4,407 research outputs found
Learning-based quantum error mitigation
If NISQ-era quantum computers are to perform useful tasks, they will need to
employ powerful error mitigation techniques. Quasi-probability methods can
permit perfect error compensation at the cost of additional circuit executions,
provided that the nature of the error model is fully understood and
sufficiently local both spatially and temporally. Unfortunately these
conditions are challenging to satisfy. Here we present a method by which the
proper compensation strategy can instead be learned ab initio. Our training
process uses multiple variants of the primary circuit where all non-Clifford
gates are substituted with gates that are efficient to simulate classically.
The process yields a configuration that is near-optimal versus noise in the
real system with its non-Clifford gate set. Having presented a range of
learning strategies, we demonstrate the power of the technique both with real
quantum hardware (IBM devices) and exactly-emulated imperfect quantum
computers. The systems suffer a range of noise severities and types, including
spatially and temporally correlated variants. In all cases the protocol
successfully adapts to the noise and mitigates it to a high degree.Comment: 28 pages, 19 figure
Dark-Ages Reionisation & Galaxy Formation Simulation XVI: The Thermal Memory of Reionisation
Intergalactic medium temperature is a powerful probe of the epoch of
reionisation, as information is retained long after reionisation itself.
However, mean temperatures are highly degenerate with the timing of
reionisation, with the amount heat injected during the epoch, and with the
subsequent cooling rates. We post-process a suite of semi-analytic galaxy
formation models to characterise how different thermal statistics of the
intergalactic medium can be used to constrain reionisation. Temperature is
highly correlated with redshift of reionisation for a period of time after the
gas is heated. However as the gas cools, thermal memory of reionisation is
lost, and a power-law temperature-density relation is formed, with . Constraining our model
against observations of electron optical depth and temperature at mean density,
we find that reionisation likely finished at with a soft spectral slope of . By
restricting spectral slope to the range motivated by population II
synthesis models, reionisation timing is further constrained to . We find that, in the future, the degeneracies between
reionisation timing and background spectrum can be broken using the scatter in
temperatures and integrated thermal history.Comment: 17 pages, 17 figures, Accepted for publication in MNRA
Dark-ages Reionization and Galaxy Formation Simulation - XIV. Gas accretion, cooling and star formation in dwarf galaxies at high redshift
We study dwarf galaxy formation at high redshift () using a suite of
high- resolution, cosmological hydrodynamic simulations and a semi-analytic
model (SAM). We focus on gas accretion, cooling and star formation in this work
by isolating the relevant process from reionization and supernova feedback,
which will be further discussed in a companion paper. We apply the SAM to halo
merger trees constructed from a collisionless N-body simulation sharing
identical initial conditions to the hydrodynamic suite, and calibrate the free
parameters against the stellar mass function predicted by the hydrodynamic
simulations at z = 5. By making comparisons of the star formation history and
gas components calculated by the two modelling techniques, we find that
semi-analytic prescriptions that are commonly adopted in the literature of
low-redshift galaxy formation do not accurately represent dwarf galaxy
properties in the hydrodynamic simulation at earlier times. We propose 3
modifications to SAMs that will provide more accurate high-redshift
simulations. These include 1) the halo mass and baryon fraction which are
overestimated by collisionless N-body simulations; 2) the star formation
efficiency which follows a different cosmic evolutionary path from the
hydrodynamic simulation; and 3) the cooling rate which is not well defined for
dwarf galaxies at high redshift. Accurate semi-analytic modelling of dwarf
galaxy formation informed by detailed hydrodynamical modelling will facilitate
reliable semi-analytic predictions over the large volumes needed for the study
of reionization.Comment: 20 pages, 10 figures; Updated to match the published version. All
results and conclusions remain unchange
Fast Deep Matting for Portrait Animation on Mobile Phone
Image matting plays an important role in image and video editing. However,
the formulation of image matting is inherently ill-posed. Traditional methods
usually employ interaction to deal with the image matting problem with trimaps
and strokes, and cannot run on the mobile phone in real-time. In this paper, we
propose a real-time automatic deep matting approach for mobile devices. By
leveraging the densely connected blocks and the dilated convolution, a light
full convolutional network is designed to predict a coarse binary mask for
portrait images. And a feathering block, which is edge-preserving and matting
adaptive, is further developed to learn the guided filter and transform the
binary mask into alpha matte. Finally, an automatic portrait animation system
based on fast deep matting is built on mobile devices, which does not need any
interaction and can realize real-time matting with 15 fps. The experiments show
that the proposed approach achieves comparable results with the
state-of-the-art matting solvers.Comment: ACM Multimedia Conference (MM) 2017 camera-read
Dark-ages Reionization & Galaxy Formation Simulation VIII. Suppressed growth of dark matter halos during the Epoch of Reionization
We investigate how the hydrostatic suppression of baryonic accretion affects
the growth rate of dark matter halos during the Epoch of Reionization. By
comparing halo properties in a simplistic hydrodynamic simulation in which gas
only cools adiabatically, with its collisionless equivalent, we find that halo
growth is slowed as hydrostatic forces prevent gas from collapsing. In our
