Quantum Fluctuations of Coulomb Potential as a Source of Flicker Noise. The Influence of External Electric Field

Fluctuations of the electromagnetic field produced by quantized matter in external electric field are investigated. A general expression for the power spectrum of fluctuations is derived within the long-range expansion. It is found that in the whole measured frequency band, the power spectrum of fluctuations exhibits an inverse frequency dependence. A general argument is given showing that for all practically relevant values of the electric field, the power spectrum of induced fluctuations is proportional to the field strength squared. As an illustration, the power spectrum is calculated explicitly using the kinetic model with the relaxation-type collision term. Finally, it is shown that the magnitude of fluctuations produced by a sample generally has a Gaussian distribution around its mean value, and its dependence on the sample geometry is determined. In particular, it is demonstrated that for geometrically similar samples, the power spectrum is inversely proportional to the sample volume. Application of the obtained results to the problem of flicker noise is discussed.Comment: 14 pages, 3 figure

On the exchange of intersection and supremum of sigma-fields in filtering theory

We construct a stationary Markov process with trivial tail sigma-field and a nondegenerate observation process such that the corresponding nonlinear filtering process is not uniquely ergodic. This settles in the negative a conjecture of the author in the ergodic theory of nonlinear filters arising from an erroneous proof in the classic paper of H. Kunita (1971), wherein an exchange of intersection and supremum of sigma-fields is taken for granted.Comment: 20 page

Wayward Youth: Trans-Beringian Movement and Differential Southward Migration by Juvenile Sharp-tailed Sandpipers

The sharp-tailed sandpiper (Calidris acuminata) is a long-distance migrant that travels each year from breeding grounds in the Russian Arctic to nonbreeding areas in Australasia. Most adults migrate rapidly from breeding grounds along a largely inland route through Asia. Here we report on the highly unusual migratory strategy of this species in which some juveniles, but virtually no adults, take a pronounced detour to western Alaska before proceeding on southward migration. We analyzed data from our own studies in this region and published and unpublished observations and specimen records of sharptailed sandpipers from the entire Pacific Basin. Each autumn, sharp-tailed sandpipers began arriving on coastal graminoid meadows and intertidal habitats throughout western Alaska during the last half of August and the last sandpipers departed from southwestern Alaska during October and November. Body mass of birds banded or collected across multiple years and sites in western Alaska (n = 330) increased by an average of 0.57 ± 0.06 g per day between mid-August and late October. Records suggest a small, regular movement of juveniles (and a very few adults) along the Asiatic coast, but we estimate from surveys that a few tens of thousands of juveniles stage in western Alaska each autumn. The distribution of sight and specimen records from the Pacific Basin during autumn suggests strongly age-segregated migration routes, with the principal migration of juveniles crossing central and western Oceania in a possible nonstop trans-Pacific flight from Alaska. This is only the second well-documented case of differential migration among birds that involves different routes for adults and juveniles, and it raises intriguing questions about how and why this system has evolved.  Le bécasseau à queue pointue (Calidris acuminata) est un migrant de longue distance qui se déplace chaque année depuis les zones de reproduction de l’Arctique russe jusqu’aux zones de non-reproduction de l’Australasie. La plupart des adultes migrent rapidement à partir des zones de reproduction le long d’un corridor largement situé à l’intérieur qui traverse l’Asie. Ici, nous faisons état de la stratégie migratoire grandement inhabituelle de cette espèce dans le cadre de laquelle certains juvéniles, mais quasiment aucun adulte, font une déviation prononcée vers l’ouest de l’Alaska avant de migrer vers le Sud. Nous avons analysé les données dérivées de nos propres études dans la région de même que des observations publiées et inédites et des enregistrements de spécimens de bécasseaux à queue pointue de tout le bassin du Pacifique. Chaque automne, les bécasseaux à queue pointue commençaient à arriver sur les prés côtiers de graminoïdes et dans les habitats intertidaux de l’ouest de l’Alaska pendant la deuxième moitié du mois d’août. Les derniers bécasseaux quittaient le sud-ouest de l’Alaska aux mois d’octobre et de novembre. La masse corporelle des oiseaux en bandes ou recueillis au cours de plusieurs années et à plusieurs emplacements de l’ouest de l’Alaska (n = 330) a augmenté en moyenne de 0,57 ± 0,06 g par jour entre la mi-août et la fin octobre. Les données laissent voir la présence d’un petit mouvement régulier de juvéniles (et très peu d’adultes) le long de la côte asiatique, mais nous avons estimé d’après les dénombrements que quelques dizaines de milliers de juvéniles passent un certain temps dans l’ouest de l’Alaska chaque automne. La répartition d’enregistrements d’observations et de spécimens du bassin du Pacifique à l’automne laisse entrevoir des routes migratoires fortement ségrégées en fonction de l’âge et que la migration principale de juvéniles traversant le centre et l’ouest de l’Océanie dans le cadre d’un vol transpacifique est susceptible d’être sans escale depuis l’Alaska. Il s’agit seulement du deuxième cas bien répertorié de migration différentielle d’oiseaux dont les trajets diffèrent chez les adultes et les juvéniles, et cela soulève des questions à savoir comment et pourquoi ce système a évolué

