11,257 research outputs found
Image Ellipticity from Atmospheric Aberrations
We investigate the ellipticity of the point-spread function (PSF) produced by
imaging an unresolved source with a telescope, subject to the effects of
atmospheric turbulence. It is important to quantify these effects in order to
understand the errors in shape measurements of astronomical objects, such as
those used to study weak gravitational lensing of field galaxies. The PSF
modeling involves either a Fourier transform of the phase information in the
pupil plane or a ray-tracing approach, which has the advantage of requiring
fewer computations than the Fourier transform. Using a standard method,
involving the Gaussian weighted second moments of intensity, we then calculate
the ellipticity of the PSF patterns. We find significant ellipticity for the
instantaneous patterns (up to more than 10%). Longer exposures, which we
approximate by combining multiple (N) images from uncorrelated atmospheric
realizations, yield progressively lower ellipticity (as 1 / sqrt(N)). We also
verify that the measured ellipticity does not depend on the sampling interval
in the pupil plane using the Fourier method. However, we find that the results
using the ray-tracing technique do depend on the pupil sampling interval,
representing a gradual breakdown of the geometric approximation at high spatial
frequencies. Therefore, ray tracing is generally not an accurate method of
modeling PSF ellipticity induced by atmospheric turbulence unless some
additional procedure is implemented to correctly account for the effects of
high spatial frequency aberrations. The Fourier method, however, can be used
directly to accurately model PSF ellipticity, which can give insights into
errors in the statistics of field galaxy shapes used in studies of weak
gravitational lensing.Comment: 9 pages, 5 color figures (some reduced in size). Accepted for
publication in the Astrophysical Journa
Ab initio theory of Fano resonances in plasmonic nanostructures and metamaterials
An ab initio theory for Fano resonances in plasmonic nanostructures and
metamaterials is developed using Feshbach formalism. It reveals the role played
by the electromagnetic modes and material losses in the system, and enables the
engineering of Fano resonances in arbitrary geometries. A general formula for
the asymmetric resonance in a non-conservative system is derived. The influence
of the electromagnetic interactions on the resonance line shape is discussed
and it is shown that intrinsic losses drive the resonance contrast, while its
width is mostly determined by the coupling strength between the non-radiative
mode and the continuum. The analytical model is in perfect agreement with
numerical simulations.Comment: 13 pages, 5 figure
First experimental demonstration of temporal hypertelescope operation with a laboratory prototype
In this paper, we report the first experimental demonstration of a Temporal
HyperTelescope (THT). Our breadboard including 8 telescopes is firstly tested
in a manual cophasing configuration on a 1D object. The Point Spread Function
(PSF) is measured and exhibits a dynamics in the range of 300. A quantitative
analysis of the potential biases demonstrates that this limitation is related
to the residual phase fluctuation on each interferometric arm. Secondly, an
unbalanced binary star is imaged demonstrating the imaging capability of THT.
In addition, 2D PSF is recorded even if the telescope array is not optimized
for this purpose.Comment: Accepted for publication in MNRAS. 11 pages, 25 figure
Photonic crystal fibre source of photon pairs for quantum information processing
We demonstrate two key components for optical quantum information processing:
a bright source of heralded single photons; and a bright source of entangled
photon pairs. A pair of pump photons produces a correlated pair of photons at
widely spaced wavelengths (583 nm and 900 nm), via a four-wave
mixing process. We demonstrate a non-classical interference between heralded
photons from independent sources with a visibility of 95%, and an entangled
photon pair source, with a fidelity of 89% with a Bell state.Comment: 4 pages, 3 figure
Factorization of finite temperature graphs in thermal QED
We extend our previous analysis of gauge and Dirac fields in the presence of
a chemical potential. We consider an alternate thermal operator which relates
in a simple way the Feynman graphs in QED at finite temperature and charge
density to those at zero temperature but non-zero chemical potential. Several
interesting features of such a factorization are discussed in the context of
the thermal photon and fermion self-energies.Comment: 4 page
Thermal Operator Representation of Finite Temperature Graphs
Using the mixed space representation (t,p) in the context of scalar field
theories, we prove in a simple manner that the Feynman graphs at finite
temperature are related to the corresponding zero temperature diagrams through
a simple thermal operator, both in the imaginary time as well as in the real
time formalisms. This result is generalized to the case when there is a
nontrivial chemical potential present. Several interesting properties of the
thermal operator are also discussed.Comment: 20 pages, seven figure
Pair Contact Process with Diffusion: Failure of Master Equation Field Theory
We demonstrate that the `microscopic' field theory representation, directly
derived from the corresponding master equation, fails to adequately capture the
continuous nonequilibrium phase transition of the Pair Contact Process with
Diffusion (PCPD). The ensuing renormalization group (RG) flow equations do not
allow for a stable fixed point in the parameter region that is accessible by
the physical initial conditions. There exists a stable RG fixed point outside
this regime, but the resulting scaling exponents, in conjunction with the
predicted particle anticorrelations at the critical point, would be in
contradiction with the positivity of the equal-time mean-square particle number
fluctuations. We conclude that a more coarse-grained effective field theory
approach is required to elucidate the critical properties of the PCPD.Comment: revtex, 8 pages, 1 figure include
The Statistics of the Number of Minima in a Random Energy Landscape
We consider random energy landscapes constructed from d-dimensional lattices
or trees. The distribution of the number of local minima in such landscapes
follows a large deviation principle and we derive the associated law exactly
for dimension 1. Also of interest is the probability of the maximum possible
number of minima; this probability scales exponentially with the number of
sites. We calculate analytically the corresponding exponent for the Cayley tree
and the two-leg ladder; for 2 to 5 dimensional hypercubic lattices, we compute
the exponent numerically and compare to the Cayley tree case.Comment: 18 pages, 8 figures, added background on landscapes and reference
The thermal operator representation for Matsubara sums
We prove in full generality the thermal operator representation for Matsubara
sums in a relativistic field theory of scalar and fermionic particles. It
states that the full result of performing the Matsubara sum associated to any
given Feynman graph, in the imaginary-time formalism of finite-temperature
field theory, can be directly obtained from its corresponding zero-temperature
energy integral, by means of a simple linear operator, which is independent of
the external Euclidean energies and whose form depends solely on the topology
of the graph.Comment: 9 pages, 1 figure, RevTe
Turbulence lifetimes: what we can learn from the physics of glasses
In this note, we critically discuss the issue of the possible finiteness of
the turbulence lifetime in subcritical transition to turbulence in shear flows,
which attracted a lot of interest recently. We briefly review recent
experimental and numerical results, as well as theoretical proposals, and
compare the difficulties arising in assessing this issue in subcritical shear
flow with that encountered in the study of the glass transition. In order to go
beyond the purely methodological similarities, we further elaborate on this
analogy and propose a qualitative mapping between these two apparently
unrelated situations, which could possibly foster new directions of research in
subcritical shear flows.Comment: 10 pages, 4 figure
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