661 research outputs found
Speech Communication
Contains reports on four research projects.National Science FoundationUnited States Air Force, Air Force Cambridge Research Center, Air Research and Development Command (Contract AF19(604)-2061
Quantum Langevin equations for semiconductor light-emitting devices and the photon statistics at a low-injection level
From the microscopic quantum Langevin equations (QLEs) we derive the
effective semiconductor QLEs and the associated noise correlations which are
valid at a low-injection level and in real devices. Applying the semiconductor
QLEs to semiconductor light-emitting devices (LEDs), we obtain a new formula
for the Fano factor of photons which gives the photon-number statistics as a
function of the pump statistics and several parameters of LEDs. Key ingredients
are non-radiative processes, carrier-number dependence of the radiative and
non-radiative lifetimes, and multimodeness of LEDs. The formula is applicable
to the actual cases where the quantum efficiency differs from the
differential quantum efficiency , whereas previous theories
implicitly assumed . It is also applicable to the cases when
photons in each mode of the cavity are emitted and/or detected inhomogeneously.
When at a running point, in particular, our formula predicts
that even a Poissonian pump can produce sub-Poissonian light. This mechanism
for generation of sub-Poissonian light is completely different from those of
previous theories, which assumed sub-Poissonian statistics for the current
injected into the active layers of LEDs. Our results agree with recent
experiments. We also discuss frequency dependence of the photon statistics.Comment: 10 pages, 8 figure
Numerical study of O(a) improved Wilson quark action on anisotropic lattice
The improved Wilson quark action on the anisotropic lattice is
investigated. We carry out numerical simulations in the quenched approximation
at three values of lattice spacing (--2 GeV) with the
anisotropy , where and are
the spatial and the temporal lattice spacings, respectively. The bare
anisotropy in the quark field action is numerically tuned by the
dispersion relation of mesons so that the renormalized fermionic anisotropy
coincides with that of gauge field. This calibration of bare anisotropy is
performed to the level of 1 % statistical accuracy in the quark mass region
below the charm quark mass. The systematic uncertainty in the calibration is
estimated by comparing the results from different types of dispersion
relations, which results in 3 % on our coarsest lattice and tends to vanish in
the continuum limit. In the chiral limit, there is an additional systematic
uncertainty of 1 % from the chiral extrapolation.
Taking the central value from the result of the
calibration, we compute the light hadron spectrum. Our hadron spectrum is
consistent with the result by UKQCD Collaboration on the isotropic lattice. We
also study the response of the hadron spectrum to the change of anisotropic
parameter, . We find that the change
of by 2 % induces a change of 1 % in the spectrum for physical quark
masses. Thus the systematic uncertainty on the anisotropic lattice, as well as
the statistical one, is under control.Comment: 27 pages, 25 eps figures, LaTe
Non-perturbative determination of anisotropy coefficients in lattice gauge theories
We propose a new non-perturbative method to compute derivatives of gauge
coupling constants with respect to anisotropic lattice spacings (anisotropy
coefficients), which are required in an evaluation of thermodynamic quantities
from numerical simulations on the lattice. Our method is based on a precise
measurement of the finite temperature deconfining transition curve in the
lattice coupling parameter space extended to anisotropic lattices by applying
the spectral density method. We test the method for the cases of SU(2) and
SU(3) gauge theories at the deconfining transition point on lattices with the
lattice size in the time direction -- 6. In both cases, there is a
clear discrepancy between our results and perturbative values. A longstanding
problem, when one uses the perturbative anisotropy coefficients, is a
non-vanishing pressure gap at the deconfining transition point in the SU(3)
gauge theory. Using our non-perturbative anisotropy coefficients, we find that
this problem is completely resolved: we obtain and
on and 6 lattices, respectively.Comment: 24pages,7figures,5table
Thermalization After Inflation and Reheating Temperature
We present a detailed examination of thermalization after inflation for
perturbative inflaton decay. Different interactions among particles in the
plasma of inflaton decay products are considered and it will be shown that 2 ->
2 scatterings and particle decay are the important ones. We show that
thermalization occurs after decays dominate scatterings, and that depending on
the typical mass scale of inflaton decay products, different situations may
arise. In particular, thermalization may be delayed until late times, in which
case the bounds from thermal gravitino production on supersymmetric models of
inflation are considerably relaxed. We will also consider the case where the
observable sector consists only of the MSSM matter content, and point out that
flat directions with large vevs may result in earlier thermalization of the
plasma and push the reheat temperature towards its upper limit.Comment: 18 pages, LaTeX, a few references added and revise
Optimal waveform estimation for classical and quantum systems via time-symmetric smoothing
Classical and quantum theories of time-symmetric smoothing, which can be used
to optimally estimate waveforms in classical and quantum systems, are derived
using a discrete-time approach, and the similarities between the two theories
are emphasized. Application of the quantum theory to homodyne phase-locked loop
design for phase estimation with narrowband squeezed optical beams is studied.
The relation between the proposed theory and Aharonov et al.'s weak value
theory is also explored.Comment: 13 pages, 5 figures, v2: changed the title to a more descriptive one,
corrected a minor mistake in Sec. IV, accepted by Physical Review
Entanglement production in Quantized Chaotic Systems
Quantum chaos is a subject whose major goal is to identify and to investigate
different quantum signatures of classical chaos. Here we study entanglement
production in coupled chaotic systems as a possible quantum indicator of
classical chaos. We use coupled kicked tops as a model for our extensive
numerical studies. We find that, in general, presence of chaos in the system
produces more entanglement. However, coupling strength between two subsystems
is also very important parameter for the entanglement production. Here we show
how chaos can lead to large entanglement which is universal and describable by
random matrix theory (RMT). We also explain entanglement production in coupled
strongly chaotic systems by deriving a formula based on RMT. This formula is
valid for arbitrary coupling strengths, as well as for sufficiently long time.
Here we investigate also the effect of chaos on the entanglement production for
the mixed initial state. We find that many properties of the mixed state
entanglement production are qualitatively similar to the pure state
entanglement production. We however still lack an analytical understanding of
the mixed state entanglement production in chaotic systems.Comment: 16 pages, 5 figures. To appear in Pramana:Journal of Physic
The Erd\H{o}s-Ko-Rado theorem for twisted Grassmann graphs
We present a "modern" approach to the Erd\H{o}s-Ko-Rado theorem for
Q-polynomial distance-regular graphs and apply it to the twisted Grassmann
graphs discovered in 2005 by van Dam and Koolen.Comment: 5 page
Preheating, Supersymmetry Breaking and Baryogenesis
Fluctuations of scalar fields produced at the stage of preheating after
inflation are so large that they can break supersymmetry much stronger than
inflation itself. These fluctuations may lead to symmetry restoration along
flat directions of the effective potential even in the theories where the usual
high temperature corrections are exponentially suppressed. Our results show
that nonthermal phase transitions after preheating may play a crucial role in
the generation of the primordial baryon asymmetry by the Affleck-Dine
mechanism. In particular, the baryon asymmetry may be generated at the very
early stage of the evolution of the Universe, at the preheating era, and not
when the Hubble parameter becomes of order the gravitino mass.Comment: 4 pages, no figure
Parametric Resonance in an Expanding Universe
Parametric resonance has been discussed as a mechanism for copious particle
production following inflation. Here we present a simple and intuitive
calculational method for estimating the efficiency of parametric amplification
as a function of parameters. This is important for determining whether resonant
amplification plays an important role in the reheating process. We find that
significant amplification occurs only for a limited range of couplings and
interactions.Comment: 18 pages, Latex, 4 figure
- …