simulations, at the high redshifts relevant for reionization (between
and ), halos that host dwarf galaxies () can be reduced by up to a factor of 2 in mass due to the
hydrostatic pressure of baryons. Consequently, the inclusion of baryonic
effects reduces the amplitude of the low mass tail of the halo mass function by
factors of 2 to 4. In addition, we find that the fraction of baryons in dark
matter halos hosting dwarf galaxies at high redshift never exceeds
of the cosmic baryon fraction. When implementing baryonic processes, including
cooling, star formation, supernova feedback and reionization, the suppression
effects become more significant with further reductions of to
60\%. Although convergence tests suggest that the suppression may become weaker
in higher resolution simulations, this suppressed growth will be important for
semi-analytic models of galaxy formation, in which the halo mass inherited from
an underlying N-body simulation directly determines galaxy properties. Based on
the adiabatic simulation, we provide tables to account for these effects in
N-body simulations, and present a modification of the halo mass function along
with explanatory analytic calculations.Comment: 17 pages, 11 figures; Updated to match the published version. Two
changes in Figures 1 and 3 in order to 1) correct bin sizes of the 10^8 and
10^8.5 Msol bins for NOSN_NOZCOOL_NoRe (was 0.5, should be 0.25); 2) include
stellar mass in baryon fraction (was missed in Fig. 3). Quantitative
description of Fig. 3 changed slightly in Section 2.2. All other results and
conclusions remain unchange
Dependence of galaxy clustering on UV-luminosity and stellar mass at
We investigate the dependence of galaxy clustering at on
UV-luminosity and stellar mass. Our sample consists of 10,000
Lyman-break galaxies (LBGs) in the XDF and CANDELS fields. As part of our
analysis, the relation is estimated for the sample,
which is found to have a nearly linear slope of . We subsequently measure the angular correlation function and
bias in different stellar mass and luminosity bins. We focus on comparing the
clustering dependence on these two properties. While UV-luminosity is only
related to recent starbursts of a galaxy, stellar mass reflects the integrated
build-up of the whole star formation history, which should make it more tightly
correlated with halo mass. Hence, the clustering segregation with stellar mass
is expected to be larger than with luminosity. However, our measurements
suggest that the segregation with luminosity is larger with
confidence (neglecting contributions from systematic errors). We compare this
unexpected result with predictions from the \textsc{Meraxes} semi-analytic
galaxy formation model. Interestingly, the model reproduces the observed
angular correlation functions, and also suggests stronger clustering
segregation with luminosity. The comparison between our observations and the
model provides evidence of multiple halo occupation in the small scale
clustering.Comment: 10 pages, 6 figures, 2 tables, accepted for publication in MNRA
To Branch Out or Stay Focused?: Affective Shifts Differentially Predict Organizational Citizenship Behavior and Task Performance
We draw from personality systems interaction theory (PSI; Kuhl, 2000) and regulatory focus theory (Higgins, 1997) to examine how dynamic positive and negative affective processes interact to predict both task and contextual performance. Using a twice-daily diary design over the course of a three-week period, results from multi-level regression analysis revealed that distinct patterns of change in positive and negative affect optimally predicted contextual and task performance among a sample of 71 individuals employed at a medium-sized technology company. Specifically, within persons, increases (upshifts) in positive affect over the course of a work day better predicted the subsequent day’s organizational citizenship behavior (OCB) when such increases were coupled with decreases (downshifts) in negative affect. The optimal pattern of change in positive and negative affect differed, however, in predicting task performance. That is, upshifts in positive affect over the course of the work day better predicted the subsequent day’s task performance when such upshifts were accompanied by upshifts in negative affect. The contribution of our findings to PSI theory and the broader affective and motivation regulation literatures, along with practical implications, are discussed
Dark-ages Reionization and Galaxy Formation Simulation - X. The small contribution of quasars to reionization
Motivated by recent measurements of the number density of faint AGN at high
redshift, we investigate the contribution of quasars to reionization by
tracking the growth of central supermassive black holes in an update of the
Meraxes semi-analytic model. The model is calibrated against the observed
stellar mass function at , the black hole mass function at
, the global ionizing emissivity at and the Thomson
scattering optical depth. The model reproduces a Magorrian relation in
agreement with observations at and predicts a decreasing black hole
mass towards higher redshifts at fixed total stellar mass. With the
implementation of an opening angle of 80 deg for quasar radiation,
corresponding to an observable fraction of per cent due to
obscuration by dust, the model is able to reproduce the observed quasar
luminosity function at . The stellar light from galaxies hosting
faint AGN contributes a significant or dominant fraction of the UV flux. At
high redshift, the model is consistent with the bright end quasar luminosity
function and suggests that the recent faint AGN sample compiled by
Giallongo et al. (2015) includes a significant fraction of stellar light.
Direct application of this luminosity function to the calculation of AGN
ionizing emissivity consequently overestimates the number of ionizing photons
produced by quasars by a factor of 3 at . We conclude that quasars are
unlikely to make a significant contribution to reionization.Comment: 21 pages, 12 figures; Updated to match the published version. All
results and conclusions remain unchange
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