4,972 research outputs found
Estimation from quantized Gaussian measurements: when and how to use dither
Subtractive dither is a powerful method for removing the signal dependence of quantization noise for coarsely quantized signals. However, estimation from dithered measurements often naively applies the sample mean or midrange, even when the total noise is not well described with a Gaussian or uniform distribution. We show that the generalized Gaussian distribution approximately describes subtractively dithered, quantized samples of a Gaussian signal. Furthermore, a generalized Gaussian fit leads to simple estimators based on order statistics that match the performance of more complicated maximum likelihood estimators requiring iterative solvers. The order statistics-based estimators outperform both the sample mean and midrange for nontrivial sums of Gaussian and uniform noise. Additional analysis of the generalized Gaussian approximation yields rules of thumb for determining when and how to apply dither to quantized measurements. Specifically, we find subtractive dither to be beneficial when the ratio between the Gaussian standard deviation and quantization interval length is roughly less than one-third. When that ratio is also greater than 0.822/K^0.930 for the number of measurements K > 20, estimators we present are more efficient than the midrange.https://arxiv.org/abs/1811.06856Accepted manuscrip
Dead Time Compensation for High-Flux Ranging
Dead time effects have been considered a major limitation for fast data
acquisition in various time-correlated single photon counting applications,
since a commonly adopted approach for dead time mitigation is to operate in the
low-flux regime where dead time effects can be ignored. Through the application
of lidar ranging, this work explores the empirical distribution of detection
times in the presence of dead time and demonstrates that an accurate
statistical model can result in reduced ranging error with shorter data
acquisition time when operating in the high-flux regime. Specifically, we show
that the empirical distribution of detection times converges to the stationary
distribution of a Markov chain. Depth estimation can then be performed by
passing the empirical distribution through a filter matched to the stationary
distribution. Moreover, based on the Markov chain model, we formulate the
recovery of arrival distribution from detection distribution as a nonlinear
inverse problem and solve it via provably convergent mathematical optimization.
By comparing per-detection Fisher information for depth estimation from high-
and low-flux detection time distributions, we provide an analytical basis for
possible improvement of ranging performance resulting from the presence of dead
time. Finally, we demonstrate the effectiveness of our formulation and
algorithm via simulations of lidar ranging.Comment: Revision with added estimation results, references, and figures, and
modified appendice
Baryon Inhomogeneity Generation in the Quark-Gluon Plasma Phase
We discuss the possibility of generation of baryon inhomogeneities in a
quark-gluon plasma phase due to moving Z(3) interfaces. By modeling the
dependence of effective mass of the quarks on the Polyakov loop order
parameter, we study the reflection of quarks from collapsing Z(3) interfaces
and estimate resulting baryon inhomogeneities in the context of the early
universe. We argue that in the context of certain low energy scale inflationary
models, it is possible that large Z(3) walls arise at the end of the reheating
stage. Collapse of such walls could lead to baryon inhomogeneities which may be
separated by large distances near the QCD scale. Importantly, the generation of
these inhomogeneities is insensitive to the order, or even the existence, of
the quark-hadron phase transition. We also briefly discuss the possibility of
formation of quark nuggets in this model, as well as baryon inhomogeneity
generation in relativistic heavy-ion collisions.Comment: 11 pages, 2 figures, revtex4, more detailed discussion added about
formation and evolution of Z(3)domain walls in the univers
Minimal Universal Two-qubit Quantum Circuits
We give quantum circuits that simulate an arbitrary two-qubit unitary
operator up to global phase. For several quantum gate libraries we prove that
gate counts are optimal in worst and average cases. Our lower and upper bounds
compare favorably to previously published results. Temporary storage is not
used because it tends to be expensive in physical implementations.
For each gate library, best gate counts can be achieved by a single universal
circuit. To compute gate parameters in universal circuits, we only use
closed-form algebraic expressions, and in particular do not rely on matrix
exponentials. Our algorithm has been coded in C++.Comment: 8 pages, 2 tables and 4 figures. v3 adds a discussion of asymetry
between Rx, Ry and Rz gates and describes a subtle circuit design problem
arising when Ry gates are not available. v2 sharpens one of the loose bounds
in v1. Proof techniques in v2 are noticeably revamped: they now rely less on
circuit identities and more on directly-computed invariants of two-qubit
operators. This makes proofs more constructive and easier to interpret as
algorithm
Classical solutions for Yang-Mills-Chern-Simons field coupled to an external source
We find wide class of exact solutions of Yang-Mills-Chern-Simons theory
coupled to an external source, in terms of doubly periodic Jacobi elliptic
functions. The obtained solutions include localized solitons, trigonometric
solutions, pure cnoidal waves, and singular solutions in certain parameter
range. Furthermore, it is observed that these solutions exist over a nonzero
background.Comment: 5 page
Noether Currents of Charged Spherical Black Holes
We calculate the Noether currents and charges for Einstein-Maxwell theory
using a version of the Wald approach. In spherical symmetry, the choice of time
can be taken as the Kodama vector. For the static case, the resulting combined
Einstein-Maxwell charge is just the mass of the black hole. Using either a
classically defined entropy or the Iyer-Wald selection rules, the entropy is
found to be just a quarter of the area of the trapping horizon. We propose
