2,835 research outputs found
Theory of dressed states in quantum optics
The dual Dyson series [M.Frasca, Phys. Rev. A {\bf 58}, 3439 (1998)], is used
to develop a general perturbative method for the study of atom-field
interaction in quantum optics. In fact, both Dyson series and its dual, through
renormalization group methods to remove secular terms from the perturbation
series, give the opportunity of a full study of the solution of the
Schr\"{o}dinger equation in different ranges of the parameters of the given
hamiltonian. In view of recent experiments with strong laser fields, this
approach seems well-suited to give a clarification and an improvement of the
applications of the dressed states as currently done through the eigenstates of
the atom-field interaction, showing that these are just the leading order of
the dual Dyson series when the Hamiltonian is expressed in the interaction
picture. In order to exploit the method at the best, a study is accomplished of
the well-known Jaynes-Cummings model in the rotating wave approximation, whose
exact solution is known, comparing the perturbative solutions obtained by the
Dyson series and its dual with the same approximations obtained by Taylor
expanding the exact solution. Finally, a full perturbative study of high-order
harmonic generation is given obtaining, through analytical expressions, a clear
account of the power spectrum using a two-level model, even if the method can
be successfully applied to a more general model that can account for ionization
too. The analysis shows that to account for the power spectrum it is needed to
go to first order in the perturbative analysis. The spectrum obtained gives a
way to measure experimentally the shift of the energy levels of the atom
interacting with the laser field by looking at the shifting of hyper-Raman
lines.Comment: Revtex, 17 page
Photospheric and chromospheric activity in four young solar-type stars
We present a photometric and spectroscopic study of four G-K dwarfs, namely
HD 166, epsilon Eri, chi1 Ori and kappa1 Cet. In three cases, we find a clear
spatial association between photospheric and chromospheric active regions. For
chi1 Ori we do not find appreciable variations of photospheric temperature, and
chromospheric Halpha emission. We applied a spot/plage model to the observed
rotational modulation of temperature and flux to derive spot/plage parameters
and to reconstruct a rough three-dimensional map of the outer atmosphere of
kappa1 Cet, HD 166 and epsilon Eri.Comment: 12 pages, 3 tables, 9 figures. Submitted to Ap
Cooperative effects in Josephson junctions in a cavity in the strong coupling regime
We analyze the behavior of systems of two and three qubits made by Josephson
junctions, treated in the two level approximation, driven by a radiation mode
in a cavity. The regime we consider is a strong coupling one recently
experimentally reached for a single junction. Rabi oscillations are obtained
with the frequency proportional to integer order Bessel functions in the limit
of a large photon number, similarly to the case of the single qubit. A
selection rule is derived for the appearance of Rabi oscillations. A quantum
amplifier built with a large number of Josephson junctions in a cavity in the
strong coupling regime is also described.Comment: 9 pages, no figures. Version accepted for publication in Physical
Review
Evidence from stellar rotation of enhanced disc dispersal: (I) The case of the triple visual system BD-21 1074 in the Pictoris association
The early stage of stellar evolution is characterized by a star-disc locking
mechanism. The disc-locking prevents the star to spin its rotation up, and its
timescale depends on the disc lifetime. Some mechanisms can significantly
shorten this lifetime, allowing a few stars to start spinning up much earlier
than other stars. In the present study, we aim to investigate how the
properties of the circumstellar environment can shorten the disc lifetime. We
have identified a few multiple stellar systems, composed of stars with similar
masses, which belong to associations with a known age. Since all parameters
that are responsible for the rotational evolution, with the exception of
environment properties and initial stellar rotation, are similar for all
components, we expect that significant differences among the rotation periods
can only arise from differences in the disc lifetimes. A photometric timeseries
allowed us to measure the rotation periods of each component, while
high-resolution spectra provided us with the fundamental parameters,
and chromospheric line fluxes. The rotation periods of the components differ
significantly, and the component B, which has a closer companion C, rotates
faster than the more distant and isolated component A. We can ascribe the
rotation period difference to either different initial rotation periods or
different disc-locking phases arising from the presence of the close companion
C. In the specific case of BD21 1074, the second scenario seems to be more
favored. In our hypothesis of different disc-locking phase, any planet orbiting
this star is likely formed very rapidly owing to a gravitational instability
mechanism, rather than core accretion. Only a large difference of initial
rotation periods alone could account for the observed period difference,
leaving comparable disc lifetimes.Comment: Accepted by Astronomy & Astrophysics on July 31, 2014; Pages 12,
Figs.
Magnetic activity, differential rotation and dynamo action in the pulsating F9IV star KIC 5955122
We present photometric spot modeling of the nearly four-year long light-curve
of the Kepler target KIC 5955122 in terms of persisting dark circular surface
features. With a Bayesian technique, we produced a plausible surface map that
shows dozens of small spots. After some artifacts are removed, the residuals
are at \,mmag. The shortest rotational period found is days. The equator-to-pole extrapolated differential rotation is rad/d. The spots are roughly half as bright as the unperturbed stellar
photosphere. Spot latitudes are restricted to the zone latitude.
