469 research outputs found
Feedback control of the fluorescence light squeezing
We consider a two-level atom stimulated by a coherent monochromatic laser and
we study how to enhance the squeezing of the fluorescence light and of the atom
itself in the presence of a Wiseman-Milburn feedback mechanism, based on the
homodyne detection of a fraction of the emitted light. Besides analyzing the
effect of the control parameters on the squeezing properties of the light and
of the atom, we also discuss the relations among these. The problem is tackled
inside the framework of quantum trajectory theory.Comment: RevTeX4, 4 pages, 2 figure
Stochastic Schr\"odinger equations with coloured noise
A natural non-Markovian extension of the theory of white noise quantum
trajectories is presented. In order to introduce memory effects in the
formalism an Ornstein-Uhlenbeck coloured noise is considered as the output
driving process. Under certain conditions a random Hamiltonian evolution is
recovered. Moreover, non-Markovian stochastic Schr\"odinger equations which
unravel non-Markovian master equations are derived.Comment: 4pages, revte
Measurements continuous in time and a posteriori states in quantum
Measurements continuous in time were consistently introduced in quantum
mechanics and applications worked out, mainly in quantum optics. In this
context a quantum filtering theory has been developed giving the reduced state
after the measurement when a certain trajectory of the measured observables is
registered (the a posteriori states). In this paper a new derivation of
filtering equations is presented, in the cases of counting processes and of
measurement processes of diffusive type. It is also shown that the equation for
the a posteriori dynamics in the diffusive case can be obtained, by a suitable
limit, from that one in the counting case. Moreover, the paper is intended to
clarify the meaning of the various concepts involved and to discuss the
connections among them. As an illustration of the theory, simple models are
worked out.Comment: 31 page. See also related papers at
http://www.maths.nott.ac.uk/personal/vpb/research/mes_fou.html and
http://www.maths.nott.ac.uk/personal/vpb/research/fil_con.htm
Quantum stochastic models of two-level atoms and electromagnetic cross sections
Quantum stochastic differential equations have been used to describe the dynamics of an atom interacting with the electromagnetic field via absorption/emission processes. Here, by using the full quantum stochastic Schroedinger equation proposed by Hudson and Parthasarathy fifteen years ago, we show that such models can be generalized to include other processes into the interaction. In the case of a two-level atom we construct a model in which the interaction with the field is due either to absorption/emission processes either to direct scattering processes, which simulate the interaction due to virtual transitions to the levels which have been eliminated from the description. To see the effects of the new terms, the total, elastic and inelastic eloctromagnetic cross sections are studied. The new power spectrum is compared with Mollow's results
Jump-diffusion unravelling of a non Markovian generalized Lindblad master equation
The "correlated-projection technique" has been successfully applied to derive
a large class of highly non Markovian dynamics, the so called non Markovian
generalized Lindblad type equations or Lindblad rate equations. In this
article, general unravellings are presented for these equations, described in
terms of jump-diffusion stochastic differential equations for wave functions.
We show also that the proposed unravelling can be interpreted in terms of
measurements continuous in time, but with some conceptual restrictions. The
main point in the measurement interpretation is that the structure itself of
the underlying mathematical theory poses restrictions on what can be considered
as observable and what is not; such restrictions can be seen as the effect of
some kind of superselection rule. Finally, we develop a concrete example and we
discuss possible effects on the heterodyne spectrum of a two-level system due
to a structured thermal-like bath with memory.Comment: 23 page
Constructing quantum measurement processes via classical stochastic calculus
AbstractA class of linear stochastic differential equations in Hilbert spaces is studied, which allows to construct probability densities and to generate changes in the probability measure one started with. Related linear equations for trace-class operators are discussed. Moreover, some analogue of filtering theory gives rise to related non-linear stochastic differential equations in Hilbert spaces and in the space of trace-class operators. Finally, it is shown how all these equations represent a new formulation and a generalization of the theory of measurements continuous in time in quantum mechanics
Retardation effects in the rotating string model
A new method to study the retardation effects in mesons is presented.
Inspired from the covariant oscillator quark model, it is applied to the
rotating string model in which a non zero value is allowed for the relative
time between the quark and the antiquark. This approach leads to a retardation
term which behaves as a perturbation of the meson mass operator. It is shown
that this term preserves the Regge trajectories for light mesons, and that a
satisfactory agreement with the experimental data can be obtained if the quark
self-energy contribution is added. The consequences of the retardation on the
Coulomb interaction and the wave function are also analyzed.Comment: 4 figure
The Dual Description of Long Distance QCD and the Effective Lagrangian for Constituent Quarks
We describe long distance QCD by a dual theory in which the fundamental
variables are dual potentials coupled to monopole fields and use this dual
theory to determine the effective Lagrangian for constituent quarks. We find
the color field distribution surrounding a quark anti-quark pair to first order
in their velocities. Using these distributions we eliminate the dual potentials
and obtain an effective interaction Lagrangian depending only upon the quark and anti-quark
coordinates and velocities, valid to second order in their velocities. We
propose as the Lagrangian describing the long distance interaction of
constituent quarks
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