695 research outputs found

    Direct imaging of a digital-micromirror device for configurable microscopic optical potentials

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    Programable spatial light modulators (SLMs) have significantly advanced the configurable optical trapping of particles. Typically, these devices are utilized in the Fourier plane of an optical system, but direct imaging of an amplitude pattern can potentially result in increased simplicity and computational speed. Here we demonstrate high-resolution direct imaging of a digital micromirror device (DMD) at high numerical apertures (NA), which we apply to the optical trapping of a Bose-Einstein condensate (BEC). We utilise a (1200 x 1920) pixel DMD and commercially available 0.45 NA microscope objectives, finding that atoms confined in a hybrid optical/magnetic or all-optical potential can be patterned using repulsive blue-detuned (532 nm) light with 630(10) nm full-width at half-maximum (FWHM) resolution, within 5% of the diffraction limit. The result is near arbitrary control of the density the BEC without the need for expensive custom optics. We also introduce the technique of time-averaged DMD potentials, demonstrating the ability to produce multiple grayscale levels with minimal heating of the atomic cloud, by utilising the high switching speed (20 kHz maximum) of the DMD. These techniques will enable the realization and control of diverse optical potentials for superfluid dynamics and atomtronics applications with quantum gases. The performance of this system in a direct imaging configuration has wider application for optical trapping at non-trivial NAs.Comment: 9 page

    Climate bifurcation during the last deglaciation?

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    There were two abrupt warming events during the last deglaciation, at the start of the Bølling-Allerød and at the end of the Younger Dryas, but their underlying dynamics are unclear. Some abrupt climate changes may involve gradual forcing past a bifurcation point, in which a prevailing climate state loses its stability and the climate tips into an alternative state, providing an early warning signal in the form of slowing responses to perturbations, which may be accompanied by increasing variability. Alternatively, short-term stochastic variability in the climate system can trigger abrupt climate changes, without early warning. Previous work has found signals consistent with slowing down during the last deglaciation as a whole, and during the Younger Dryas, but with conflicting results in the run-up to the Bølling-Allerød. Based on this, we hypothesise that a bifurcation point was approached at the end of the Younger Dryas, in which the cold climate state, with weak Atlantic overturning circulation, lost its stability, and the climate tipped irreversibly into a warm interglacial state. To test the bifurcation hypothesis, we analysed two different climate proxies in three Greenland ice cores, from the Last Glacial Maximum to the end of the Younger Dryas. Prior to the Bølling warming, there was a robust increase in climate variability but no consistent slowing down signal, suggesting this abrupt change was probably triggered by a stochastic fluctuation. The transition to the warm Bølling-Allerød state was accompanied by a slowing down in climate dynamics and an increase in climate variability. We suggest that the Bølling warming excited an internal mode of variability in Atlantic meridional overturning circulation strength, causing multi-centennial climate fluctuations. However, the return to the Younger Dryas cold state increased climate stability. We find no consistent evidence for slowing down during the Younger Dryas, or in a longer spliced record of the cold climate state before and after the Bølling-Allerød. Therefore, the end of the Younger Dryas may also have been triggered by a stochastic perturbation

    Extent of partial ice cover due to carbon cycle feedback in a zonal energy balance model

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    International audienceA global carbon cycle is introduced into a zonally averaged energy balance climate model. The physical model components are similar to those of Budyko (1969) and Sellers (1969). The new carbon components account for atmospheric carbon dioxide concentrations and the terrestrial and oceanic storage of carbon. Prescribing values for the sum of these carbon components, it is found that inclusion of a closed carbon cycle reduces the range of insolation over which stable partial ice cover solutions may occur. This highly simplified climate model also predicts that the estimated release of carbon from fossil fuel burning over the next hundred years could result in the eventual melting of the ice sheets. Keywords: climate, carbon cycle,zonal model, earth system modellin

    Early warning of climate tipping points from critical slowing down: comparing methods to improve robustness

