243 research outputs found
Quasi-Stationary Distributions for Models of Heterogeneous Catalysis
We construct the quasi-stationary (QS) distribution for two models of
heterogeneous catalysis having two absorbing states: the ZGB model for the
oxidation of CO, and a version with noninstantaneous reactions. Using a
mean-field-like approximation, we study the quasi-stationary surface coverages,
moment ratios and the lifetime of the QS state. We also derive an improved,
consistent one-site mean-field theory for the ZGB model.Comment: 18 pages, 13 figure
Quantum Computing
Quantum mechanics---the theory describing the fundamental workings of
nature---is famously counterintuitive: it predicts that a particle can be in
two places at the same time, and that two remote particles can be inextricably
and instantaneously linked. These predictions have been the topic of intense
metaphysical debate ever since the theory's inception early last century.
However, supreme predictive power combined with direct experimental observation
of some of these unusual phenomena leave little doubt as to its fundamental
correctness. In fact, without quantum mechanics we could not explain the
workings of a laser, nor indeed how a fridge magnet operates. Over the last
several decades quantum information science has emerged to seek answers to the
question: can we gain some advantage by storing, transmitting and processing
information encoded in systems that exhibit these unique quantum properties?
Today it is understood that the answer is yes. Many research groups around the
world are working towards one of the most ambitious goals humankind has ever
embarked upon: a quantum computer that promises to exponentially improve
computational power for particular tasks. A number of physical systems,
spanning much of modern physics, are being developed for this task---ranging
from single particles of light to superconducting circuits---and it is not yet
clear which, if any, will ultimately prove successful. Here we describe the
latest developments for each of the leading approaches and explain what the
major challenges are for the future.Comment: 26 pages, 7 figures, 291 references. Early draft of Nature 464, 45-53
(4 March 2010). Published version is more up-to-date and has several
corrections, but is half the length with far fewer reference
Long-term cycles in the history of life: Periodic biodiversity in the Paleobiology Database
Time series analysis of fossil biodiversity of marine invertebrates in the
Paleobiology Database (PBDB) shows a significant periodicity at approximately
63 My, in agreement with previous analyses based on the Sepkoski database. I
discuss how this result did not appear in a previous analysis of the PBDB. The
existence of the 63 My periodicity, despite very different treatment of
systematic error in both PBDB and Sepkoski databases strongly argues for
consideration of its reality in the fossil record. Cross-spectral analysis of
the two datasets finds that a 62 My periodicity coincides in phase by 1.6 My,
equivalent to better than the errors in either measurement. Consequently, the
two data sets not only contain the same strong periodicity, but its peaks and
valleys closely correspond in time. Two other spectral peaks appear in the PBDB
analysis, but appear to be artifacts associated with detrending and with the
increased interval length. Sampling-standardization procedures implemented by
the PBDB collaboration suggest that the signal is not an artifact of sampling
bias. Further work should focus on finding the cause of the 62 My periodicity.Comment: Published in PLoS ONE. 5 pages, 3 figures. Version with live links,
discussion available at
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004044#to
Gene Flow between the Korean Peninsula and Its Neighboring Countries
SNP markers provide the primary data for population structure analysis. In this study, we employed whole-genome autosomal SNPs as a marker set (54,836 SNP markers) and tested their possible effects on genetic ancestry using 320 subjects covering 24 regional groups including Northern ( = 16) and Southern ( = 3) Asians, Amerindians ( = 1), and four HapMap populations (YRI, CEU, JPT, and CHB). Additionally, we evaluated the effectiveness and robustness of 50K autosomal SNPs with various clustering methods, along with their dependencies on recombination hotspots (RH), linkage disequilibrium (LD), missing calls and regional specific markers. The RH- and LD-free multi-dimensional scaling (MDS) method showed a broad picture of human migration from Africa to North-East Asia on our genome map, supporting results from previous haploid DNA studies. Of the Asian groups, the East Asian group showed greater differentiation than the Northern and Southern Asian groups with respect to Fst statistics. By extension, the analysis of monomorphic markers implied that nine out of ten historical regions in South Korea, and Tokyo in Japan, showed signs of genetic drift caused by the later settlement of East Asia (South Korea, Japan and China), while Gyeongju in South East Korea showed signs of the earliest settlement in East Asia. In the genome map, the gene flow to the Korean Peninsula from its neighboring countries indicated that some genetic signals from Northern populations such as the Siberians and Mongolians still remain in the South East and West regions, while few signals remain from the early Southern lineages
The Pioneer Anomaly
Radio-metric Doppler tracking data received from the Pioneer 10 and 11
spacecraft from heliocentric distances of 20-70 AU has consistently indicated
the presence of a small, anomalous, blue-shifted frequency drift uniformly
changing with a rate of ~6 x 10^{-9} Hz/s. Ultimately, the drift was
interpreted as a constant sunward deceleration of each particular spacecraft at
the level of a_P = (8.74 +/- 1.33) x 10^{-10} m/s^2. This apparent violation of
the Newton's gravitational inverse-square law has become known as the Pioneer
anomaly; the nature of this anomaly remains unexplained. In this review, we
summarize the current knowledge of the physical properties of the anomaly and
the conditions that led to its detection and characterization. We review
various mechanisms proposed to explain the anomaly and discuss the current
