5,769 research outputs found
Disagreement between correlations of quantum mechanics and stochastic electrodynamics in the damped parametric oscillator
Intracavity and external third order correlations in the damped nondegenerate
parametric oscillator are calculated for quantum mechanics and stochastic
electrodynamics (SED), a semiclassical theory. The two theories yield greatly
different results, with the correlations of quantum mechanics being cubic in
the system's nonlinear coupling constant and those of SED being linear in the
same constant. In particular, differences between the two theories are present
in at least a mesoscopic regime. They also exist when realistic damping is
included. Such differences illustrate distinctions between quantum mechanics
and a hidden variable theory for continuous variables.Comment: accepted by PR
First-principles quantum dynamics in interacting Bose gases I: The positive P representation
The performance of the positive P phase-space representation for exact
many-body quantum dynamics is investigated. Gases of interacting bosons are
considered, where the full quantum equations to simulate are of a
Gross-Pitaevskii form with added Gaussian noise. This method gives tractable
simulations of many-body systems because the number of variables scales
linearly with the spatial lattice size. An expression for the useful simulation
time is obtained, and checked in numerical simulations. The dynamics of first-,
second- and third-order spatial correlations are calculated for a uniform
interacting 1D Bose gas subjected to a change in scattering length. Propagation
of correlations is seen. A comparison is made to other recent methods. The
positive P method is particularly well suited to open systems as no
conservation laws are hard-wired into the calculation. It also differs from
most other recent approaches in that there is no truncation of any kind.Comment: 21 pages, 7 figures, 2 tables, IOP styl
A variance-minimization scheme for optimizing Jastrow factors
We describe a new scheme for optimizing many-electron trial wave functions by
minimizing the unreweighted variance of the energy using stochastic integration
and correlated-sampling techniques. The scheme is restricted to parameters that
are linear in the exponent of a Jastrow correlation factor, which are the most
important parameters in the wave functions we use. The scheme is highly
efficient and allows us to investigate the parameter space more closely than
has been possible before. We search for multiple minima of the variance in the
parameter space and compare the wave functions obtained using reweighted and
unreweighted variance minimization.Comment: 19 pages; 12 figure
Gaussian phase-space representations for fermions
We introduce a positive phase-space representation for fermions, using the
most general possible multi-mode Gaussian operator basis. The representation
generalizes previous bosonic quantum phase-space methods to Fermi systems. We
derive equivalences between quantum and stochastic moments, as well as operator
correspondences that map quantum operator evolution onto stochastic processes
in phase space. The representation thus enables first-principles quantum
dynamical or equilibrium calculations in many-body Fermi systems. Potential
applications are to strongly interacting and correlated Fermi gases, including
coherent behaviour in open systems and nanostructures described by master
equations. Examples of an ideal gas and the Hubbard model are given, as well as
a generic open system, in order to illustrate these ideas.Comment: More references and examples. Much less mathematical materia
Supersonic combustor modeling
The physical phenomena involved when a supersonic flow undergoes chemical reaction are discussed. Detailed physical models of convective and diffusive mixing, and finite rate chemical reaction in supersonic flow are presented. Numerical algorithms used to solve the equations governing these processes are introduced. Computer programs using these algorithms are used to analyze the structure of the reacting mixing layer. It is concluded that, as in subsonic flow, exothermic heat release in unconfined supersonic flows retards fuel/air mixing. Non mixing is shown to be a potential problem in reducing the efficiency of supersonic as well as subsonic combustion. Techniques for enhancing fuel/air mixing and combustion are described
Electron Emission from Diamondoids: A Diffusion Quantum Monte Carlo Study
We present density-functional theory (DFT) and quantum Monte Carlo (QMC)
calculations designed to resolve experimental and theoretical controversies
over the optical properties of H-terminated C nanoparticles (diamondoids). The
QMC results follow the trends of well-converged plane-wave DFT calculations for
