19,233 research outputs found
Phonon number quantum jumps in an optomechanical system
We describe an optomechanical system in which the mean phonon number of a
single mechanical mode conditionally displaces the amplitude of the optical
field. Using homodyne detection of the output field we establish the conditions
under which phonon number quantum jumps can be inferred from the measurement
record: both the cavity damping rate and the measurement rate of the phonon
number must be much greater than the thermalization rate of the mechanical
mode. We present simulations of the conditional dynamics of the measured system
using the stochastic master equation. In the good-measurement limit, the
conditional evolution of the mean phonon number shows quantum jumps as phonons
enter and exit the mechanical resonator via the bath.Comment: 13 pages, 4 figures. minor revisions since first versio
Entropic particle transport: higher order corrections to the Fick-Jacobs diffusion equation
Transport of point-size Brownian particles under the influence of a constant
and uniform force field through a three-dimensional channel with smoothly
varying periodic cross-section is investigated. Here, we employ an asymptotic
analysis in the ratio between the difference of the widest and the most narrow
constriction divided through the period length of the channel geometry. We
demonstrate that the leading order term is equivalent to the Fick-Jacobs
approximation. By use of the higher order corrections to the probability
density we derive an expression for the spatially dependent diffusion
coefficient D(x) which substitutes the constant diffusion coefficient present
in the common Fick-Jacobs equation. In addition, we show that in the diffusion
dominated regime the average transport velocity is obtained as the product of
the zeroth-order Fick-Jacobs result and the expectation value of the spatially
dependent diffusion coefficient . The analytic findings are corroborated
with the precise numerical results of a finite element calculation of the
Smoluchowski diffusive particle dynamics occurring in a reflection symmetric
sinusoidal-shaped channel.Comment: 9 pages, 3 figure
Spin transport in a unitary Fermi gas close to the BCS transition
We consider spin transport in a two-component ultracold Fermi gas with
attractive interspecies interactions close to the BCS pairing transition. In
particular, we consider the spin-transport relaxation rate and the
spin-diffusion constant. Upon approaching the transition, the scattering
amplitude is enhanced by pairing fluctuations. However, as the system
approaches the transition, the spectral weight for excitations close to the
Fermi level is decreased by the formation of a pseudogap. To study the
consequence of these two competing effects, we determine the spin-transport
relaxation rate and the spin-diffusion constant using both a Boltzmann approach
and a diagrammatic approach. The former ignores pseudogap physics and finite
lifetime effects. In the latter, we incorporate the full pseudogap physics and
lifetime effects, but we ignore vertex corrections, so that we effectively
calculate single-particle relaxation rates instead of transport relaxation
rates. We find that there is qualitative agreement between these two approaches
although the results for the transport coefficients differ quantitatively.Comment: 9 pages, 10 figure
Evaluation of SMAP Freeze/Thaw Retrieval Accuracy at Core Validation Sites in the Contiguous United States
Seasonal freeze-thaw (FT) impacts much of the northern hemisphere and is an important control on its water, energy, and carbon cycle. Although FT in natural environments extends south of 45°N, FT studies using the L-band have so far been restricted to boreal or greater latitudes. This study addresses this gap by applying a seasonal threshold algorithm to Soil Moisture Active Passive (SMAP) data (L3_SM_P) to obtain a FT product south of 45°N (‘SMAP FT’), which is then evaluated at SMAP core validation sites (CVS) located in the contiguous United States (CONUS). SMAP landscape FT retrievals are usually in good agreement with 0–5 cm soil temperature at SMAP grids containing CVS stations (\u3e70%). The accuracy could be further improved by taking into account specific overpass time (PM), the grid-specific seasonal scaling factor, the data aggregation method, and the sampling error. Annual SMAP FT extent maps compared to modeled soil temperatures derived from the Goddard Earth Observing System Model Version 5 (GEOS-5) show that seasonal FT in CONUS extends to latitudes of about 35–40°N, and that FT varies substantially in space and by year. In general, spatial and temporal trends between SMAP and modeled FT were similar
Intermediate states at structural phase transition: Model with a one-component order parameter coupled to strains
We study a Ginzburg-Landau model of structural phase transition in two
dimensions, in which a single order parameter is coupled to the tetragonal and
dilational strains. Such elastic coupling terms in the free energy much affect
the phase transition behavior particularly near the tricriticality. A
characteristic feature is appearance of intermediate states, where the ordered
and disordered regions coexist on mesoscopic scales in nearly steady states in
a temperature window. The window width increases with increasing the strength
of the dilational coupling. It arises from freezing of phase ordering in
inhomogeneous strains. No impurity mechanism is involved. We present a simple
theory of the intermediate states to produce phase diagrams consistent with
simulation results.Comment: 16 pages, 14 figure
Probabilistic Search for Object Segmentation and Recognition
The problem of searching for a model-based scene interpretation is analyzed
within a probabilistic framework. Object models are formulated as generative
models for range data of the scene. A new statistical criterion, the truncated
object probability, is introduced to infer an optimal sequence of object
hypotheses to be evaluated for their match to the data. The truncated
probability is partly determined by prior knowledge of the objects and partly
learned from data. Some experiments on sequence quality and object segmentation
and recognition from stereo data are presented. The article recovers classic
concepts from object recognition (grouping, geometric hashing, alignment) from
the probabilistic perspective and adds insight into the optimal ordering of
object hypotheses for evaluation. Moreover, it introduces point-relation
densities, a key component of the truncated probability, as statistical models
of local surface shape.Comment: 18 pages, 5 figure
Superlinear Scaling for Innovation in Cities
Superlinear scaling in cities, which appears in sociological quantities such
as economic productivity and creative output relative to urban population size,
has been observed but not been given a satisfactory theoretical explanation.
Here we provide a network model for the superlinear relationship between
population size and innovation found in cities, with a reasonable range for the
exponent.Comment: 5 pages, 5 figures, 1 table, submitted to Phys. Rev. E; references
corrected; figures corrected, references and brief discussion adde
On the validity of the reduced Salpeter equation
We adapt a general method to solve both the full and reduced Salpeter
equations and systematically explore the conditions under which these two
equations give equivalent results in meson dynamics. The effects of constituent
mass, angular momentum state, type of interaction, and the nature of
confinement are all considered in an effort to clearly delineate the range of
validity of the reduced Salpeter approximations. We find that for
the solutions are strikingly similar for all
constituent masses. For zero angular momentum states the full and reduced
Salpeter equations give different results for small quark mass especially with
a large additive constant coordinate space potential. We also show that
corrections to heavy-light energy levels can be accurately
computed with the reduced equation.Comment: Latex (uses epsf macro), 24 pages of text, 12 postscript figures
included. Slightly revised version, to appear in Phys. Rev.
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