3,031 research outputs found
Experimental quantum verification in the presence of temporally correlated noise
Growth in the complexity and capabilities of quantum information hardware
mandates access to practical techniques for performance verification that
function under realistic laboratory conditions. Here we experimentally
characterise the impact of common temporally correlated noise processes on both
randomised benchmarking (RB) and gate-set tomography (GST). We study these
using an analytic toolkit based on a formalism mapping noise to errors for
arbitrary sequences of unitary operations. This analysis highlights the role of
sequence structure in enhancing or suppressing the sensitivity of quantum
verification protocols to either slowly or rapidly varying noise, which we
treat in the limiting cases of quasi-DC miscalibration and white noise power
spectra. We perform experiments with a single trapped Yb ion as a
qubit and inject engineered noise () to probe protocol
performance. Experiments on RB validate predictions that the distribution of
measured fidelities over sequences is described by a gamma distribution varying
between approximately Gaussian for rapidly varying noise, and a broad, highly
skewed distribution for the slowly varying case. Similarly we find a strong
gate set dependence of GST in the presence of correlated errors, leading to
significant deviations between estimated and calculated diamond distances in
the presence of correlated errors. Numerical simulations demonstrate
that expansion of the gate set to include negative rotations can suppress these
discrepancies and increase reported diamond distances by orders of magnitude
for the same error processes. Similar effects do not occur for correlated
or errors or rapidly varying noise processes,
highlighting the critical interplay of selected gate set and the gauge
optimisation process on the meaning of the reported diamond norm in correlated
noise environments.Comment: Expanded and updated analysis of GST, including detailed examination
of the role of gauge optimization in GST. Full GST data sets and
supplementary information available on request from the authors. Related
results available from
http://www.physics.usyd.edu.au/~mbiercuk/Publications.htm
Oral human papillomavirus (HPV) infection in men who have sex with men: prevalence and lack of anogenital concordance.
To estimate the prevalence of oral detectable human papillomavirus (HPV) DNA in HIV-negative men who have sex with men (MSM) attending a sexual health clinic in London and concordance with anogenital HPV infection. Such data are important to improve our understanding of the epidemiology of oral HPV and the potential use of vaccines to prevent oropharyngeal cancers
Diffuse X-ray emission in spiral galaxies
We compare the soft diffuse X-ray emission from Chandra images of 12 nearby
intermediate inclination spiral galaxies to the morphology seen in Halpha,
molecular gas, and mid-infrared emission. We find that diffuse X-ray emission
is often located along spiral arms in the outer parts of spiral galaxies but
tends to be distributed in a rounder morphology in the center. The X-ray
morphology in the spiral arms matches that seen in the mid-infrared or Halpha
and so implies that the X-ray emission is associated with recent active star
formation. We see no strong evidence for X-ray emission trailing the location
of high mass star formation in spiral arms. However, population synthesis
models predict a high mechanical energy output rate from supernovae for a time
period that is about 10 times longer than the lifetime of massive ionizing
stars, conflicting with the narrow appearance of the arms in X-rays. The
fraction of supernova energy that goes into heating the ISM must depend on
environment and is probably higher near sites of active star formation. The
X-ray estimated emission measures suggest that the volume filling factors and
scale heights are high in the galaxy centers but low in the outer parts of
these galaxies. The differences between the X-ray properties and morphology in
the centers and outer parts of these galaxies suggest that galactic fountains
operate in outer galaxy disks but that winds are primarily driven from galaxy
centers.Comment: 28 pages, 4 figures, to be submitted to Ap
Metallicity Evolution of Damped Lyman-Alpha Galaxies
We have reanalyzed the existing data on Zinc abundances in damped Ly-alpha
(DLA) absorbers to investigate whether their mean metallicity evolves with
time. Most models of cosmic chemical evolution predict that the mass- weighted
mean interstellar metallicity of galaxies should rise with time from a low
value ~ 1/30 solar at z ~ 3 to a nearly solar value at z ~ 0. However, several
previous analyses have suggested that there is little or no evolution in the
global metallicity of DLAs. We have used a variety of statistical techniques to
quantify the global metallicity-redshift relation and its uncertainties, taking
into account both measurement and sampling errors. Three new features of our
analysis are: (a) an unbinned N(H I)-weighted nonlinear chi-square fit to an
exponential relation; (b) survival analysis to treat the large number of limits
in the existing data; and (c) a comparison of the data with several models of
cosmic chemical evolution based on an unbinned N(H I)-weighted chi-square. We
find that a wider range of evolutionary rates is allowed by the present data
than claimed in previous studies. The slope of the exponential fit to the N(H
I)-weighted mean Zn metallicity vs. redshift relation is -0.20 plus minus 0.11
counting limits as detections and -0.27 plus minus 0.12 counting limits as
zeros. Similar results are also obtained if the data are binned in redshift,
and if survival analysis is used. These slopes are marginally consistent with
no evolution, but are also consistent with the rates predicted by several
models of cosmic chemical evolution. Finally, we outline some future
measurements necessary to improve the statistics of the global
metallicity-redshift relation.Comment: 25 pages, 1 figure, accepted for publication in the Astrophysical
Journa
The impact of distribution locational marginal prices on distributed energy resources : an aggregated approach
