9,697 research outputs found
Calibration biases in measurements of weak lensing
As recently shown by Viola et al., the common (KSB) method for measuring weak
gravitational shear creates a non-linear relation between the measured and the
true shear of objects. We investigate here what effect such a non-linear
calibration relation may have on cosmological parameter estimates from weak
lensing if a simpler, linear calibration relation is assumed. We show that the
non-linear relation introduces a bias in the shear-correlation amplitude and
thus a bias in the cosmological parameters Omega_matter and sigma_8. Its
direction and magnitude depends on whether the point-spread function is narrow
or wide compared to the galaxy images from which the shear is measured.
Substantial over- or underestimates of the cosmological parameters are equally
possible, depending also on the variant of the KSB method. Our results show
that for trustable cosmological-parameter estimates from measurements of weak
lensing, one must verify that the method employed is free from
ellipticity-dependent biases or monitor that the calibration relation inferred
from simulations is applicable to the survey at hand.Comment: 5 pages, 3 figures, submitted to A&
MARKOV DIFFUSIONS IN COMOVING COORDINATES AND STOCHASTIC QUANTIZATION OF THE FREE RELATIVISTIC SPINLESS PARTICLE
We revisit the classical approach of comoving coordinates in relativistic
hydrodynamics and we give a constructive proof for their global existence under
suitable conditions which is proper for stochastic quantization. We show that
it is possible to assign stochastic kinematics for the free relativistic
spinless particle as a Markov diffusion globally defined on . Then
introducing dynamics by means of a stochastic variational principle with
Einstein's action, we are lead to positive-energy solutions of Klein-Gordon
equation. The procedure exhibits relativistic covariance properties.Comment: 31 pages + 1 figure available upon request; Plain REVTe
Dynamically Error-Corrected Gates for Universal Quantum Computation
Scalable quantum computation in realistic devices requires that precise
control can be implemented efficiently in the presence of decoherence and
operational errors. We propose a general constructive procedure for designing
robust unitary gates on an open quantum system without encoding or measurement
overhead. Our results allow for a low-level error correction strategy solely
based on Hamiltonian engineering using realistic bounded-strength controls and
may substantially reduce implementation requirements for fault-tolerant quantum
computing architectures.Comment: 5 pages, 3 figure
Weak gravitational lensing with DEIMOS
We introduce a novel method for weak-lensing measurements, which is based on
a mathematically exact deconvolution of the moments of the apparent brightness
distribution of galaxies from the telescope's PSF. No assumptions on the shape
of the galaxy or the PSF are made. The (de)convolution equations are exact for
unweighted moments only, while in practice a compact weight function needs to
be applied to the noisy images to ensure that the moment measurement yields
significant results. We employ a Gaussian weight function, whose centroid and
ellipticity are iteratively adjusted to match the corresponding quantities of
the source. The change of the moments caused by the application of the weight
function can then be corrected by considering higher-order weighted moments of
the same source. Because of the form of the deconvolution equations, even an
incomplete weighting correction leads to an excellent shear estimation if
galaxies and PSF are measured with a weight function of identical size. We
demonstrate the accuracy and capabilities of this new method in the context of
weak gravitational lensing measurements with a set of specialized tests and
show its competitive performance on the GREAT08 challenge data. A complete C++
implementation of the method can be requested from the authors.Comment: 7 pages, 3 figures, fixed typo in Eq. 1
Description of Quantum Entanglement with Nilpotent Polynomials
We propose a general method for introducing extensive characteristics of
quantum entanglement. The method relies on polynomials of nilpotent raising
operators that create entangled states acting on a reference vacuum state. By
introducing the notion of tanglemeter, the logarithm of the state vector
represented in a special canonical form and expressed via polynomials of
nilpotent variables, we show how this description provides a simple criterion
for entanglement as well as a universal method for constructing the invariants
characterizing entanglement. We compare the existing measures and classes of
entanglement with those emerging from our approach. We derive the equation of
motion for the tanglemeter and, in representative examples of up to four-qubit
systems, show how the known classes appear in a natural way within our
framework. We extend our approach to qutrits and higher-dimensional systems,
and make contact with the recently introduced idea of generalized entanglement.
Possible future developments and applications of the method are discussed.Comment: 40 pages, 7 figures, 1 table, submitted for publication. v2: section
II.E has been changed and the Appendix on "Four qubit sl-entanglement
measure" has been removed. There are changes in the notation of section IV.
Typos and language mistakes has been corrected. A figure has been added and a
figure has been replaced. The references have been update
How does gas cool in DM halos?
In order to study the process of cooling in dark-matter (DM) halos and assess
how well simple models can represent it, we run a set of radiative SPH
hydrodynamical simulations of isolated halos, with gas sitting initially in
hydrostatic equilibrium within Navarro-Frenk-White (NFW) potential wells. [...]
After having assessed the numerical stability of the simulations, we compare
the resulting evolution of the cooled mass with the predictions of the
classical cooling model of White & Frenk and of the cooling model proposed in
the MORGANA code of galaxy formation. We find that the classical model predicts
fractions of cooled mass which, after about two central cooling times, are
about one order of magnitude smaller than those found in simulations. Although
this difference decreases with time, after 8 central cooling times, when
simulations are stopped, the difference still amounts to a factor of 2-3. We
ascribe this difference to the lack of validity of the assumption that a mass
shell takes one cooling time, as computed on the initial conditions, to cool to
very low temperature. [...] The MORGANA model [...] better agrees with the
cooled mass fraction found in the simulations, especially at early times, when
the density profile of the cooling gas is shallow. With the addition of the
simple assumption that the increase of the radius of the cooling region is
counteracted by a shrinking at the sound speed, the MORGANA model is also able
to reproduce for all simulations the evolution of the cooled mass fraction to
within 20-50 per cent, thereby providing a substantial improvement with respect
to the classical model. Finally, we provide a very simple fitting function
which accurately reproduces the cooling flow for the first ~10 central cooling
times. [Abridged]Comment: 15 pages, accepted by MNRA
Beyond the Platonic Brain: facing the challenge of individual differences in function-structure mapping
Single-bit Feedback and Quantum Dynamical Decoupling
Synthesizing an effective identity evolution in a target system subjected to
unwanted unitary or non-unitary dynamics is a fundamental task for both quantum
control and quantum information processing applications. Here, we investigate
how single-bit, discrete-time feedback capabilities may be exploited to enact
or to enhance quantum procedures for effectively suppressing unwanted dynamics
in a finite-dimensional open quantum system. An explicit characterization of
the joint unitary propagators correctable by a single-bit feedback strategy for
arbitrary evolution time is obtained. For a two-dimensional target system, we
show how by appropriately combining quantum feedback with dynamical decoupling
methods, concatenated feedback-decoupling schemes may be built, which can
operate under relaxed control assumptions and can outperform purely closed-loop
and open-loop protocols.Comment: 12 pages, 2 figure
Dynamical Generation of Noiseless Quantum Subsystems
We present control schemes for open quantum systems that combine decoupling
and universal control methods with coding procedures. By exploiting a general
algebraic approach, we show how appropriate encodings of quantum states result
in obtaining universal control over dynamically-generated noise-protected
subsystems with limited control resources. In particular, we provide an
efficient scheme for performing universal encoded quantum computation in a wide
class of systems subjected to linear non-Markovian quantum noise and supporting
Heisenberg-type internal Hamiltonians.Comment: 4 pages, no figures; REVTeX styl
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