854 research outputs found
Collective Uncertainty in Partially-Polarized and Partially-Decohered Spin-1/2 Systems
It has become common practice to model large spin ensembles as an effective
pseudospin with total angular momentum J = N x j, where j is the spin per
particle. Such approaches (at least implicitly) restrict the quantum state of
the ensemble to the so-called symmetric Hilbert space. Here, we argue that
symmetric states are not generally well-preserved under the type of decoherence
typical of experiments involving large clouds of atoms or ions. In particular,
symmetric states are rapidly degraded under models of decoherence that act
identically but locally on the different members of the ensemble. Using an
approach [Phys. Rev. A 78, 052101 (2008)] that is not limited to the symmetric
Hilbert space, we explore potential pitfalls in the design and interpretation
of experiments on spin-squeezing and collective atomic phenomena when the
properties of the symmetric states are extended to systems where they do not
apply.Comment: 13 pages, 7 figure
Efficient feedback controllers for continuous-time quantum error correction
We present an efficient approach to continuous-time quantum error correction
that extends the low-dimensional quantum filtering methodology developed by van
Handel and Mabuchi [quant-ph/0511221 (2005)] to include error recovery
operations in the form of real-time quantum feedback. We expect this paradigm
to be useful for systems in which error recovery operations cannot be applied
instantaneously. While we could not find an exact low-dimensional filter that
combined both continuous syndrome measurement and a feedback Hamiltonian
appropriate for error recovery, we developed an approximate reduced-dimensional
model to do so. Simulations of the five-qubit code subjected to the symmetric
depolarizing channel suggests that error correction based on our approximate
filter performs essentially identically to correction based on an exact quantum
dynamical model
Collective processes of an ensemble of spin-1/2 particles
When the dynamics of a spin ensemble are expressible solely in terms of
symmetric processes and collective spin operators, the symmetric collective
states of the ensemble are preserved. These many-body states, which are
invariant under particle relabeling, can be efficiently simulated since they
span a subspace whose dimension is linear in the number of spins. However, many
open system dynamics break this symmetry, most notably when ensemble members
undergo identical, but local, decoherence. In this paper, we extend the
definition of symmetric collective states of an ensemble of spin-1/2 particles
in order to efficiently describe these more general collective processes. The
corresponding collective states span a subspace which grows quadratically with
the number of spins. We also derive explicit formulae for expressing arbitrary
identical, local decoherence in terms of these states.Comment: 12 pages, see 0805.2910 for simulations using these method
Generalized Limits for Single-Parameter Quantum Estimation
We develop generalized bounds for quantum single-parameter estimation
problems for which the coupling to the parameter is described by intrinsic
multi-system interactions. For a Hamiltonian with -system
parameter-sensitive terms, the quantum limit scales as where is the
number of systems. These quantum limits remain valid when the Hamiltonian is
augmented by any parameter independent interaction among the systems and when
adaptive measurements via parameter-independent coupling to ancillas are
allowed.Comment: 4 pages, 1 figure. v2 typos correcte
Associations of neighborhood characteristics with active park use : an observational study in two cities in the USA and Belgium
Background: Public parks can be an important setting for physical activity promotion, but to increase park use and the activity levels of park users, the crucial attributes related to active park use need to be defined. Not only user characteristics and structural park attributes, but also characteristics of the surrounding neighborhood are important to examine. Furthermore, internationally comparable studies are needed, to find out if similar intervention strategies might be effective worldwide. The main aim of this study was to examine whether the overall number of park visitors and their activity levels depend on study site, neighborhood walkability and neighborhood income.
Methods: Data were collected in 20 parks in Ghent, Belgium and San Diego, USA. Two trained observers systematically coded park characteristics using the Environmental Assessment of Public Recreation Spaces (EAPRS) tool, and park user characteristics using the System for Observing Play and recreation in Communities (SOPARC) tool. Multilevel multiple regression models were conducted in MLwiN 2.25.
Results: In San Diego parks, activity levels of park visitors and number of vigorously active visitors were higher than in Ghent, while the number of visitors walking and the overall number of park visitors were lower. Neighborhood walkability was positively associated with the overall number of visitors, the number of visitors walking, number of sedentary visitors and mean activity levels of visitors. Neighborhood income was positively associated with the overall number of visitors, but negatively with the number of visitors being vigorously active.
Conclusions: Neighborhood characteristics are important to explain park use. Neighborhood walkability-related attributes should be taken into account when promoting the use of existing parks or creating new parks. Because no strong differences were found between parks in high-and low-income neighborhoods, it seems that promoting park use might be a promising strategy to increase physical activity in low-income populations, known to be at higher risk for overweight and obesity
Late Effects of Disturbed IGF Signaling in Congenital Diseases.
The biologic effects of insulin-like growth factor-1 (IGF-1) are mediated by specific
cell surface receptors. IGF-1 binding to the extracellular -subunits activates the tyrosine
kinase intrinsic to the cytoplasmic portion of the IGF-1 receptor, leading to autophosphorylation
of specific tyrosine residues in the receptor -subunit. One early molecular event that
links the receptor kinase to the biologic actions of IGF-1 is tyrosine phosphorylation of the
insulin receptor substrate family (IRS-1 to -4). IRS acts as a multisite ‘docking’ protein by
binding to downstream signal-transducing molecules. Phosphorylation of multiple tyrosine
residues results in the association of IRS-1 with the Src homology 2 (SH2) domains of other
cytoplasmic signaling proteins, including phosphatidylinositol 3 kinase, Syp, Grb2 and
Nck. By binding to Grb2, IRS proteins couple the IGF-1 receptor to the Ras/mitogenactivated
protein kinase pathway. This pathway regulates cell growth, differentiation and
proliferation. Severe pre- and postnatal growth retardation may arise from abnormalities of
IGF-1 signaling such as IGF-1-binding alterations and IGF-1 receptor mutations. Knockout
studies have shown severe growth impairment in mice lacking IRS family components or
Akt. Finally, in human placentas from pregnancies complicated by intrauterine growth
retardation, multiple alterations of IGF-1-signaling molecules have recently been described
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