812 research outputs found
Demonstration of non-Markovian process characterisation and control on a quantum processor
In the scale-up of quantum computers, the framework underpinning
fault-tolerance generally relies on the strong assumption that environmental
noise affecting qubit logic is uncorrelated (Markovian). However, as physical
devices progress well into the complex multi-qubit regime, attention is turning
to understanding the appearance and mitigation of correlated -- or
non-Markovian -- noise, which poses a serious challenge to the progression of
quantum technology. This error type has previously remained elusive to
characterisation techniques. Here, we develop a framework for characterising
non-Markovian dynamics in quantum systems and experimentally test it on
multi-qubit superconducting quantum devices. Where noisy processes cannot be
accounted for using standard Markovian techniques, our reconstruction predicts
the behaviour of the devices with an infidelity of . Our results show
this characterisation technique leads to superior quantum control and extension
of coherence time by effective decoupling from the non-Markovian environment.
This framework, validated by our results, is applicable to any controlled
quantum device and offers a significant step towards optimal device operation
and noise reduction
Cosmic Gravitational Shear from the HST Medium Deep Survey
We present a measurement of cosmic shear on scales ranging from 10\arcsec
to 2\arcmin in 347 WFPC2 images of random fields. Our result is based on
shapes measured via image fitting and on a simple statistical technique;
careful calibration of each step allows us to quantify our systematic
uncertainties and to measure the cosmic shear down to very small angular
scales. The WFPC2 images provide a robust measurement of the cosmic shear
signal decreasing from at 10\arcsec to at 130\arcsec .Comment: 4 pages 2 Postscript figures, uses emulateapj.cls Astrophysical
Journal Letters, December 1, 200
Compact Nuclei in Moderately Redshifted Galaxies
The Hubble Space Telescope WFPC2 is being used to obtain high-resolution
images in the V and I bands for several thousand distant galaxies as part of
the Medium Deep Survey (MDS). An important scientific aim of the MDS is to
identify possible AGN candidates from these images in order to measure the
faint end of the AGN luminosity function as well as to study the host galaxies
of AGNs and nuclear starburst systems. We are able to identify candidate
objects based on morphology. Candidates are selected by fitting bulge+disk
models and bulge+disk+point source nuclei models to HST imaged galaxies and
determining the best model fit to the galaxy light profile. We present results
from a sample of MDS galaxies with I less than 21.5 mag that have been searched
for AGN/starburst nuclei in this manner. We identify 84 candidates with
unresolved nuclei in a sample of 825 galaxies. For the expected range of galaxy
redshifts, all normal bulges are resolved. Most of the candidates are found in
galaxies displaying exponential disks with some containing an additional bulge
component. 5% of the hosts are dominated by an r^-1/4 bulge. The V-I color
distribution of the nuclei is consistent with a dominant population of
Seyfert-type nuclei combined with an additional population of starbursts. Our
results suggest that 10% +/- 1% of field galaxies at z less than 0.6 may
contain AGN/starburst nuclei that are 1 to 5 magnitudes fainter than the host
galaxies.Comment: 12 pages AASTeX manuscript, 3 separate Postscript figures, to be
published in ApJ Letter
Compact Nuclei in Galaxies at Moderate Redshift: I. Imaging and Spectroscopy
This study explores the space density and properties of active galaxies to
z=0.8. We have investigated the frequency and nature of unresolved nuclei in
galaxies at moderate redshift as indicators of nuclear activity such as Active
Galactic Nuclei (AGN) or starbursts. Candidates are selected by fitting imaged
galaxies with multi-component models using maximum likelihood estimate
techniques to determine the best model fit. We select those galaxies requiring
an unresolved, point source component in the galaxy nucleus, in addition to a
disk and/or bulge component, to adequately model the galaxy light. We have
searched 70 WFPC2 images primarily from the Medium Deep Survey for galaxies
containing compact nuclei. In our survey of 1033 galaxies, the fraction
containing an unresolved nuclear component greater than 3% of the total galaxy
light is 16+/-3% corrected for incompleteness and 9+/-1% for nuclei greater
than 5% of the galaxy light. Spectroscopic redshifts have been obtained for 35
of our AGN/starburst candidates and photometric redshifts are estimated to an
accuracy of sigma_z=0.1 for the remaining sample. In this paper, the first of
two in this series, we present the selected HST imaged galaxies having
unresolved nuclei and discuss the selection procedure. We also present the
ground-based spectroscopy for these galaxies as well as the photometric
redshifts estimated for those galaxies without spectra.Comment: 56 pages, 22 figures, to appear in ApJ Supplement Series, April 199
Filtering crosstalk from bath non-Markovianity via spacetime classical shadows
From an open system perspective non-Markovian effects due to a nearby bath or
neighbouring qubits are dynamically equivalent. However, there is a conceptual
distinction to account for: neighbouring qubits may be controlled. We combine
recent advances in non-Markovian quantum process tomography with the framework
of classical shadows to characterise spatiotemporal quantum correlations.
