2,235 research outputs found
On the occurrence times of componentwise maxima and bias in likelihood inference for multivariate max-stable distributions
Full likelihood-based inference for high-dimensional multivariate extreme
value distributions, or max-stable processes, is feasible when incorporating
occurrence times of the maxima; without this information, -dimensional
likelihood inference is usually precluded due to the large number of terms in
the likelihood. However, some studies have noted bias when performing
high-dimensional inference that incorporates such event information,
particularly when dependence is weak. We elucidate this phenomenon, showing
that for unbiased inference in moderate dimensions, dimension should be of
a magnitude smaller than the square root of the number of vectors over which
one takes the componentwise maximum. A bias reduction technique is suggested
and illustrated on the extreme value logistic model.Comment: 7 page
Accounting for choice of measurement scale in extreme value modeling
We investigate the effect that the choice of measurement scale has upon
inference and extrapolation in extreme value analysis. Separate analyses of
variables from a single process on scales which are linked by a nonlinear
transformation may lead to discrepant conclusions concerning the tail behavior
of the process. We propose the use of a Box--Cox power transformation
incorporated as part of the inference procedure to account parametrically for
the uncertainty surrounding the scale of extrapolation. This has the additional
feature of increasing the rate of convergence of the distribution tails to an
extreme value form in certain cases and thus reducing bias in the model
estimation. Inference without reparameterization is practicably infeasible, so
we explore a reparameterization which exploits the asymptotic theory of
normalizing constants required for nondegenerate limit distributions. Inference
is carried out in a Bayesian setting, an advantage of this being the
availability of posterior predictive return levels. The methodology is
illustrated on both simulated data and significant wave height data from the
North Sea.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS333 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
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
Photon pair generation using four-wave mixing in a microstructured fibre: theory versus experiment
We develop a theoretical analysis of four-wave mixing used to generate photon
pairs useful for quantum information processing. The analysis applies to a
single mode microstructured fibre pumped by an ultra-short coherent pulse in
the normal dispersion region. Given the values of the optical propagation
constant inside the fibre, we can estimate the created number of photon pairs
per pulse, their central wavelength and their respective bandwidth. We use the
experimental results from a picosecond source of correlated photon pairs using
a micro-structured fibre to validate the model. The fibre is pumped in the
normal dispersion regime at 708nm and phase matching is satisfied for widely
spaced parametric wavelengths of 586nm and 894nm. We measure the number of
photons per pulse using a loss-independent coincidence scheme and compare the
results with the theoretical expectation. We show a good agreement between the
theoretical expectations and the experimental results for various fibre lengths
and pump powers.Comment: 23 pages, 9 figure
Study to establish cost projections for production of Redox chemicals
A cost study of four proposed manufacturing processes for redox chemicals for the NASA REDOX Energy Storage System yielded favorable selling prices in the range 1.91/kg of chromic chloride, anhydrous basis, including ferrous chloride. The prices corresponded to specific energy storage costs from under 17/kWh. A refined and expanded cost analysis of the most favored process yielded a price estimate corresponding to a storage cost of $11/kWh. The findings supported the potential economic viability of the NASA REDOX system
An All Optical Fibre Quantum Controlled-NOT Gate
We report the first experimental demonstration of an optical controlled-NOT
gate constructed entirely in fibre. We operate the gate using two heralded
optical fibre single photon sources and find an average logical fidelity of 90%
and an average process fidelity of 0.83<F<0.91. On the basis of a simple model
we are able to conclude that imperfections are primarily due to the photon
sources, meaning that the gate itself works with very high fidelity.Comment: 4 pages, 4 figures, comments welcom
Two-photon interference between disparate sources for quantum networking
Quantum networks involve entanglement sharing between multiple users.
Ideally, any two users would be able to connect regardless of the type of
photon source they employ, provided they fulfill the requirements for
two-photon interference. From a theoretical perspective, photons coming from
different origins can interfere with a perfect visibility, provided they are
made indistinguishable in all degrees of freedom. Previous experimental
demonstrations of such a scenario have been limited to photon wavelengths below
900 nm, unsuitable for long distance communication, and suffered from low
interference visibility. We report two-photon interference using two disparate
heralded single photon sources, which involve different nonlinear effects,
operating in the telecom wavelength range. The measured visibility of the
two-photon interference is 80+/-4%, which paves the way to hybrid universal
quantum networks
On the Estimation of Bivariate Return Curves for Extreme Values
In the multivariate setting, defining extremal risk measures is important in
many contexts, such as finance, environmental planning and structural
engineering. In this paper, we review the literature on extremal bivariate
return curves, a risk measure that is the natural bivariate extension to a
return level, and propose new estimation methods based on multivariate extreme
value models that can account for both asymptotic dependence and asymptotic
independence. We identify gaps in the existing literature and propose novel
tools for testing and validating return curves and comparing estimates from a
range of multivariate models. These tools are then used to compare a selection
of models through simulation and case studies. We conclude with a discussion
and list some of the challenges.Comment: 41 pages (without supplementary), 11 figures, 2 table
Improving estimation for asymptotically independent bivariate extremes via global estimators for the angular dependence function
Modelling the extremal dependence of bivariate variables is important in a
wide variety of practical applications, including environmental planning,
catastrophe modelling and hydrology. The majority of these approaches are based
on the framework of bivariate regular variation, and a wide range of literature
is available for estimating the dependence structure in this setting. However,
this framework is only applicable to variables exhibiting asymptotic
dependence, even though asymptotic independence is often observed in practice.
In this paper, we consider the so-called `angular dependence function'; this
quantity summarises the extremal dependence structure for asymptotically
independent variables. Until recently, only pointwise estimators of the angular
dependence function have been available. We introduce a range of global
estimators and compare them to another recently introduced technique for global
estimation through a systematic simulation study, and a case study on river
flow data from the north of England, UK
The narrative potential of the British Birth Cohort Studies
This paper draws attention to the narrative potential of longitudinal studies such as the British Birth Cohort Studies (BBCS), and explores the possibility of creating narrative case histories and conducting narrative analysis based on information available from the studies. The BBCS have historically adopted a quantitative research design and used structured interviews and questionnaires to collect data from large samples of individuals born in specific years. However, the longitudinal nature of these studies means that they follow the same sample of individuals from birth through childhood into adult life, and this leads to the creation of data that can be understood as a quantitative auto/biography
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