2,322 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
Investigation of DC-8 nacelle modifications to reduce fan-compressor noise in airport communities. Part 2 - Design studies and duct-lining investigations, May 1967 - October 1969
Modifications to reduce fan-compressor noise level of DC-8 aircraft - Part
Gaia DR2 view of the Lupus V-VI clouds: the candidate diskless young stellar objects are mainly background contaminants
Extensive surveys of star-forming regions with Spitzer have revealed
populations of disk-bearing young stellar objects. These have provided crucial
constraints, such as the timescale of dispersal of protoplanetary disks,
obtained by carefully combining infrared data with spectroscopic or X-ray data.
While observations in various regions agree with the general trend of
decreasing disk fraction with age, the Lupus V and VI regions appeared to have
been at odds, having an extremely low disk fraction. Here we show, using the
recent Gaia data release 2 (DR2), that these extremely low disk fractions are
actually due to a very high contamination by background giants. Out of the 83
candidate young stellar objects (YSOs) in these clouds observed by Gaia, only
five have distances of 150 pc, similar to YSOs in the other Lupus clouds, and
have similar proper motions to other members in this star-forming complex. Of
these five targets, four have optically thick (Class II) disks. On the one
hand, this result resolves the conundrum of the puzzling low disk fraction in
these clouds, while, on the other hand, it further clarifies the need to
confirm the Spitzer selected diskless population with other tracers, especially
in regions at low galactic latitude like Lupus V and VI. The use of Gaia
astrometry is now an independent and reliable way to further assess the
membership of candidate YSOs in these, and potentially other, star-forming
regions.Comment: Accepted for publication on Astronomy&Astrophysics Letter
A study of turbofan-engine compressor noise suppression techniques
Nacelle design for turbofan engine compressor noise suppression on Douglas DC-8 aircraf
The Spitzer c2d survey of large, nearby, interstellar clouds. X. The Chamaeleon II pre-main-sequence population as observed with IRAC and MIPS
We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy survey observations and complementary optical and NIR data of the Chamaeleon II (Cha II) dark cloud. We perform a census of the young population in an area of similar to 1.75 deg^(2) and study the spatial distribution and properties of the cloud members and candidate pre-main-sequence (PMS) objects and their circumstellar matter. Our census is complete down to the substellar regime (M approximate to 0.03 M☉). From the analysis of the volume density of the PMS objects and candidates we find two groups of objects with volume densities higher than 25 M☉ pc^(-3) and 5-10 members each. A multiplicity fraction of about 13% +/- 3% is observed for objects with separations 0.8" < θ < 6.0" (142-1065 AU). No evidence for variability between the two epochs of the c2d IRAC data set, Δt ~ 6 hr, is detected. We estimate a star formation efficiency of 1%-4%, consistent with the estimates for Taurus and Lupus, but lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds reflect a different history of star formation. We also find that Cha II is turning some 6-7 M☉ into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. The disk fraction of 70%-80% that we estimate in Cha II is much higher than in other star-forming regions and indicates that the population in this cloud is dominated by objects with active accretion. Finally, the Cha II outflows are discussed; a new Herbig-Haro outflow, HH 939, driven by the classical T Tauri star Sz 50, has been discovered
The Gaia-ESO Survey: N-body modelling of the Gamma Velorum cluster
The Gaia-ESO Survey has recently unveiled the complex kinematic signature of
the Gamma Velorum cluster: this cluster is composed of two kinematically
distinct populations (hereafter, population A and B), showing two different
velocity dispersions and a relative ~2 km s^-1 radial velocity (RV) shift. In
this paper, we propose that the two populations of the Gamma Velorum cluster
originate from two different sub-clusters, born from the same parent molecular
cloud. We investigate this possibility by means of direct-summation N-body
simulations. Our scenario is able to reproduce not only the RV shift and the
different velocity dispersions, but also the different centroid (~0.5 pc), the
different spatial concentration and the different line-of-sight distance (~5
pc) of the two populations. The observed 1-2 Myr age difference between the two
populations is also naturally explained by our scenario, in which the two
sub-clusters formed in two slightly different star formation episodes. Our
simulations suggest that population B is strongly supervirial, while population
A is close to virial equilibrium. We discuss the implications of our models for