Observables here constitute operations applied to the system, where the free
operation is the maximally depolarising channel. Using this as a causal break,
we systematically erase causal pathways to narrow down the progenitors of
temporal correlations. We show that one application of this is to filter out
the effects of crosstalk and probe only non-Markovianity from an inaccessible
bath. It also provides a lens on spatiotemporally spreading correlated noise
throughout a lattice from common environments. We demonstrate both examples on
synthetic data. Owing to the scaling of classical shadows, we can erase
arbitrarily many neighbouring qubits at no extra cost. Our procedure is thus
efficient and amenable to systems even with all-to-all interactions.Comment: 5 pages, 4 figure
Combining computational fluid dynamics and neural networks to characterize microclimate extremes: Learning the complex interactions between meso-climate and urban morphology
The urban form and extreme microclimate events can have an important impact on the energy performance of buildings, urban comfort and human health. State-of-the-art building energy simulations require information on the urban microclimate, but typically rely on ad-hoc numerical simulations, expensive in-situ measurements, or data from nearby weather stations. As such, they do not account for the full range of possible urban microclimate variability and findings cannot be generalized across urban morphologies. To bridge this knowledge gap, this study proposes two data-driven models to downscale climate variables from the meso to the micro scale in arbitrary urban morphologies, with a focus on extreme climate conditions. The models are based on a feedforward and a deep neural network (NN) architecture, and are trained using results from computational fluid dynamics (CFD) simulations of flow over a series of idealized but representative urban environments, spanning a realistic range of urban morphologies. Both models feature a relatively good agreement with corresponding CFD training data, with a coefficient of determination R2 = 0.91 (R2 = 0.89) and R2 = 0.94 (R2 = 0.92) for spatially-distributed wind magnitude and air temperature for the deep NN (feedforward NN). The models generalize well for unseen urban morphologies and mesoscale input data that are within the training bounds in the parameter space, with a R2 = 0.74 (R2 = 0.69) and R2 = 0.81 (R2 = 0.74) for wind magnitude and air temperature for the deep NN (feedforward NN). The accuracy and efficiency of the proposed CFD-NN models makes them well suited for the design of climate-resilient buildings at the early design stage
On the sampling complexity of open quantum systems
Open quantum systems are ubiquitous in the physical sciences, with widespread
applications in the areas of chemistry, condensed matter physics, material
science, optics, and many more. Not surprisingly, there is significant interest
in their efficient simulation. However, direct classical simulation quickly
becomes intractable with coupling to an environment whose effective dimension
grows exponentially. This raises the question: can quantum computers help model
these complex dynamics? A first step in answering this question requires
understanding the computational complexity of this task. Here, we map the
temporal complexity of a process to the spatial complexity of a many-body state
using a computational model known as the process tensor framework. With this,
we are able to explore the simulation complexity of an open quantum system as a
dynamic sampling problem: a system coupled to an environment can be probed at
successive points in time -- accessing multi-time correlations. The complexity
of multi-time sampling, which is an important and interesting problem in its
own right, contains the complexity of master equations and stochastic maps as a
special case. Our results show how the complexity of the underlying quantum
stochastic process corresponds to the complexity of the associated family of
master equations for the dynamics. We present both analytical and numerical
examples whose multi-time sampling is as complex as sampling from a many-body
state that is classically hard. This also implies that the corresponding family
of master equations are classically hard. Our results pave the way for studying
open quantum systems from a complexity-theoretic perspective, highlighting the
role quantum computers will play in our understanding of quantum dynamics
Hydrographic Surveys at Seven Chutes and Three Backwaters on the Missouri River in Nebraska, Iowa, and Missouri, 2011-13
