552 research outputs found
Non-Markovian master equation for a damped driven two-state system
We present a detailed microscopic derivation for a non-Markovian master
equation for a driven two-state system interacting with a general structured
reservoir. The master equation is derived using the time-convolutionless
projection operator technique in the limit of weak coupling between the
two-state quantum system and its environment. We briefly discuss the Markov
approximation, the secular approximation and their validity.Comment: 6 pages, submitted to proceedings of CEWQO200
Break-down of the single-active-electron approximation for one-photon ionization of the B state of H exposed to intense laser fields
Ionization, excitation, and de-excitation to the ground state is studied
theoretically for the first excited singlet state B of H
exposed to intense laser fields with photon energies in between about 3 eV and
13 eV. A parallel orientation of a linear polarized laser and the molecular
axis is considered. Within the dipole and the fixed-nuclei approximations the
time-dependent Schr\"odinger equation describing the electronic motion is
solved in full dimensionality and compared to simpler models. A dramatic
break-down of the single-active-electron approximation is found and explained
to be due to the inadequate description of the final continuum states.Comment: 9 pages, 4 figure
Non-Markovian dynamics in atom-laser outcoupling from a double-well Bose-Einstein condensate
We investigate the dynamics of a continuous atom laser based on the merging
of independently formed atomic condensates. In a first attempt to understand
the dynamics of the system, we consider two independent elongated Bose-Einstein
condensates which approach each other and focus on intermediate inter-trap
distances so that a two-mode model is well justified. In the framework of a
mean-field theory, we discuss the quasi steady-state population of the traps as
well as the energy distribution of the outcoupled atoms.Comment: 21 pages, 9 figure, to appear in J. Phys.
Time-dependent calculation of ionization in Potassium at mid-infrared wavelengths
We study the dynamics of the Potassium atom in the mid-infrared, high
intensity, short laser pulse regime. We ascertain numerical convergence by
comparing the results obtained by the direct expansion of the time-dependent
Schroedinger equation onto B-Splines, to those obtained by the eigenbasis
expansion method. We present ionization curves in the 12-, 13-, and 14-photon
ionization range for Potassium. The ionization curve of a scaled system, namely
Hydrogen starting from the 2s, is compared to the 12-photon results. In the
13-photon regime, a dynamic resonance is found and analyzed in some detail. The
results for all wavelengths and intensities, including Hydrogen, display a
clear plateau in the peak-heights of the low energy part of the Above Threshold
Ionization (ATI) spectrum, which scales with the ponderomotive energy Up, and
extends to 2.8 +- 0.5 Up.Comment: 15 two-column pages with 15 figures, 3 tables. Accepted for
publication in Phys. Rev A. Improved figures, language and punctuation, and
made minor corrections. We also added a comparison to the ADK theor
Non-Markovian Dynamics of Entanglement for Multipartite Systems
Entanglement dynamics for a couple of two-level atoms interacting with
independent structured reservoirs is studied using a non-perturbative approach.
It is shown that the revival of atom entanglement is not necessarily
accompanied by the sudden death of reservoir entanglement, and vice versa. In
fact, atom entanglement can revive before, simultaneously or even after the
disentanglement of reservoirs. Using a novel method based on the population
analysis for the excited atomic state, we present the quantitative criteria for
the revival and death phenomena. For giving a more physically intuitive
insight, the quasimode Hamiltonian method is applied. Our quantitative analysis
is helpful for the practical engineering of entanglement.Comment: 10 pages and 4 figure
The relationship between Higher Education and labour market in Greece : the weakest link?
The high level of graduate unemployment, even though it is acknowledged as one of the most distinctive characteristics of the Greek labour market, it has not attracted enough attention in the academic literature. This paper utilizes micro-data from the Labour Force Survey in order to investigate how the employment situation of young (aged 35 and below) graduates varies across fields of study. The findings suggest that graduates of disciplines that have high levels of private sector employment, such as Polytechnics and Computer Science, are in general better off in the Greek labour market. On the other hand, graduates of disciplines that are traditionally related to the needs of the public sector, such as Sociology and Humanities, face poor employment prospects. The findings of this study highlight the need for drastic reforms of the Higher Education system
Existence criteria for stabilization from the scaling behaviour of ionization probabilities
We provide a systematic derivation of the scaling behaviour of various
quantities and establish in particular the scale invariance of the ionization
probability. We discuss the gauge invariance of the scaling properties and the
manner in which they can be exploited as consistency check in explicit
analytical expressions, in perturbation theory, in the Kramers-Henneberger and
Floquet approximation, in upper and lower bound estimates and fully numerical
solutions of the time dependent Schroedinger equation. The scaling invariance
leads to a differential equation which has to be satisfied by the ionization
probability and which yields an alternative criterium for the existence of
atomic bound state stabilization.Comment: 12 pages of Latex, one figur
Effects of interatomic collisions on atom laser outcoupling
We present a computational approach to the outcoupling in a simple
one-dimensional atom laser model, the objective being to circumvent
mathematical difficulties arising from the breakdown of the Born and Markov
approximations. The approach relies on the discretization of the continuum
representing the reservoir of output modes, which allows the treatment of
arbitrary forms of outcoupling as well as the incorporation of non-linear terms
in the Hamiltonian, associated with interatomic collisions. By considering a
single-mode trapped condensate, we study the influence of elastic collisions
between trapped and free atoms on the quasi steady-state population of the
trap, as well as the energy distribution and the coherence of the outcoupled
atoms.Comment: 25 pages, 11 figures, to appear in J. Phys.
Photon-photon correlations and entanglement in doped photonic crystals
We consider a photonic crystal (PC) doped with four-level atoms whose
intermediate transition is coupled near-resonantly with a photonic band-gap
edge. We show that two photons, each coupled to a different atomic transition
in such atoms, can manifest strong phase or amplitude correlations: One photon
can induce a large phase shift on the other photon or trigger its absorption
and thus operate as an ultrasensitive nonlinear photon-switch. These features
allow the creation of entangled two-photon states and have unique advantages
over previously considered media: (i) no control lasers are needed; (ii) the
system parameters can be chosen to cause full two-photon entanglement via
absorption; (iii) a number of PCs can be combined in a network.Comment: Modified, expanded text; added reference
Linear atomic quantum coupler
In this paper, we develop the notion of the linear atomic quantum coupler.
This device consists of two modes propagating into two waveguides, each of them
includes a localized and/or a trapped atom. These waveguides are placed close
enough to allow exchanging energy between them via evanescent waves. Each mode
interacts with the atom in the same waveguide in the standard way, i.e. as the
Jaynes-Cummings model (JCM), and with the atom-mode in the second waveguide via
evanescent wave. We present the Hamiltonian for the system and deduce the exact
form for the wavefunction. We investigate the atomic inversions and the
second-order correlation function. In contrast to the conventional linear
coupler, the atomic quantum coupler is able to generate nonclassical effects.
The atomic inversions can exhibit long revival-collapse phenomenon as well as
subsidiary revivals based on the competition among the switching mechanisms in
the system. Finally, under certain conditions, the system can yield the results
of the two-mode JCM.Comment: 14 pages, 3 figures; comments are most welcom
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