There is no indication for any near-pole spots. In addition, the p-mode
pulsations enabled us to determine the evolutionary status of the star, the
extension of the convective zone, and its radius and mass. We discuss the
possibility that the clear signature of active regions in the light curve of
the F9IV star KIC 5955122 is produced by a flux-transport dynamo action at the
base of the convection zone. In particular, we argue that this star has evolved
from an active to a quiet status during the Q0--Q16 period of observation, and
we predict, according to our dynamo model, that the characteristic activity
cycle is of the order of the solar one.Comment: 9 pages, 12 figures, to be published on A&
Dynamical decoherence in a cavity with a large number of two-level atoms
We consider a large number of two-level atoms interacting with the mode of a
cavity in the rotating-wave approximation (Tavis-Cummings model). We apply the
Holstein-Primakoff transformation to study the model in the limit of the number
of two-level atoms, all in their ground state, becoming very large. The unitary
evolution that we obtain in this approximation is applied to a macroscopic
superposition state showing that, when the coherent states forming the
superposition are enough distant, then the state collapses on a single coherent
state describing a classical radiation mode. This appear as a true dynamical
effect that could be observed in experiments with cavities.Comment: 9 pages, no figures. This submission substitutes paper
quant-ph/0212148 that was withdrawn. Version accepted for publication in
Journal of Physics B: Atomic, Molecular & Optical Physic
An extensive VLT/X-Shooter library of photospheric templates of pre-main sequence stars
Studies of the formation and evolution of young stars and their disks rely on
the knowledge of the stellar parameters of the young stars. The derivation of
these parameters is commonly based on comparison with photospheric template
spectra. Furthermore, chromospheric emission in young active stars impacts the
measurement of mass accretion rates, a key quantity to study disk evolution.
Here we derive stellar properties of low-mass pre-main sequence stars without
disks, which represent ideal photospheric templates for studies of young stars.
We also use these spectra to constrain the impact of chromospheric emission on
the measurements of mass accretion rates. The spectra in reduced,
flux-calibrated, and corrected for telluric absorption form are made available
to the community. We derive the spectral type for our targets by analyzing the
photospheric molecular features present in their VLT/X-Shooter spectra by means
of spectral indices and comparison of the relative strength of photospheric
absorption features. We also measure effective temperature, gravity, projected
rotational velocity, and radial velocity from our spectra by fitting them with
synthetic spectra with the ROTFIT tool. The targets have negligible extinction
and spectral type from G5 to M8. We perform synthetic photometry on the spectra
to derive the typical colors of young stars in different filters. We measure
the luminosity of the emission lines present in the spectra and estimate the
noise due to chromospheric emission in the measurements of accretion luminosity
in accreting stars. We provide a calibration of the photospheric colors of
young PMS stars as a function of their spectral type in a set of standard
broad-band optical and near-infrared filters. For stars with masses of ~
1.5Msun and ages of ~1-5 Myr, the chromospheric noise converts to a limit of
measurable mass accretion rates of ~ 3x10^-10 Msun/yr.Comment: Accepted for publication on Astronomy & Astrophysics. The spectra of
the photospheric templates will be uploaded to Vizier, but are already
available on request. Abstract shortened for arxiv constraints. Language
edited versio
Modulation of human corticospinal excitability by paired associative stimulation in patients with amyotrophic lateral sclerosis and effects of Riluzole
BACKGROUND Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that causes an impairment in both the upper and lower motor neurons. The recent description of numerous non-motor signs points to an involvement of the neocortex networks that is more complex than was previously believed. Paired associative stimulation (PAS), a combination of transcranial magnetic stimulation (TMS) and peripheral nerve stimulation, can enhance motor output in the contralateral hand through an NMDA-mediated sensorimotor mechanism.
OBJECTIVE To describe the effects of PAS on ALS patients before and after Riluzole intake compared with healthy subjects.
METHODS PAS was used to detect differences between 24 newly-diagnosed ALS patients and 25 age-matched healthy controls. MEP amplitude from the abductor pollicis brevis was considered before PAS, immediately after (T0) and after 10 (T10), 20 (T20), 30 (T30) and 60 (T60) minutes. Statistical significance was calculated using RM-ANOVA.
RESULTS In healthy controls, PAS significantly increased MEP amplitude at T10, T20 and T30 (p < 0.05). In ALS patients, a significant increase in MEP amplitude was also observed after 60 min (p < 0.05), thus demonstrating NMDA-mediated enhanced facilitatory plasticity. After two weeks of riluzole intake, no MEP amplitude increase was evident after PAS at any time point. In three monomelic-onset ALS patients, sensorimotor facilitation was evident only in the hemisphere corresponding to the affected side and appeared in the opposite hemisphere when the patients manifested contralateral symptoms.
CONCLUSIONS PAS may be considered a useful tool when investigating NMDA-mediated neocortical networks in ALS patients and the modulation of such networks after anti-glutamatergic drug intake
Reachability analysis in stochastic directed graphs by reinforcement learning
We characterize the reachability probabilities in stochastic directed graphs by means of reinforcement learning methods. In particular, we show that the dynamics of the transition probabilities in a stochastic digraph can be modeled via a difference inclusion, which, in turn, can be interpreted as a Markov decision process. Using the latter framework, we offer a methodology to design reward functions to provide upper and lower bounds on the reachability probabilities of a set of nodes for stochastic digraphs. The effectiveness of the proposed technique is demonstrated by application to the diffusion of epidemic diseases over time-varying contact networks generated by the proximity patterns of mobile agents
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