37,365 research outputs found
Precision laboratory UV and IR wavelengths for cosmological and astrophysical applications
The quality of astronomical spectra is now so high that the accuracy of the
laboratory data is getting more and more important for the analysis and
interpretation. Both in astrophysics and cosmology the needs for accurate
laboratory wavelengths have increased with the development of new ground-based
and air-borne telescopes and spectrographs. The high resolution UV Fourier
Transform spectrometer at Lund Observatory is being used for studying
laboratory spectra of astrophysically important elements. Measurements of
accurate laboratory UV and IR wavelengths have been made for cosmological and
astrophysical applications.Comment: To appear in the proceedings of "Precision Spectroscopy in
Astrophysics", Aveiro, Portugal, Sep. 2006, eds Pasquini et al., ESO
Astrophysics Symposia. 2 pages, 2 figure
The dynamical Casimir effect in superconducting microwave circuits
We theoretically investigate the dynamical Casimir effect in electrical
circuits based on superconducting microfabricated waveguides with tunable
boundary conditions. We propose to implement a rapid modulation of the boundary
conditions by tuning the applied magnetic flux through superconducting quantum
interference devices (SQUIDs) that are embedded in the waveguide circuits. We
consider two circuits: (i) An open waveguide circuit that corresponds to a
single mirror in free space, and (ii) a resonator coupled to a microfabricated
waveguide, which corresponds to a single-sided cavity in free space. We analyze
the properties of the dynamical Casimir effect in these two setups by
calculating the generated photon-flux density, output-field correlation
functions, and the quadrature squeezing spectra. We show that these properties
of the output field exhibit signatures unique to the radiation due to the
dynamical Casimir effect, and could therefore be used for distinguishing the
dynamical Casimir effect from other types of radiation in these circuits. We
also discuss the similarities and differences between the dynamical Casimir
effect, in the resonator setup, and downconversion of pump photons in
parametric oscillators.Comment: 18 pages, 14 figure
Accurate laboratory ultraviolet wavelengths for quasar absorption-line constraints on varying fundamental constants
The most precise method of investigating possible space-time variations of
the fine-structure constant, using high-redshift quasar absorption lines, is
the many-multiplet (MM) method. For reliable results this method requires very
accurate relative laboratory wavelengths for a number of UV resonance
transitions from several different ionic species. For this purpose laboratory
wavelengths and wavenumbers of 23 UV lines from MgI, MgII, TiII, CrII, MnII,
FeII and ZnII have been measured using high-resolution Fourier Transform (FT)
spectrometry. The spectra of the different ions (except for one FeII line, one
MgI line and the TiII lines) are all measured simultaneously in the same FT
spectrometry recording by using a composite hollow cathode as a light source.
This decreases the relative uncertainties of all the wavelengths. In addition
to any measurement uncertainty, the wavelength uncertainty is determined by
that of the ArII calibration lines, by possible pressure shifts and by
illumination effects. The absolute wavenumbers have uncertainties of typically
0.001 to 0.002 cm^(-1) (0.06 to 0.1 mAA at 2500 AA), while the relative
wavenumbers for strong, symmetric lines in the same spectral recording have
uncertainties of 0.0005 cm^(-1) (0.03 mAA at 2500 AA) or better, depending
mostly on uncertainties in the line fitting procedure. This high relative
precision greatly reduces the potential for systematic effects in the MM
method, while the new TiII measurements now allow these transitions to be used
in MM analyses.Comment: Accepted for publication in MNRAS, 10 pages, 9 figure
Nonclassical microwave radiation from the dynamical Casimir effect
We investigate quantum correlations in microwave radiation produced by the
dynamical Casimir effect in a superconducting waveguide terminated and
modulated by a superconducting quantum interference device. We apply
nonclassicality tests and evaluate the entanglement for the predicted field
states. For realistic circuit parameters, including thermal background noise,
the results indicate that the produced radiation can be strictly nonclassical
and can have a measurable amount of intermode entanglement. If measured
experimentally, these nonclassicalilty indicators could give further evidence
of the quantum nature of the dynamical Casimir radiation in these circuits.Comment: 5 pages, 3 figure
A population of extreme mid-to-near-infrared sources: obscured AGN and dusty starbursts
We present a sample of mid-infrared detected sources from the European Large
Area ISO Survey (ELAIS) regions characterised by strong mid-IR radiation with
faint near-IR and optical counterparts. These extreme mid-to-near-IR objects
(EMNOs) are defined here by a flux ratio of f_15um / f_2.2um > 25. This
population is not obvious in deeper small area ISO surveys, though it produces
more than 20% of the observed cosmic IR background radiation (CIRB) at 15um
above 1 mJy. Near-future large area deep mid-IR surveys with the Spitzer Space
Telescope, however, are bound to uncover large amounts of these objects, which
we argue to most likely be obscured AGN, based on SED shapes and X-ray data.
