Cold atoms in micromachined waveguides: A new platform for atom-photon interactions

Hybrid quantum devices, incorporating both atoms and photons, can exploit the benefits of both to enable scalable architectures for quantum computing and quantum communication, as well as chip-scale sensors and single-photon sources. Production of such devices depends on the development of an interface between their atomic and photonic components. This should be compact, robust, and compatible with existing technologies from both fields. Here we demonstrate such an interface. Cold cesium atoms are trapped inside a transverse, 30μm-diameter through hole in an optical fiber, created via laser micromachining. When the guided light is on resonance with the cesium D2 line, up to 87% of it is absorbed by the atoms. The corresponding optical depth per unit length is ∼700 cm−1, higher than any reported for a comparable system. This is important for miniaturization and scalability. The technique can be equally effective in optical waveguide chips and other existing photonic systems, providing a promising platform for fundamental research

Tests for Substructure in Gravitational Lenses

The flux anomalies in four-image gravitational lenses can be interpreted as evidence for the dark matter substructure predicted by cold dark matter (CDM) halo models. In principle, these flux anomalies could arise from alternate sources such as absorption, scattering or scintillation by the interstellar medium (ISM) of the lens galaxy, problems in the ellipsoidal macro models used to fit lens systems, or stellar microlensing. We apply several tests to the data that appear to rule out these alternate explanations. First, the radio flux anomalies show no significant dependence on wavelength, as would be expected for almost any propagation effect in the ISM or microlensing by the stars. Second, the flux anomaly distributions show the characteristic demagnifications of the brightest saddle point relative to the other images expected for low optical depth substructure, which cannot be mimicked by either the ISM or problems in the macro models. Microlensing by stars also cannot reproduce the suppression of the bright saddle points if the radio source sizes are consistent with the Compton limit for their angular sizes. Third, while it is possible to change the smooth lens models to fit the flux anomalies in some systems, we can rule out the necessary changes in all systems where we have additional lens constraints to check the models. Moreover, the parameters of these models are inconsistent with our present observations and expectations for the structure of galaxies. We conclude that low-mass halos remain the best explanation of the phenomenon.Comment: Submitted to Ap

Benthic suspension feeders, key players in Antartic marine ecosystems?

Ja fa un cert temps que hom accepta, de manera general i a partir d'estudis de la columna d'aigua en mar obert, que en les comunitats marines de l'oceà Antàrtic hi ha un període prolongat d'activitat hivernal mínima que s'estén al llarg de sis mesos com a mínim, que és el temps que dura l'hivern austral. Tanmateix, investigacions recents en algunes espècies d'invertebrats suspensívors (filtradors) bentònics litorals han suggerit que el període d'inactivitat potser dura només unes poques setmanes. Això planteja una interessant qüestió: què és el que permet que aquests organismes antàrtics mengin, i per tant romanguin actius, durant períodes molt més llargs del que fins ara s'havia cregut? Hom ha suggerit adaptacions especials a menjar en concentracions molt baixes d'aliment, o bé a usar eficientment l'abundància ocasional d'aliment enmig de llargs períodes d'escassedat. Molts animals suspensívors podrien usar fonts alternatives al seu aliment usual, el fito- i el zooplàncton; per exemple, la ?fracció fina' de matèria orgànica, o microplàncton (bacteris, ciliats i flagel·lats, junt amb carboni inorgànic particulat d'orígens diversos), que hom ha demostrat recentment que té un paper molt més significatiu del que hom creia a l'hora de complementar la dieta i de subvenir a les demandes energètiques de moltes espècies de suspensívors. També s'ha esmentat el possible paper de la resuspensió de sediments i, atès l'elevat valor alimentari del component orgànic dels sediments antàrtics, àdhuc de fons pregons, serà interessant investigar més en aquesta línia. Observacions recents relacionades amb la dieta natural (captura de preses) d'algunes espècies d'aigües somes porten a suposar que alguns suspensívors antàrtics, si més no, tenen taxes de creixement i reproducció que són similars a les assenyalades per a espècies d'aigües temperades.For quite some time it was generally accepted, from water column and open sea studies, that in Antarctic marine communities there is a prolonged period of minimal winter activity which lasts for at least six months during the Southern winter. However, recent studies on certain littoral benthic suspension feeders have suggested that the period of inactivity may last only a few weeks. This raises the question of what allows these organisms to feed, and hence remain active, for considerably longer periods than previously thought. Special adaptations to feeding at low levels of food concentration, or to using occasional food abundance followed by long periods of starvation have been suggested. Many suspension feeders might use alternative food sources to phytoand zooplankton, e.g., the «fine fraction» of organic matter, or microplankton (bacteria, ciliates, and flagellates including POC of several origins), which has recently been shown to play a much more significant role than expected in complementing the diet and meeting energy demands in many species of suspension feeders. The possible role of sediment resuspension has also been reported, and given the high food value of the organic component of sediments even on deep bottoms further research would be of interest. Recent observations related to the natural diet (prey capture) of several shallow species lead to the assumption that part of Antarctic suspension feeders have similar growth and reproduction rates to those reported for temperate waters. Efficient food assimilation may contribute to our understanding of the continuous reproduction state observed in octocorals and, also, of the development of tridimensionally structured communities which are suspension-feeder dominated, highly diversified, and have a high biomass