state of efforts to determine its nature. A comprehensive new investigation of
the anomalous behavior of the two Pioneers has begun recently. The new efforts
rely on the much-extended set of radio-metric Doppler data for both spacecraft
in conjunction with the newly available complete record of their telemetry
files and a large archive of original project documentation. As the new study
is yet to report its findings, this review provides the necessary background
for the new results to appear in the near future. In particular, we provide a
significant amount of information on the design, operations and behavior of the
two Pioneers during their entire missions, including descriptions of various
data formats and techniques used for their navigation and radio-science data
analysis. As most of this information was recovered relatively recently, it was
not used in the previous studies of the Pioneer anomaly, but it is critical for
the new investigation.Comment: 165 pages, 40 figures, 16 tables; accepted for publication in Living
Reviews in Relativit
A pilot survey of post-deployment health care needs in small community-based primary care clinics
<p>Abstract</p> <p>Background</p> <p>Relatively little is known regarding to what extent community-based primary care physicians are encountering post-deployment health care needs among veterans of the Afghanistan or Iraq conflicts and their family members.</p> <p>Methods</p> <p>This pilot study conducted a cross-sectional survey of 37 primary care physicians working at small urban and suburban clinics belonging to a practice-based research network in the south central region of Texas.</p> <p>Results</p> <p>Approximately 80% of the responding physicians reported caring for patients who have been deployed to the Afghanistan or Iraq war zones, or had a family member deployed. Although these physicians noted a variety of conditions related to physical trauma, mental illnesses and psychosocial disruptions such as marital, family, financial, and legal problems appeared to be even more prevalent among their previously deployed patients and were also noted among family members of deployed veterans.</p> <p>Conclusions</p> <p>Community-based primary care physicians should be aware of common post-deployment health conditions and the resources that are available to meet these needs.</p
Noise Reduction by Diffusional Dissipation in a Minimal Quorum Sensing Motif
Cellular interactions are subject to random fluctuations (noise) in quantities of interacting molecules. Noise presents a major challenge for the robust function of natural and engineered cellular networks. Past studies have analyzed how noise is regulated at the intracellular level. Cell–cell communication, however, may provide a complementary strategy to achieve robust gene expression by enabling the coupling of a cell with its environment and other cells. To gain insight into this issue, we have examined noise regulation by quorum sensing (QS), a mechanism by which many bacteria communicate through production and sensing of small diffusible signals. Using a stochastic model, we analyze a minimal QS motif in Gram-negative bacteria. Our analysis shows that diffusion of the QS signal, together with fast turnover of its transcriptional regulator, attenuates low-frequency components of extrinsic noise. We term this unique mechanism “diffusional dissipation” to emphasize the importance of fast signal turnover (or dissipation) by diffusion. We further show that this noise attenuation is a property of a more generic regulatory motif, of which QS is an implementation. Our results suggest that, in a QS system, an unstable transcriptional regulator may be favored for regulating expression of costly proteins that generate public goods
On computational approaches for size-and-shape distributions from sedimentation velocity analytical ultracentrifugation
Sedimentation velocity analytical ultracentrifugation has become a very popular technique to study size distributions and interactions of macromolecules. Recently, a method termed two-dimensional spectrum analysis (2DSA) for the determination of size-and-shape distributions was described by Demeler and colleagues (Eur Biophys J 2009). It is based on novel ideas conceived for fitting the integral equations of the size-and-shape distribution to experimental data, illustrated with an example but provided without proof of the principle of the algorithm. In the present work, we examine the 2DSA algorithm by comparison with the mathematical reference frame and simple well-known numerical concepts for solving Fredholm integral equations, and test the key assumptions underlying the 2DSA method in an example application. While the 2DSA appears computationally excessively wasteful, key elements also appear to be in conflict with mathematical results. This raises doubts about the correctness of the results from 2DSA analysis
Prediction by Promoter Logic in Bacterial Quorum Sensing
Quorum-sensing systems mediate chemical communication between bacterial cells, coordinating cell-density-dependent processes like biofilm formation and virulence-factor expression. In the proteobacterial LuxI/LuxR quorum sensing paradigm, a signaling molecule generated by an enzyme (LuxI) diffuses between cells and allosterically stimulates a transcriptional regulator (LuxR) to activate its cognate promoter (pR). By expressing either LuxI or LuxR in positive feedback from pR, these versatile systems can generate smooth (monostable) or abrupt (bistable) density-dependent responses to suit the ecological context. Here we combine theory and experiment to demonstrate that the promoter logic of pR – its measured activity as a function of LuxI and LuxR levels – contains all the biochemical information required to quantitatively predict the responses of such feedback loops. The interplay of promoter logic with feedback topology underlies the versatility of the LuxI/LuxR paradigm: LuxR and LuxI positive-feedback systems show dramatically different responses, while a dual positive/negative-feedback system displays synchronized oscillations. These results highlight the dual utility of promoter logic: to probe microscopic parameters and predict macroscopic phenotype
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