Very strong dusty starbursts at z>1 may also have high mid-to-near-IR flux
ratios, but using the MIR/NIR and FIR/MIR ratios these may be separated. Most
of our EMNOs appear to be ULIRGs, half are also extremely red objects (ERO). A
curious case of a low redshift, less luminous object with a very young stellar
population is also found. We predict that the simple broad band selection
method makes EMNOs a useful window into high-redshift obscured nuclear activity
and its sought after relation to star-formation, in a similar way that EROs
have been used to define samples of high-redshift early type galaxies.Comment: 8 pages, 3 figures. A&A accepted version. Results unchanged but
discussion is significantly expande
A Survey on Graph Kernels
Graph kernels have become an established and widely-used technique for
solving classification tasks on graphs. This survey gives a comprehensive
overview of techniques for kernel-based graph classification developed in the
past 15 years. We describe and categorize graph kernels based on properties
inherent to their design, such as the nature of their extracted graph features,
their method of computation and their applicability to problems in practice. In
an extensive experimental evaluation, we study the classification accuracy of a
large suite of graph kernels on established benchmarks as well as new datasets.
We compare the performance of popular kernels with several baseline methods and
study the effect of applying a Gaussian RBF kernel to the metric induced by a
graph kernel. In doing so, we find that simple baselines become competitive
after this transformation on some datasets. Moreover, we study the extent to
which existing graph kernels agree in their predictions (and prediction errors)
and obtain a data-driven categorization of kernels as result. Finally, based on
our experimental results, we derive a practitioner's guide to kernel-based
graph classification
Kalman Filtering with Uncertain Process and Measurement Noise Covariances with Application to State Estimation in Sensor Networks
Distributed state estimation under uncertain process
and measurement noise covariances is considered. An
algorithm based on sensor fusion using Kalman filtering is
investigated. It is shown that if the covariances are decomposed into a known nominal covariance plus an uncertainty term, then the uncertainty of the actual estimation error covariance for the Kalman filter grows linearly with the size of the uncertainty term. This result is extended to the sensor fusion scheme to give an upper bound on the actual error covariance for the fused state estimate. Examples are provided to illustrate how the theory can be applied in practice
Detecting itinerant single microwave photons
Single photon detectors are fundamental tools of investigation in quantum
optics and play a central role in measurement theory and quantum informatics.
Photodetectors based on different technologies exist at optical frequencies and
much effort is currently being spent on pushing their efficiencies to meet the
demands coming from the quantum computing and quantum communication proposals.
In the microwave regime however, a single photon detector has remained elusive
although several theoretical proposals have been put forth. In this article, we
review these recent proposals, especially focusing on non-destructive detectors
of propagating microwave photons. These detection schemes using superconducting
artificial atoms can reach detection efficiencies of 90\% with existing
technologies and are ripe for experimental investigations.Comment: 11 pages, 8 figure
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