With the growth in Distributed Energy Resources (DERs) and the trend towards electrification of heat and transport, distribution networks will be increasingly challenged and will need to be more actively managed. Distribution Locational Marginal Prices (DLMPs) offer a method of clearing markets at distribution level and providing information on transmission losses and congestion in a network due to transmission constraints. This paper examines the application of DLMPs to a region of the South West of England. The resulting DER penetration from applying DLMPs to different voltage levels was considered. It was found that by applying DLMPs down to 11 kV, distributed generation capacity and output could be increased significantly. Applying DLMPs down to 11 kV had a less pronounced effect on flexible demand dispatch due to the coincidence of renewable curtailment and the lowest daily system prices
A motif-based approach to network epidemics
Networks have become an indispensable tool in modelling infectious diseases, with the structure of epidemiologically relevant contacts known to affect both the dynamics of the infection process and the efficacy of intervention strategies. One of the key reasons for this is the presence of clustering in contact networks, which is typically analysed in terms of prevalence of triangles in the network. We present a more general approach, based on the prevalence of different four-motifs, in the context of ODE approximations to network dynamics. This is shown to outperform existing models for a range of small world networks
The Relative Orientation of Nuclear Accretion and Galaxy Stellar Disks in Seyfert Galaxies
We use the difference (delta) between the position angles of the nuclear
radio emission and the host galaxy major axis to investigate the distribution
of the angle (beta) between the axes of the nuclear accretion disk and the host
galaxy disk in Seyfert galaxies. We provide a critical appraisal of the quality
of all measurements, and find that the data are limited by observational
uncertainties and biases, such as the well known deficiency of Seyfert galaxies
of high inclination. There is weak evidence that the distribution of delta for
Seyfert 2 galaxies may be different (at the 90% confidence level) from a
uniform distribution, while the Seyfert 1 delta distribution is not
significantly different from a uniform distribution or from the Seyfert 2 delta
distribution. The cause of the possible non-uniformity in the distribution of
delta for Seyfert 2 galaxies is discussed. Seyfert nuclei in late-type spiral
galaxies may favor large values of delta (at the ~96% confidence level), while
those in early-type galaxies show a more or less random distribution of delta.
This may imply that the nuclear accretion disk in non-interacting late-type
spirals tends to align with the stellar disk, while that in early-type galaxies
is more randomly oriented, perhaps as a result of accretion following a galaxy
merger.
We point out that biases in the distribution of inclination translate to
biased estimates of beta in the context of the unified scheme. When this effect
is taken into account, the distributions of beta for all Seyferts together, and
of Seyfert 1's and 2's separately, agree with the hypothesis that the radio
jets are randomly oriented with respect to the galaxy disk. The data are
consistent with the expectations of the unified scheme, but do not demand it.Comment: To appear in the Astrophysical Journal, Vol 516 #1, May 1, 1999.
Corrected figure placement within pape
Abundances in the Neutral Interstellar Medium of I Zw 18 from FUSE Observations
We report on new FUSE far-UV spectroscopy of the most metal-poor blue compact
dwarf galaxy I Zw 18. The new data represent an improvement over previous FUSE
spectra by a factor of 1.7 in the signal-to-noise. Together with a larger
spectral coverage (917-1188 angstroms), this allows us to characterize
absorption lines in the interstellar medium with unprecedented accuracy. The
kinematics averaged over the large sampled region shows no clear evidence of
gas inflows or outflows. The H I absorption is interstellar with a column
density of 2.2 (+0.6,-0.5} * 10^21 cm^(-2). A conservative 3 sigma upper limit
of 5.25 * 10^(14) cm^(-2) is derived for the column density of diffuse H_2.
From a simultaneous fitting of metal absorption lines in the interstellar
medium, we infer the following abundances: [Fe/H] = -1.76 +/- 0.12, [O/H] =
-2.06 +/- 0.28, [Si/H] = -2.09 +/- 0.12, [Ar/H] = -2.27 +/- 0.13, and [N/H] =
-2.88 +/- 0.11. This is in general several times lower than in the H II
regions. The only exception is iron, whose abundance is the same. The abundance
pattern of the interstellar medium suggests ancient star-formation activity
with an age of at least a Gyr that enriched the H I phase. Around 470 SNe Ia
are required to produce the iron content. A more recent episode that started 10
to several 100 Myr ago is responsible for the additional enrichment of
alpha-elements and nitrogen in the H II regions.Comment: 48 pages including 3 tables (Latex) and 7 figures (postscript).
Accepted for publication in the Astrophysical Journa
Congestion management with aggregated delivery of flexibility using distributed energy resources
Increasing penetrations of small scale electricity generation and storage technologies are making an important contribution to the decentralisation and decarbonisation of power system control and operation. Although not currently realised, coordination of local distributed energy resources (DERs) and a greater degree of demand flexibility through digital aggregation, offer the potential to lower the cost of energy at source and to enable remuneration for consumer participation, addressing the rising costs of energy supply, which impacts strongly on all consumers. Methods are required to manage potential distribution network constraints caused by flexible DERs, as well as for determining the risk to delivery of flexibility from these DERs for aggregators. A heuristic network flexibility dispatch methodology is proposed, which can be used to calculate the probability of constraints, and any required adjustments of flexible agent positions to resolve them, at half hourly resolution. The aggregator can use this methodology to manage their portfolio risk, while a distribution system operator can estimate required flexibility to manage constraints down to low voltage level
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