The United States Geological Survey (USGS) cooperated with the United States Army Corps of Engineers (USACE), Omaha District, to complete hydrographic surveys of seven chutes and three backwaters on the Missouri River yearly during 2011–13. These chutes and backwaters were constructed by the USACE to increase the amount of available shallow water habitat (SWH) to support threatened and endangered species, as required by the amended “2000 Biological Opinion” on the operation of the Missouri River main-stem reservoir system. Chutes surveyed included Council chute, Plattsmouth chute, Tobacco chute, Upper Hamburg chute, Lower Hamburg chute, Kansas chute, and Deroin chute. Backwaters surveyed included Ponca backwater, Plattsmouth backwater, and Langdon backwater. Hydrographic data from these chute and backwater surveys will aid the USACE to assess the current (2011–13) amount of available SWH, the effects river flow have had on evolving morphology of the chutes and backwaters, and the functionality of the chute and backwater designs. Chutes and backwaters were surveyed from August through November 2011, June through November 2012, and May through October 2013. During the 2011 surveys, high water was present at all sites because of the major flooding on the Missouri River. The hydrographic survey data are published along with this report in comma-separated-values (csv) format with associated metadata.Hydrographic surveys included bathymetric and Real-Time Kinematic Global Navigation Satellite System surveys. Hydrographic data were collected along transects extending across the channel from top of bank to top of bank. Transect segments with water depths greater than 1 meter were surveyed using a single-beam echosounder to measure depth and a differentially corrected global positioning system to measure location. These depth soundings were converted to elevation using water-surface-elevation information collected with a Real-Time Kinematic Global Navigation Satellite System. Transect segments with water depths less than 1 meter were surveyed using Real-Time Kinematic Global Navigation Satellite Systems. Surveyed features included top of bank, toe of bank, edge of water, sand bars, and near-shore areas.Discharge was measured at chute survey sites, in both the main channel of the Missouri River upstream from the chute and the chute. Many chute entrances and control structures were damaged by floodwater during the 2011 Missouri River flood, allowing a larger percentage of the total Missouri River discharge to flow through the chute than originally intended in the chute design. Measured discharge split between the main channel and the chute at most chutes was consistent with effects of the 2011 Missouri River flood damages and a larger percent of the total Missouri River discharge was flowing through the chute than originally intended. The US Army Corps of Engineers repaired many of these chutes in 2012 and 2013, and the resulting hydraulic changes are reflected in the discharge splits
Влияние магнитомеханического резонанса на амплитудно- и фазочастотные зависимости переменных составляющих эффекта Фарадея
В рамках модели продольных колебаний тонкого стержня из магнитооптического кристалла в условиях магнитомеханического резонанса показано, что вынужденные колебания могут создавать не только амплитудные, но и фазочастотные зависимости переменных составляющих эффекта Фарадея.У рамках моделі поздовжніх коливань тонкого стрижня із магнітооптичного кристала в умовах магнітомеханічного резонансу показано, що вимушені коливання можуть створювати не тільки амплітудні, але й фазочастотні залежності змінних складових ефекту Фарадея.In the framework of a model of longitudinal vibrations of a thin rod fabricated of the magneto-optical crystal under a magnetomechanical resonance, it is shown that the forced oscillations can create not only the amplitude but also phase-frequency dependences of the variable components of the Faraday effect
Luminosity Functions of Elliptical Galaxies at z < 1.2
The luminosity functions of E/S0 galaxies are constructed in 3 different
redshift bins (0.2 < z < 0.55, 0.55 < z < 0.8, 0.8 < z < 1.2), using the data
from the Hubble Space Telescope Medium Deep Survey (HST MDS) and other HST
surveys. These independent luminosity functions show the brightening in the
luminosity of E/S0s by about 0.5~1.0 magnitude at z~1, and no sign of
significant number evolution.
This is the first direct measurement of the luminosity evolution of E/S0
galaxies, and our results support the hypothesis of a high redshift of
formation (z > 1) for elliptical galaxies, together with weak evolution of the
major merger rate at z < 1.Comment: To be published in ApJ Letters, 4 pages, AAS Latex, 4 figures, and 2
table
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