A family of complex potentials with real spectrum

We consider a two-parameter non hermitean quantum-mechanical hamiltonian that is invariant under the combined effects of parity and time reversal transformation. Numerical investigation shows that for some values of the potential parameters the hamiltonian operator supports real eigenvalues and localized eigenfunctions. In contrast with other PT symmetric models, which require special integration paths in the complex plane, our model is integrable along a line parallel to the real axis.Comment: Six figures and four table

Integrability and chaos: the classical uncertainty

In recent years there has been a considerable increase in the publishing of textbooks and monographs covering what was formerly known as random or irregular deterministic motion, now named by the more fashionable term of deterministic chaos. There is still substantial interest in a matter that is included in many graduate and even undergraduate courses on classical mechanics. Based on the Hamiltonian formalism, the main objective of this article is to provide, from the physicist's point of view, an overall and intuitive review of this broad subject (with some emphasis on the KAM theorem and the stability of planetary motions) which may be useful to both students and instructors.Comment: 24 pages, 10 figure

The kinematics in the pc-scale jets of AGN The case of S5 1803+784

We present a kinematic analysis of jet component motion in the VLBI jet of the BL Lac object S5 1803+784, which does not reveal long-term outward motion for most of the components. Understanding the complex kinematic phenomena can possibly provide insights into the differences between quasars and BL Lac objects. The blazar S5 1803+784 has been studied with VLBI at $\nu$ =1.6, 2.3, 5, 8.4, and 15 GHz between 1993.88 and 2005.68 in 26 observing runs. We (re)analyzed the data and present Gaussian model-fits. We collected the already published kinematic information for this source from the literature and re-identified the components according to the new scenario presented in this paper. Altogether, 94 epochs of observations have been investigated. A careful study of the long-term kinematics reveals a new picture for component motion in S5 1803+784. In contrast to previously discussed motion scenarios, we find that the jet structure within 12 mas of the core can most easily be described by the coexistence of several bright jet features that remain on the long-term at roughly constant core separations (in addition to the already known {\it stationary} jet component $\sim$ 1.4 mas) and one faint component moving with an apparent superluminal speed ($\sim$ 19c, based on 3 epochs). While most of the components maintain long-term roughly constant distances from the core, we observe significant, smooth changes in their position angles. We report on an evolution of the whole jet ridge line with time over the almost 12 years of observations. The width of the jet changes periodically with a period of $\sim$ 8 to 9 years. We find a correlation between changes in the position angle and maxima in the total flux-density. We present evidence for a geometric origin of the phenomena and discuss possible models.Comment: The manuscript will be published by A&

Adult beginner distance language learner perceptions and use of assignment feedback

This qualitative study examines perceptions and use of assignment feedback among adult beginner modern foreign language learners on higher education distance learning courses. A survey of responses to feedback on assignments by 43 Open University students on beginner language courses in Spanish, French, and German indicated that respondents can be classified into three groups: those who use feedback strategically by integrating it into the learning process and comparing it with, for example, informal feedback from interaction with native speakers, those who take note of feedback, but seem not to use it strategically, and those who appear to take little account of either marks or feedback. The first group proved to be the most confident and most likely to maintain their motivation in the longer term. The conclusion discusses some of the pedagogical and policy implications of the findings

23 GHz VLBI Observations of SN 2008ax

We report on phase-referenced 23 GHz Very-Long-Baseline-Interferometry (VLBI) observations of the type IIb supernova SN 2008ax, made with the Very Long Baseline Array (VLBA) on 2 April 2008 (33 days after explosion). These observations resulted in a marginal detection of the supernova. The total flux density recovered from our VLBI image is 0.8$\pm$0.3 mJy (one standard deviation). As it appears, the structure may be interpreted as either a core-jet or a double source. However, the supernova structure could be somewhat confused with a possible close by noise peak. In such a case, the recovered flux density would decrease to 0.48$\pm$0.12 mJy, compatible with the flux densities measured with the VLA at epochs close in time to our VLBI observations. The lowest average expansion velocities derived from our observations are $(1.90 \pm 0.30) \times 10^5$ km s$^{-1}$ (case of a double source) and $(5.2 \pm 1.3) \times 10^4$ km s$^{-1}$ (taking the weaker source component as a spurious, close by, noise peak, which is the more likely interpretation). These velocities are 7.3 and 2 times higher, respectively, than the maximum ejecta velocity inferred from optical-line observations.Comment: 4 pages, 3 figures. Accepted in A&A on 24/03/200