Langevin Thermostat for Rigid Body Dynamics

We present a new method for isothermal rigid body simulations using the quaternion representation and Langevin dynamics. It can be combined with the traditional Langevin or gradient (Brownian) dynamics for the translational degrees of freedom to correctly sample the NVT distribution in a simulation of rigid molecules. We propose simple, quasi-symplectic second-order numerical integrators and test their performance on the TIP4P model of water. We also investigate the optimal choice of thermostat parameters.Comment: 15 pages, 13 figures, 1 tabl

A discrete invitation to quantum filtering and feedback control

The engineering and control of devices at the quantum-mechanical level--such as those consisting of small numbers of atoms and photons--is a delicate business. The fundamental uncertainty that is inherently present at this scale manifests itself in the unavoidable presence of noise, making this a novel field of application for stochastic estimation and control theory. In this expository paper we demonstrate estimation and feedback control of quantum mechanical systems in what is essentially a noncommutative version of the binomial model that is popular in mathematical finance. The model is extremely rich and allows a full development of the theory, while remaining completely within the setting of finite-dimensional Hilbert spaces (thus avoiding the technical complications of the continuous theory). We introduce discretized models of an atom in interaction with the electromagnetic field, obtain filtering equations for photon counting and homodyne detection, and solve a stochastic control problem using dynamic programming and Lyapunov function methods.Comment: 76 pages, 12 figures. A PDF file with high resolution figures can be found at http://minty.caltech.edu/papers.ph

Introduction : contemporary orientations in African cultural studies

Abstract: This paper offers a glimpse of work generated by the 2014 John Douglas Taylor conference on ‘Contemporary Orientations in African Cultural Studies’. The conference generated a number of inquiries into the time and place of contemporary African cultural work, many of which theorized beyond the frameworks that postcolonial and globalization studies frequently offer. Under the shifting paradigms of cultural studies, the work of this conference, as well as the current project, moves away from reading the African everyday as exclusively a construction out of a series of colonial histories and relationalities, or global cultural flows. In line with Jean and John Comaroffs’ Theory From the South, this issue is instead dedicated to relocating the global centres from which cultural studies emanates and positing African work’s challenge to normative zones of cultural critique. ‘Contemporary orientations’ attempts to relocate the time and space of critique in African studies, but it resists the gesture to posit a stable trajectory through which time moves. Rather, the terms of the contemporary and the orientation depend on how they are read in relation to a multitude of other temporalities, orientations, and objects

Leukocyte adhesion: reconceptualizing chemokine presentation by glycosaminoglycans

Recruitment of immune cells from the vasculature relies on the presentation of glycosaminoglycan-bound chemokines on the luminal side of vascular endothelial cells. However, the current model of chemokine–glycosaminoglycan interactions, and its implications for receptor interactions, remains poorly developed. We propose a refined ‘Chemokine Cloud’ model, arguing that chemokines are not presented to leukocytes bound to glycosaminoglycans, but rather, in solution while sequestered within the hydrated glycocalyx. We posit that glycosaminoglycans provide an immobilized chemokine depot maintaining a ‘cloud’ of ‘solution-phase’ chemokines within the glycocalyx, and that it is this soluble form of any given chemokine that interacts with leukocyte-bound receptors. Our proposition clarifies certain anomalies associated with the current model of chemokine–glycosaminoglycan interactions, with implications for the design of blockers of chemokine function

Neuraminidase Inhibitor Resistance in Influenza: Assessing the Danger of Its Generation and Spread

Neuraminidase Inhibitors (NI) are currently the most effective drugs against influenza. Recent cases of NI resistance are a cause for concern. To assess the danger of NI resistance, a number of studies have reported the fraction of treated patients from which resistant strains could be isolated. Unfortunately, those results strongly depend on the details of the experimental protocol. Additionally, knowing the fraction of patients harboring resistance is not too useful by itself. Instead, we want to know how likely it is that an infected patient can generate a resistant infection in a secondary host, and how likely it is that the resistant strain subsequently spreads. While estimates for these parameters can often be obtained from epidemiological data, such data is lacking for NI resistance in influenza. Here, we use an approach that does not rely on epidemiological data. Instead, we combine data from influenza infections of human volunteers with a mathematical framework that allows estimation of the parameters that govern the initial generation and subsequent spread of resistance. We show how these parameters are influenced by changes in drug efficacy, timing of treatment, fitness of the resistant strain, and details of virus and immune system dynamics. Our study provides estimates for parameters that can be directly used in mathematical and computational models to study how NI usage might lead to the emergence and spread of resistance in the population. We find that the initial generation of resistant cases is most likely lower than the fraction of resistant cases reported. However, we also show that the results depend strongly on the details of the within-host dynamics of influenza infections, and most importantly, the role the immune system plays. Better knowledge of the quantitative dynamics of the immune response during influenza infections will be crucial to further improve the results