8,126 research outputs found
Time Evolution of Two-Level Systems Driven by Periodic Fields
In this paper we study the time evolution of a class of two-level systems
driven by periodic fields in terms of new convergent perturbative expansions
for the associated propagator U(t). The main virtue of these expansions is that
they do not contain secular terms, leading to a very convenient method for
quantitatively studying the long-time behaviour of that systems. We present a
complete description of an algorithm to numerically compute the perturbative
expansions. In particular, we applied the algorithm to study the case of an
ac-dc field (monochromatic interaction), exploring various situations and
showing results on (time-dependent) observable quantities, like transition
probabilities. For a simple ac field, we analised particular situations where
an approximate effect of dynamical localisation is exhibited by the driven
system. The accuracy of our calculations was tested measuring the unitarity of
the propagator U(t), resulting in very small deviations, even for very long
times compared to the cycle of the driving field.Comment: 1 table, 5 figures. Version 2 contains minor correction
Electrically and Magnetically Charged States and Particles in the 2+1-Dimensional Z_N-Higgs Gauge Model
Electrically as well as magnetically charged states are constructed in the
2+1-dimensional Euclidean Z_N-Higgs lattice gauge model, the former following
ideas of Fredenhagen and Marcu and the latter using duality transformations on
the algebra of observables. The existence of electrically and of magnetically
charged particles is also established. With this work we prepare the ground for
the constructive study of anyonic statistics of multiparticle scattering states
of electrically and magnetically charged particles in this model (work in
progress).Comment: 57 pages, Sfb 288 Preprint No. 109. To appear in Commun. Math. Phys.
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authors at [email protected] or Freie Universitaet Berlin.
Institut fuer Theoretische Physik. Arnimallee 14. Berlin 14195 German
Darboux transformation for two-level systems
We develop the Darboux procedure for the case of the two-level system. In
particular, it is demonstrated that one can construct the Darboux intertwining
operator that does not violate the specific structure of the equations of the
two-level system, transforming only one real potential into another real
potential. We apply the obtained Darboux transformation to known exact
solutions of the two-level system. Thus, we find three classes of new solutions
for the two-level system and the corresponding new potentials that allow such
solutions.Comment: 10 page
Stability for quasi-periodically perturbed Hill's equations
We consider a perturbed Hill's equation of the form , where is real analytic and
periodic, is real analytic and quasi-periodic and \eps is a ``small''
real parameter. Assuming Diophantine conditions on the frequencies of the
decoupled system, i.e. the frequencies of the external potentials and
and the proper frequency of the unperturbed () Hill's
equation, but without making non-degeneracy assumptions on the perturbing
potential , we prove that quasi-periodic solutions of the unperturbed
equation can be continued into quasi-periodic solutions if lies in a
Cantor set of relatively large measure in , where
is small enough. Our method is based on a resummation procedure of
a formal Lindstedt series obtained as a solution of a generalized Riccati
equation associated to Hill's problem.Comment: 40 pages, 4 figure
Localizability in de Sitter space
An analogue of the Newton-Wigner position operator is defined for a massive
neutral scalar field in de Sitter space. The one-particle subspace of the
theory, consisting of positive-energy solutions of the Klein-Gordon equation
selected by the Hadamard condition, is identified with an irreducible
representation of de Sitter group. Postulates of localizability analogous to
those written by Wightman for fields in Minkowski space are formulated on it,
and a unique solution is shown to exist. A simple expression for the
time-evolution of the operator is presented.Comment: Presentation improved; references adde
New Shop Floor Control Approaches for Virtual Enterprises
The virtual enterprise paradigm seems a fit response to face market instability and the volatile nature of business opportunities increasing enterprise’s interest in similar forms of networked organisations. The dynamic environment of a virtual enterprise requires that partners in the consortium own reconfigurable shop floors. This paper presents new approaches to shop floor control that meet the requirements of the new industrial paradigms and argues on work re-organization at shop floor level.virtual enterprise; networked organisations
Ways to open innovation: main agents and sources in the Portuguese case
Facing increasing open innovation trends, Portuguese enterprises are considering the related processes and impacts. Thus, this work aims to identify the sectors whose enterprises most engage in open innovation (such as cooperation on this issue) and which sources/agents are most used. This is analyzed by sector and type of innovation as an interesting way of differentiation for better open innovation strategy delineation. Using the data from the Community Innovation Survey (CIS-2012), it first appraises the nature of the innovation process, either cooperative or firm-based, as the starting level of analysis. Then, it differentiates the results by sector illustrating which cooperation sources/agents are most used (scope) and relative intensity of use (scale). This is important to assess levels of openness and related factors. Results show that main innovating sectors in Portugal are of three types: research-based, knowledge-based and service-based. They reveal an increasing focus on knowledge and services, trends that have been leading to more active openness towards innovation. For instance, health and construction are increasing their openness for innovating and internationalizing processes. However, Portuguese innovation is still more firm-based (in-house) than cooperation-based, especially concerning new products' launching. This work and future analyzes around it can contribute to encourage the open innovation strategy in more sectors of the economy as an easy and effective way to cope with rapid trends and changes. (C) 2017 Elsevier Ltd. All rights reserved.info:eu-repo/semantics/publishedVersio
Spin equation and its solutions
The aim of the present article is to study in detail the so-called spin
equation (SE) and present both the methods of generating new solution and a new
set of exact solutions. We recall that the SE with a real external field can be
treated as a reduction of the Pauli equation to the (0+1)-dimensional case.
Two-level systems can be described by an SE with a particular form of the
external field. In this article, we also consider associated equations that are
equivalent or (in one way or another) related to the SE. We describe the
general solution of the SE and solve the inverse problem for this equation. We
construct the evolution operator for the SE and consider methods of generating
new sets of exact solutions. Finally, we find a new set of exact solutions of
the SE.Comment: 29 page
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