identifying the combined Noether charge as an energy associated with the Kodama
time. For the extremal black hole case, we discuss the problem of Wald's
rescaling of the surface gravity to define the entropy.Comment: 4 page
Band inversion driven by electronic correlations at the (111) LaAlO/SrTiO interface
Quantum confinement at complex oxide interfaces establishes an intricate
hierarchy of the strongly correlated -orbitals which is widely recognized as
a source of emergent physics. The most prominent example is the (001)
LaAlO/SrTiO(LAO/STO) interface, which features a dome-shaped phase
diagram of superconducting critical temperature and spin-orbit coupling (SOC)
as a function of electrostatic doping, arising from a selective occupancy of
orbitals of different character. Here we study (111)-oriented LAO/STO
interfaces - where the three orbitals contribute equally to the
sub-band states caused by confinement - and investigate the impact of this
unique feature on electronic transport. We show that transport occurs through
two sets of electron-like sub-bands, and the carrier density of one of the sets
shows a non-monotonic dependence on the sample conductance. Using tight-binding
modeling, we demonstrate that this behavior stems from a band inversion driven
by on-site Coulomb interactions. The balanced contribution of all
orbitals to electronic transport is shown to result in strong SOC with reduced
electrostatic modulation.Comment: 5 pages, 4 figures, (+ supplemental material
Universality of the single-particle spectra of cuprate superconductors
All the available data for the dispersion and linewidth of the
single-particle spectra above the superconducting gap and the pseudogap in
metallic cuprates for any doping has universal features. The linewidth is
linear in energy below a scale and constant above. The cusp in the
linewidth at mandates, due to causality, a "waterfall", i.e., a
vertical feature in the dispersion. These features are predicted by a recent
microscopic theory. We find that all data can be quantitatively fitted by the
theory with a coupling constant and an upper cutoff at
which vary by less than 50% among the different cuprates and for varying
dopings. The microscopic theory also gives these values to within factors of
O(2).Comment: 4 pages, 4 figures; accepted by Phys. Rev. Let
Probing the time variability of five Fe low broad absorption line quasars
We study the time variability of five Fe Low ionization Broad Absorption Line
(FeLoBAL) QSOs using repeated spectroscopic observations with the 2m telescope
at IUCAA Girawali observatory (IGO) spanning an interval of upto 10 years. We
report a dramatic variation in Al III and Fe III fine-structure lines in the
spectra of SDSS J221511.93-004549.9 (z_em ~ 1.478). However, there is no such
strong variability shown by the C IV absorption. This source is known to be
unusual with (i) the continuum emission dominated by Fe emission lines, (ii) Fe
III absorption being stronger than Fe II and (iii) the apparent ratio of Fe III
UV 48 to Fe III UV 34 absorption suggesting an inverted population ratio. This
is the first reported detection of time variability in the Fe III
fine-structure lines in QSO spectra. There is a strong reduction in the
absorption strength of these lines between year 2000 and 2008. Using the
template fitting techniques, we show that the apparent inversion of strength of
UV lines could be related to the complex spectral energy distribution of this
QSO. The observed variability can be related to change in the ionization state
of the gas or due to transverse motion of this absorbing gas. The shortest
variability timescale of Al III line gives a lower limit on the electron
density of the absorbing gas as n_e >= 1.1 x 10^4 cm^-3. The remaining 4
FeLoBALs do not show any changes beyond the measurement uncertainties either in
optical depth or in the velocity structure. We present the long-term
photometric light curve for all of our sources. Among them only SDSS
J221511.93-004549.9 shows significant (>= 0.2 mag) variability.Comment: 15 pages, 9 figures, 3 tables, Accepted for publication in MNRA
Abnormal wave reflections and left ventricular hypertrophy late after coarctation of the aorta repair
Patients with repaired coarctation of the aorta are thought to have increased afterload due to abnormalities in vessel structure and function. We have developed a novel cardiovascular magnetic resonance protocol that allows assessment of central hemodynamics, including central aortic systolic blood pressure, resistance, total arterial compliance, pulse wave velocity, and wave reflections. The main study aims were to (1) characterize group differences in central aortic systolic blood pressure and peripheral systolic blood pressure, (2) comprehensively evaluate afterload (including wave reflections) in the 2 groups, and (3) identify possible biomarkers among covariates associated with elevated left ventricular mass (LVM). Fifty adult patients with repaired coarctation and 25 age- and sex-matched controls were recruited. Ascending aorta area and flow waveforms were obtained using a high temporal-resolution spiral phase-contrast cardiovascular magnetic resonance flow sequence. These data were used to derive central hemodynamics and to perform wave intensity analysis noninvasively. Covariates associated with LVM were assessed using multivariable linear regression analysis. There were no significant group differences (P≥0.1) in brachial systolic, mean, or diastolic BP. However central aortic systolic blood pressure was significantly higher in patients compared with controls (113 versus 107 mm Hg, P=0.002). Patients had reduced total arterial compliance, increased pulse wave velocity, and larger backward compression waves compared with controls. LVM index was significantly higher in patients than controls (72 versus 59 g/m(2), P<0.0005). The magnitude of the backward compression waves was independently associated with variation in LVM (P=0.01). Using a novel, noninvasive hemodynamic assessment, we have shown abnormal conduit vessel function after coarctation of the aorta repair, including abnormal wave reflections that are associated with elevated LVM
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