the size dependence of the optical gap, but they predict gaps that are 1-2 eV
higher. They confirm that quantum confinement effects disappear in diamondoids
larger than 1 nm, which have gaps below that of bulk diamond. Our QMC
calculations predict a small exciton binding energy and a negative electron
affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized
nature of the lowest unoccupied molecular orbital. The NEA suggests a range of
possible applications of diamondoids as low-voltage electron emitters
The time-reversal test for stochastic quantum dynamics
The calculation of quantum dynamics is currently a central issue in
theoretical physics, with diverse applications ranging from ultra-cold atomic
Bose-Einstein condensates (BEC) to condensed matter, biology, and even
astrophysics. Here we demonstrate a conceptually simple method of determining
the regime of validity of stochastic simulations of unitary quantum dynamics by
employing a time-reversal test. We apply this test to a simulation of the
evolution of a quantum anharmonic oscillator with up to
(Avogadro's number) of particles. This system is realisable as a Bose-Einstein
condensate in an optical lattice, for which the time-reversal procedure could
be implemented experimentally.Comment: revtex4, two figures, four page
Quantum many-body simulations using Gaussian phase-space representations
Phase-space representations are of increasing importance as a viable and
successful means to study exponentially complex quantum many-body systems from
first principles. This review traces the background of these methods, starting
from the early work of Wigner, Glauber and Sudarshan. We focus on modern
phase-space approaches using non-classical phase-space representations. These
lead to the Gaussian representation, which unifies bosonic and fermionic
phase-space. Examples treated include quantum solitons in optical fibers,
colliding Bose-Einstein condensates, and strongly correlated fermions on
lattices.Comment: Short Review (10 pages); Corrected typo in eq (14); Added a few more
reference
Crystallization and characterization of Y2O3-SiO2 glasses
Glasses in the yttria-silica system with 20 to 40 mol pct Y2O3 were subjected to recrystallization studies after melting at 1900 to 2100 C in W crucibles in 1 and 50 atm N2. The TEM and XRD results obtained indicate the presence of the delta, gamma, gamma prime, and beta-Y2Si2O7 crystalline phases, depending on melting and quenching conditions. Heat treatment in air at 1100 to 1600 C increased the amount of crystallization, and led to the formation of Y2SiO5, cristabalite, and polymorphs of Y2Si2O7. Also investigated were the effects of 5 and 10 wt pct zirconia additions
The triaxial ellipsoid dimensions, rotational pole, and bulk density of ESA Rosetta target asteroid (21) Lutetia
We seek the best size estimates of the asteroid (21) Lutetia, the direction
of its spin axis, and its bulk density, assuming its shape is well described by
a smooth featureless triaxial ellipsoid, and to evaluate the deviations from
this assumption. Methods. We derive these quantities from the outlines of the
asteroid in 307 images of its resolved apparent disk obtained with adaptive
optics (AO) at Keck II and VLT, and combine these with recent mass
determinations to estimate a bulk density. Our best triaxial ellipsoid
diameters for Lutetia, based on our AO images alone, are a x b x c = 132 x 101
x 93 km, with uncertainties of 4 x 3 x 13 km including estimated systematics,
with a rotational pole within 5 deg. of ECJ2000 [long,lat] = [45, -7], or
EQJ2000 [RA, DEC] = [44, +9]. The AO model fit itself has internal precisions
of 1 x 1 x 8 km, but it is evident, both from this model derived from limited
viewing aspects and the radius vector model given in a companion paper, that
Lutetia has significant departures from an idealized ellipsoid. In particular,
the long axis may be overestimated from the AO images alone by about 10 km.
Therefore, we combine the best aspects of the radius vector and ellipsoid model
into a hybrid ellipsoid model, as our final result, of 124 +/- 5 x 101 +/- 4 x
93 +/- 13 km that can be used to estimate volumes, sizes, and projected areas.
The adopted pole position is within 5 deg. of [long, lat] = [52, -6] or[RA DEC]
= [52, +12]. Using two separately determined masses and the volume of our
hybrid model, we estimate a density of 3.5 +/- 1.1 or 4.3 +/- 0.8 g cm-3 . From
the density evidence alone, we argue that this favors an enstatite-chondrite
composition, although other compositions are formally allowed at the extremes
(low-porosity CV/CO carbonaceous chondrite or high-porosity metallic). We
discuss this in the context of other evidence.Comment: 9 pages, 8 figures, 5 tables, submitted to Astronomy and Astrophysic
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