27,750 research outputs found
Oscillations of neutrinos and mesons in quantum field theory
This report deals with the quantum field theory of particle oscillations in
vacuum. We first review the various controversies regarding quantum-mechanical
derivations of the oscillation formula, as well as the different
field-theoretical approaches proposed to settle them. We then clear up the
contradictions between the existing field-theoretical treatments by a thorough
study of the external wave packet model. In particular, we show that the latter
includes stationary models as a subcase. In addition, we explicitly compute
decoherence terms, which destroy interferences, in order to prove that the
coherence length can be increased without bound by more accurate energy
measurements. We show that decoherence originates not only in the width and in
the separation of wave packets, but also in their spreading through space-time.
In this review, we neither assume the relativistic limit nor the stability of
oscillating particles, so that the oscillation formula derived with
field-theoretical methods can be applied not only to neutrinos but also to
neutral K and B mesons. Finally, we discuss oscillations of correlated
particles in the same framework.Comment: v2, 124 pages, 10 figures (7 more); updated review of the literature;
complete derivation of the oscillation probability at short and large
distance; more details on the influence of the spreading of the amplitude on
decoherence; submitted to Physics Report
Interplay between localization and absorption in disordered waveguides
This work presents results of ab-initio simulations of continuous wave
transport in disordered absorbing waveguides. Wave interference effects cause
deviations from diffusive picture of wave transport and make the diffusion
coefficient position- and absorption-dependent. As a consequence, the true
limit of a zero diffusion coefficient is never reached in an absorbing random
medium of infinite size, instead, the diffusion coefficient saturates at some
finite constant value. Transition to this absorption-limited diffusion exhibits
a universality which can be captured within the framework of the
self-consistent theory (SCT) of localization. The results of this work (i)
justify use of SCT in analyses of experiments in localized regime, provided
that absorption is not weak; (ii) open the possibility of diffusive description
of wave transport in the saturation regime even when localization effects are
strong.Comment: 10 pages, 3 figure
Disorder and interference: localization phenomena
The specific problem we address in these lectures is the problem of transport
and localization in disordered systems, when interference is present, as
characteristic for waves, with a focus on realizations with ultracold atoms.Comment: Notes of a lecture delivered at the Les Houches School of Physics on
"Ultracold gases and quantum information" 2009 in Singapore. v3: corrected
mistakes, improved script for numerics, Chapter 9 in "Les Houches 2009 -
Session XCI: Ultracold Gases and Quantum Information" edited by C. Miniatura
et al. (Oxford University Press, 2011
Weak localization with nonlinear bosonic matter waves
We investigate the coherent propagation of dilute atomic Bose-Einstein
condensates through irregularly shaped billiard geometries that are attached to
uniform incoming and outgoing waveguides. Using the mean-field description
based on the nonlinear Gross-Pitaevskii equation, we develop a diagrammatic
theory for the self-consistent stationary scattering state of the interacting
condensate, which is combined with the semiclassical representation of the
single-particle Green function in terms of chaotic classical trajectories
within the billiard. This analytical approach predicts a universal dephasing of
weak localization in the presence of a small interaction strength between the
atoms, which is found to be in good agreement with the numerically computed
reflection and transmission probabilities of the propagating condensate. The
numerical simulation of this quasi-stationary scattering process indicates that
this interaction-induced dephasing mechanism may give rise to a signature of
weak antilocalization, which we attribute to the influence of non-universal
short-path contributions.Comment: 67 pages, 19 figure
Effect of nonlinearity on the dynamics of a particle in dc field-induced systems
Dynamics of a particle in a perfect chain with one nonlinear impurity and in
a perfect nonlinear chain under the action of dc field is studied numerically.
The nonlinearity appears due to the coupling of the electronic motion to
optical oscillators which are treated in adiabatic approximation.
We study for both the low and high values of field strength. Three different
range of nonlinearity is obtained where the dynamics is different. In low and
intermediate range of nonlinearity, it reduces the localization. In fact in the
intermediate range subdiffusive behavior in the perfect nonlinear chain is
obtained for a long time. In all the cases a critical value of nonlinear
strength exists where self-trapping transition takes place. This critical value
depends on the system and the field strength. Beyond the self-trapping
transition nonlinearity enhances the localization.Comment: 9 pages, Revtex, 6 ps figures include
Wave propagation through a coherently amplifying random medium
We report a detailed and systematic numerical study of wave propagation
through a coherently amplifying random one-dimensional medium. The coherent
amplification is modeled by introducing a uniform imaginary part in the site
energies of the disordered single-band tight binding Hamiltonian. Several
distinct length scales (regimes), most of them new, are identified from the
behavior of transmittance and reflectance as a function of the material
parameters. We show that the transmittance is a non-self-averaging quantity
with a well defined mean value. The stationary distribution of the super
reflection differs qualitatively from the analytical results obtained within
the random phase approximation in strong disorder and amplification regime. The
study of the stationary distribution of the phase of the reflected wave reveals
the reason for this discrepancy. The applicability of random phase
approximation is discussed. We emphasize the dual role played by the lasing
medium, as an amplifier as well as a reflector.Comment: 33 pages RevTex, 14 EPS figures included, Accepted for publication in
IJMP-
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