4,462 research outputs found
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
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
Hybrid phase-space simulation method for interacting Bose fields
We introduce an approximate phase-space technique to simulate the quantum
dynamics of interacting bosons. With the future goal of treating Bose-Einstein
condensate systems, the method is designed for systems with a natural
separation into highly occupied (condensed) modes and lightly occupied modes.
The method self-consistently uses the Wigner representation to treat highly
occupied modes and the positive-P representation for lightly occupied modes. In
this method, truncation of higher-derivative terms from the Fokker-Planck
equation is usually necessary. However, at least in the cases investigated
here, the resulting systematic error, over a finite time, vanishes in the limit
of large Wigner occupation numbers. We tested the method on a system of two
interacting anharmonic oscillators, with high and low occupations,
respectively. The Hybrid method successfully predicted atomic quadratures to a
useful simulation time 60 times longer than that of the positive-P method. The
truncated Wigner method also performed well in this test. For the prediction of
the correlation in a quantum nondemolition measurement scheme, for this same
system, the Hybrid method gave excellent agreement with the exact result, while
the truncated Wigner method showed a large systematic error.Comment: 13 pages; 6 figures; references added; figures correcte
Quantum squeezing of optical dissipative structures
We show that any optical dissipative structure supported by degenerate
optical parametric oscillators contains a special transverse mode that is free
from quantum fluctuations when measured in a balanced homodyne detection
experiment. The phenomenon is not critical as it is independent of the system
parameters and, in particular, of the existence of bifurcations. This result is
a consequence of the spatial symmetry breaking introduced by the dissipative
structure. Effects that could degrade the squeezing level are considered.Comment: 4 pages and a half, 1 fugure. Version to appear in Europhysics
Letter
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
Bell inequalities for Continuous-Variable Measurements
Tests of local hidden variable theories using measurements with continuous
variable (CV) outcomes are developed, and a comparison of different methods is
presented. As examples, we focus on multipartite entangled GHZ and cluster
states. We suggest a physical process that produces the states proposed here,
and investigate experiments both with and without binning of the continuous
variable. In the former case, the Mermin-Klyshko inequalities can be used
directly. For unbinned outcomes, the moment-based CFRD inequalities are
extended to functional inequalities by considering arbitrary functions of the
measurements at each site. By optimising these functions, we obtain more robust
violations of local hidden variable theories than with either binning or
moments. Recent inequalities based on the algebra of quaternions and octonions
are compared with these methods. Since the prime advantage of CV experiments is
to provide a route to highly efficient detection via homodyne measurements, we
analyse the effect of noise and detection losses in both binned and unbinned
cases. The CV moment inequalities with an optimal function have greater
robustness to both loss and noise. This could permit a loophole-free test of
Bell inequalities.Comment: 17 pages, 6 figure
Assessment of practicality of remote sensing techniques for a study of the effects of strip mining in Alabama
Because of the volume of coal produced by strip mining, the proximity of mining operations, and the diversity of mining methods (e.g. contour stripping, area stripping, multiple seam stripping, and augering, as well as underground mining), the Warrior Coal Basin seemed best suited for initial studies on the physical impact of strip mining in Alabama. Two test sites, (Cordova and Searles) representative of the various strip mining techniques and environmental problems, were chosen for intensive studies of the correlation between remote sensing and ground truth data. Efforts were eventually concentrated in the Searles Area, since it is more accessible and offers a better opportunity for study of erosional and depositional processes than the Cordova Area
Evaluation of the simulated solar spectrum in the jpl 25-ft space simulator
Simulation of solar spectrum in space simulato
Continuum Derrida Approach to Drift and Diffusivity in Random Media
By means of rather general arguments, based on an approach due to Derrida
that makes use of samples of finite size, we analyse the effective diffusivity
and drift tensors in certain types of random medium in which the motion of the
particles is controlled by molecular diffusion and a local flow field with
known statistical properties. The power of the Derrida method is that it uses
the equilibrium probability distribution, that exists for each {\em finite}
sample, to compute asymptotic behaviour at large times in the {\em infinite}
medium. In certain cases, where this equilibrium situation is associated with a
vanishing microcurrent, our results demonstrate the equality of the
renormalization processes for the effective drift and diffusivity tensors. This
establishes, for those cases, a Ward identity previously verified only to
two-loop order in perturbation theory in certain models. The technique can be
applied also to media in which the diffusivity exhibits spatial fluctuations.
We derive a simple relationship between the effective diffusivity in this case
and that for an associated gradient drift problem that provides an interesting
constraint on previously conjectured results.Comment: 18 pages, Latex, DAMTP-96-8
Quantum dynamics in ultra-cold atomic physics
We review recent developments in the theory of quantum dynamics in ultra-cold
atomic physics, including exact techniques, but focusing on methods based on
phase-space mappings that are appli- cable when the complexity becomes
exponentially large. These phase-space representations include the truncated
Wigner, positive-P and general Gaussian operator representations which can
treat both bosons and fermions. These phase-space methods include both
traditional approaches using a phase-space of classical dimension, and more
recent methods that use a non-classical phase-space of increased
dimensionality. Examples used include quantum EPR entanglement of a four-mode
BEC, time-reversal tests of dephasing in single-mode traps, BEC quantum
collisions with up to 106 modes and 105 interacting particles, quantum
interferometry in a multi-mode trap with nonlinear absorp- tion, and the theory
of quantum entropy in phase-space. We also treat the approach of variational
optimization of the sampling error, giving an elementary example of a nonlinear
oscillator
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