235 research outputs found
Quantum erasure within the Optical Stern-Gerlach Model
In the optical Stern-Gerlach effect the two branches in which the incoming
atomic packet splits up can display interference pattern outside the cavity
when a field measurement is made which erases the which-way information on the
quantum paths the system can follow. On the contrary, the mere possibility to
acquire this information causes a decoherence effect which cancels out the
interference pattern. A phase space analysis is also carried out to investigate
on the negativity of the Wigner function and on the connection between its
covariance matrix and the distinguishability of the quantum paths.Comment: 7 pages, 3 figure
Non-perturbative results for the spectrum of surface-disordered waveguides
We calculated the spectrum of normal scalar waves in a planar waveguide with
absolutely soft randomly rough boundaries beyond the perturbation theories in
the roughness heights and slopes, basing on the exact boundary scattering
potential. The spectrum is proved to be a nearly real non-analytic function of
the dispersion of the roughness heights (with square-root
singularity) as . The opposite case of large boundary defects is
summarized.Comment: REVTEX 3, OSA style, 9 pages, no figures. Submitted to Optics Letter
Effective Dielectric Tensor for Electromagnetic Wave Propagation in Random Media
We derive exact strong-contrast expansions for the effective dielectric
tensor \epeff of electromagnetic waves propagating in a two-phase composite
random medium with isotropic components explicitly in terms of certain
integrals over the -point correlation functions of the medium. Our focus is
the long-wavelength regime, i.e., when the wavelength is much larger than the
scale of inhomogeneities in the medium. Lower-order truncations of these
expansions lead to approximations for the effective dielectric constant that
depend upon whether the medium is below or above the percolation threshold. In
particular, we apply two- and three-point approximations for \epeff to a
variety of different three-dimensional model microstructures, including
dispersions of hard spheres, hard oriented spheroids and fully penetrable
spheres as well as Debye random media, the random checkerboard, and
power-law-correlated materials. We demonstrate the importance of employing
-point correlation functions of order higher than two for high
dielectric-phase-contrast ratio. We show that disorder in the microstructure
results in an imaginary component of the effective dielectric tensor that is
directly related to the {\it coarseness} of the composite, i.e., local
volume-fraction fluctuations for infinitely large windows. The source of this
imaginary component is the attenuation of the coherent homogenized wave due to
scattering. We also remark on whether there is such attenuation in the case of
a two-phase medium with a quasiperiodic structure.Comment: 40 pages, 13 figure
Exact positivity of the Wigner and P-functions of a Markovian open system
We discuss the case of a Markovian master equation for an open system, as it
is frequently found from environmental decoherence. We prove two theorems for
the evolution of the quantum state. The first one states that for a generic
initial state the corresponding Wigner function becomes strictly positive after
a finite time has elapsed. The second one states that also the P-function
becomes exactly positive after a decoherence time of the same order. Therefore
the density matrix becomes exactly decomposable into a mixture of Gaussian
pointer states.Comment: 11 pages, references added, typo corrected, to appear in J. Phys.
Hidden Breit-Wigner distribution and other properties of random matrices with preferential basis
We study statistical properties of a class of band random matrices which
naturally appears in systems of interacting particles. The local spectral
density is shown to follow the Breit-Wigner distribution in both localized and
delocalized regimes with width independent on the band/system size. We analyse
the implications of this distribution to the inverse participation ratio, level
spacing statistics and the problem of two interacting particles in a random
potential.Comment: 4 pages, 4 postscript figures appended, new version with minor
change
Quantum dynamics in canonical and micro-canonical ensembles. Part I. Anderson localization of electrons
The new numerical approach for consideration of quantum dynamics and
calculations of the average values of quantum operators and time correlation
functions in the Wigner representation of quantum statistical mechanics has
been developed. The time correlation functions have been presented in the form
of the integral of the Weyl's symbol of considered operators and the Fourier
transform of the product of matrix elements of the dynamic propagators. For the
last function the integral Wigner- Liouville's type equation has been derived.