the formation of young star clusters and OB associations in the Milky Way.Comment: 12 pages, 7 figures, 2 tables, Astronomy and Astrophysics, in pres
The Master Stability Function for Synchronization in Simplicial Complexes
All interesting and fascinating collective properties of a complex system
arise from the intricate way in which its components interact. Various systems
in physics, biology, social sciences and engineering have been successfully
modelled as networks of coupled dynamical systems, where the graph links
describe pairwise interactions. This is, however, too strong a limitation, as
recent studies have revealed that higher-order many-body interactions are
present in social groups, ecosystems and in the human brain, and they actually
affect the emergent dynamics of all these systems. Here, we introduce a general
framework that allows to study coupled dynamical systems accounting for the
precise microscopic structure of their interactions at any possible order. We
consider the most general ensemble of identical dynamical systems, organized on
the nodes of a simplicial complex, and interacting through
synchronization-non-invasive coupling function. The simplicial complex can be
of any dimension, meaning that it can account, at the same time, for pairwise
interactions, three-body interactions and so on. In such a broad context, we
show that complete synchronization exists as an invariant solution, and we give
the necessary condition for it to be observed as a stable state in terms of a
Master Stability Function. This generalizes the existing results valid for
pairwise interactions (i.e. graphs) to the case of complex systems with the
most general possible architecture. Moreover, we show how the approach can be
simplified for specific, yet frequently occurring, instances, and we verify all
our theoretical predictions in synthetic and real-world systems. Given the
completely general character of the method proposed, our results contribute to
the theory of dynamical systems with many-body interactions and can find
applications in an extremely wide range of practical cases
The Gaia-ESO Survey: membership and Initial Mass Function of the Gamma Velorum cluster
Understanding the properties of young open clusters, such as the Initial Mass
Function (IMF), star formation history and dynamic evolution, is crucial to
obtain reliable theoretical predictions of the mechanisms involved in the star
formation process. We want to obtain a list, as complete as possible, of
confirmed members of the young open cluster Gamma Velorum, with the aim of
deriving general cluster properties such as the IMF. We used all available
spectroscopic membership indicators within the Gaia-ESO public archive together
with literature photometry and X-ray data and, for each method, we derived the
most complete list of candidate cluster members. Then, we considered
photometry, gravity and radial velocities as necessary conditions to select a
subsample of candidates whose membership was confirmed by using the lithium and
H lines and X-rays as youth indicators. We found 242 confirmed and 4
possible cluster members for which we derived masses using very recent stellar
evolutionary models. The cluster IMF in the mass range investigated in this
study shows a slope of for and
for and is consistent with a standard
IMF. The similarity of the IMF of the young population around Vel to
that in other star forming regions and the field suggests it may have formed
through very similar processes.Comment: Accepted for publication in A&A; 18 pages, 11 figures, 5 table
Stability of Synchronization in Simplicial Complexes
Networks with higher order interactions, relevant to social groups, ecosystems and human brain, require new tools and instruments for their analysis. Gambuzza et al. propose an analytical approach which allows to find conditions for stable synchronization in many-body interaction networks
Mathematical Structure of Rabi Oscillations in the Strong Coupling Regime
In this paper we generalize the Jaynes--Cummings Hamiltonian by making use of
some operators based on Lie algebras su(1,1) and su(2), and study a
mathematical structure of Rabi floppings of these models in the strong coupling
regime. We show that Rabi frequencies are given by matrix elements of
generalized coherent operators (quant--ph/0202081) under the rotating--wave
approximation.
In the first half we make a general review of coherent operators and
generalized coherent ones based on Lie algebras su(1,1) and su(2). In the
latter half we carry out a detailed examination of Frasca (quant--ph/0111134)
and generalize his method, and moreover present some related problems.
We also apply our results to the construction of controlled unitary gates in
Quantum Computation. Lastly we make a brief comment on application to Holonomic
Quantum Computation.Comment: Latex file, 24 pages. I added a new section (Quantum Computation), so
this paper became self-contained in a certain sens
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