicate an increase of the order of 5% in the HST parallax.Comment: accepted for publication on ApJ Letter
Non-local quantum correlations and detection processes in QFT
Quantum detection processes in QFT must play a key role in the description of
quantum field correlations, such as the appearance of entanglement, and of
causal effects. We consider the detection in the case of a simple QFT model
with a suitable interaction to exact treatment, consisting of a quantum scalar
field coupled linearly to a classical scalar source. We then evaluate the
response function to the field quanta of two-level point-like quantum model
detectors, and analyze the effects of the approximation adopted in standard
detection theory. We show that the use of the RWA, that characterizes the
Glauber detection model, leads in the detector response to non-local terms
corresponding to an instantaneously spreading of source effects over the whole
space. Other detector models, obtained with non-standard or the no-application
of RWA, give instead local responses to field quanta, apart from source
independent vacuum contribution linked to preexisting correlations of
zero-point field.Comment: 23 page
Quantum dynamics of a vibrational mode of a membrane within an optical cavity
Optomechanical systems are a promising candidate for the implementation of
quantum interfaces for storing and redistributing quantum information. Here we
focus on the case of a high-finesse optical cavity with a thin vibrating
semitransparent membrane in the middle. We show that robust and stationary
optomechanical entanglement could be achieved in the system, even in the
presence of nonnegligible optical absorption in the membrane. We also present
some preliminary experimental data showing radiation-pressure induced optical
bistability.Comment: 6 pages, 2 figures. Work presented at the conference QCMC 2010 held
on 19-23 July 2010 at the University of Queensland, Brisbane, Australi
Quantum dynamics of a high-finesse optical cavity coupled with a thin semi-transparent membrane
We study the quantum dynamics of the cavity optomechanical system formed by a
Fabry-Perot cavity with a thin vibrating membrane at its center. We first
derive the general multimode Hamiltonian describing the radiation pressure
interaction between the cavity modes and the vibrational modes of the membrane.
We then restrict the analysis to the standard case of a single cavity mode
interacting with a single mechanical resonator and we determine to what extent
optical absorption by the membrane hinder reaching a quantum regime for the
cavity-membrane system. We show that membrane absorption does not pose serious
limitations and that one can simultaneously achieve ground state cooling of a
vibrational mode of the membrane and stationary optomechanical entanglement
with state-of-the-art apparatuses.Comment: 14 pages, 7 figure
A generalized Kramers-Kronig transform for Casimir effect computations
Recent advances in experimental techniques now permit to measure the Casimir
force with unprecedented precision. In order to achieve a comparable precision
in the theoretical prediction of the force, it is necessary to accurately
determine the electric permittivity of the materials constituting the plates
along the imaginary frequency axis. The latter quantity is not directly
accessible to experiments, but it can be determined via dispersion relations
from experimental optical data. In the experimentally important case of
conductors, however, a serious drawback of the standard dispersion relations
commonly used for this purpose, is their strong dependence on the chosen
low-frequency extrapolation of the experimental optical data, which introduces
a significant and not easily controllable uncertainty in the result. In this
paper we show that a simple modification of the standard dispersion relations,
involving suitable analytic window functions, resolves this difficulty, making
it possible to reliably determine the electric permittivity at imaginary
frequencies solely using experimental optical data in the frequency interval
where they are available, without any need of uncontrolled data extrapolations.Comment: 10 pages, 6 encapsulated figures. A few typos corrected, some
references added. The new version matches the one accepted for publication on
Phys. Rev.
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