The numerical procedure for solving this equation combining both molecular
dynamics and Monte Carlo methods has been developed. For electrons in
disordered systems of scatterers the numerical results have been obtained for
series of the average values of the quantum operators including position and
momentum dispersions, average energy, energy distribution function as well as
for the frequency dependencies of tensor of electron conductivity and
permittivity according to quantum Kubo formula. Zero or very small value of
static conductivity have been considered as the manifestation of Anderson
localization of electrons in 1D case. Independent evidence of Anderson
localization comes from the behaviour of the calculated time dependence of
position dispersion.Comment: 8 pages, 10 figure
Accurate seeing measurements with MASS and DIMM
Astronomical seeing is quantified by a single parameter, turbulence integral,
in the framework of the Kolmogorov turbulence model. This parameter can be
routinely measured by a Differential Image Motion Monitor, DIMM. A new
instrument, Multi-Aperture Scintillation Sensor (MASS), permits to measure the
seeing in the free atmosphere above ~0.5km and, together with a DIMM, to
estimate the ground-layer seeing. The absolute accuracy of both methods is
studied here using analytical theory, numerical simulation, and experiments. A
modification of the MASS data processing to compensate for partially saturated
scintillation is developed. We find that the DIMM can be severely biased by
optical aberrations (e.g. defocus) and propagation. Seeing measurements with
DIMM and MASS can reach absolute accuracy of ~10% when their biases are
carefully controlled. Pushing this limit to 1% appears unrealistic because the
seeing itself is just a model-dependent parameter of a non-stationary random
process.Comment: 13 pages, 14 figures. Accepted for publication in MNRA
Asymmetry Function of Interstellar Scintillations of Pulsars
A new method for separating intensity variations of a source's radio emission
having various physical natures is proposed. The method is based on a joint
analysis of the structure function of the intensity variations and the
asymmetry function, which is a generalization of the asymmetry coefficient and
characterizes the asymmetry of the distribution function of the intensity
fluctuations on various scales for the inhomogeneities in the diffractive
scintillation pattern. Relationships for the asymmetry function in the cases of
a logarithmic normal distribution of the intensity fluctuations and a normal
distribution of the field fluctuations are derived. Theoretical relationships
and observational data on interstellar scintillations of pulsars (refractive,
diffractive, and weak scintillations) are compared. Pulsar scintillations match
the behavior expected for a normal distribution of the field fluctuations
(diffractive scintillation) or logarithmic normal distribution of the intensity
fluctuations (refractive and weak scintillation). Analysis of the asymmetry
function is a good test for distinguishing scintillations against the
background of variations that have different origins
Photon Distribution Function for Long-Distance Propagation of Partially Coherent Beams through the Turbulent Atmosphere
The photon density operator function is used to calculate light beam
propagation through turbulent atmosphere. A kinetic equation for the photon
distribution function is derived and solved using the method of
characteristics. Optical wave correlations are described in terms of photon
trajectories that depend on fluctuations of the refractive index. It is shown
that both linear and quadratic disturbances produce sizable effects for
long-distance propagation. The quadratic terms are shown to suppress the
correlation of waves with different wave vectors. We examine the intensity
fluctuations of partially coherent beams (beams whose initial spatial coherence
is partially destroyed). Our calculations show that it is possible to
significantly reduce the intensity fluctuations by using a partially coherent
beam. The physical mechanism responsible for this pronounced reduction is
similar to that of the Hanbury-Braun, Twiss effect.Comment: 28 pages, 4 figure
Scintillation Reduction for Laser Beams Propagating Through Turbulent Atmosphere
We numerically examine the spatial evolution of the structure of coherent and
partially coherent laser beams, including the optical vortices, propagating in
turbulent atmospheres. The influence of beam fragmentation and wandering
relative to the axis of propagation (z-axis) on the value of the scintillation
index (SI) of the signal at the detector is analyzed. These studies were
performed for different dimensions of the detector, distances of propagation,
and strengths of the atmospheric turbulence. Methods for significantly reducing
the scintillation index are described. These methods utilize averaging of the
signal at the detector over a set of partially coherent beams (PCBs). It is
demonstrated that the most effective approach is using a set of PCBs with
definite initial directions of propagation relative to the z-axis. This
approach results in a significant compensation of the beam wandering which in
many cases is the main contributor to the SI. A novel method is to generate the
PCBs by combining two laser beams - Gaussian and vortex beams, with different
frequencies (the difference between these two frequencies being significantly
smaller than the frequencies themselves). In this case, the effective
suppression of the SI does not require high-frequency modulators. This result
is important for achieving gigabit data-rates in long-distance laser
communication through turbulent atmospheres.Comment: 35 pages, 29 figure
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