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    We address whether robust early warning signals can, in principle, be provided before a climate tipping point is reached, focusing on methods that seek to detect critical slowing down as a precursor of bifurcation. As a test bed, six previously analysed datasets are reconsidered, three palaeoclimate records approaching abrupt transitions at the end of the last ice age and three models of varying complexity forced through a collapse of the Atlantic thermohaline circulation. Approaches based on examining the lag-1 autocorrelation function or on detrended fluctuation analysis are applied together and compared. The effects of aggregating the data, detrending method, sliding window length and filtering bandwidth are examined. Robust indicators of critical slowing down are found prior to the abrupt warming event at the end of the Younger Dryas, but the indicators are less clear prior to the Bølling-Allerød warming, or glacial termination in Antarctica. Early warnings of thermohaline circulation collapse can be masked by inter-annual variability driven by atmospheric dynamics. However, rapidly decaying modes can be successfully filtered out by using a long bandwidth or by aggregating data. The two methods have complementary strengths and weaknesses and we recommend applying them together to improve the robustness of early warnings

    Daisyworld: a review

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    Daisyworld is a simple planetary model designed to show the long-term effects of coupling between life and its environment. Its original form was introduced by James Lovelock as a defense against criticism that his Gaia theory of the Earth as a self-regulating homeostatic system requires teleological control rather than being an emergent property. The central premise, that living organisms can have major effects on the climate system, is no longer controversial. The Daisyworld model has attracted considerable interest from the scientific community and has now established itself as a model independent of, but still related to, the Gaia theory. Used widely as both a teaching tool and as a basis for more complex studies of feedback systems, it has also become an important paradigm for the understanding of the role of biotic components when modeling the Earth system. This paper collects the accumulated knowledge from the study of Daisyworld and provides the reader with a concise account of its important properties. We emphasize the increasing amount of exact analytic work on Daisyworld and are able to bring together and summarize these results from different systems for the first time. We conclude by suggesting what a more general model of life-environment interaction should be based on

    Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering.

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    types: JOURNAL ARTICLEA shift toward higher atmospheric oxygen concentration during the late Proterozoic has been inferred from multiple indirect proxies and is seen by many as a prerequisite for the emergence of complex animal life. However, the mechanisms controlling the level of oxygen throughout the Proterozoic and its eventual rise remain uncertain. Here we use a simple biogeochemical model to show that the balance between long-term carbon removal fluxes via terrestrial silicate weathering and ocean crust alteration plays a key role in determining atmospheric oxygen concentration. This balance may be shifted by changes in terrestrial weatherability or in the generation rate of oceanic crust. As a result, the terrestrial chemical weathering flux may be permanently altered-contrasting with the conventional view that the global silicate weathering flux must adjust to equal the volcanic CO2 degassing flux. Changes in chemical weathering flux in turn alter the long-term supply of phosphorus to the ocean, and therefore the flux of organic carbon burial, which is the long-term source of atmospheric oxygen. Hence we propose that increasing solar luminosity and a decrease in seafloor spreading rate over 1,500-500 Ma drove a gradual shift from seafloor weathering to terrestrial weathering, and a corresponding steady rise in atmospheric oxygen. Furthermore, increased terrestrial weatherability during the late Neoproterozoic may explain low temperature, increases in ocean phosphate, ocean sulfate, and atmospheric oxygen concentration at this time.NER

    Understanding global patterns of domestic cannabis cultivation

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    Purpose – Unlike other plant-based drugs, cannabis is increasingly grown within the country of consumption, requires minimal processing before consumption, and can be easily grown almost anywhere using indoor or outdoor cultivation techniques. Developments in agronomic technologies have led to global growth in domestic cultivation, both by cannabis users for self- and social-supply, and by more commercially-oriented growers. Cross-national research is needed to better understand who is involved in domestic cultivation, the diversity in cultivation practices and motivations, and cultivators’ interaction with the criminal justice system and cannabis control policies. Design/methodology/approach – The article introduces the Global Cannabis Cultivation Research Consortium (GCCRC), describes its evolution and aims, and outlines the methodology of its ongoing cross-national online survey of cannabis cultivation. Findings – Despite differing national contexts, the GCCRC successfully developed a core questionnaire to be used in different countries. It accommodates varying research interests through the addition of optional survey sections. The benefits to forming an international consortium to conduct web-based survey research include the sharing of expertise, recruitment efforts and problem-solving. Research limitations/implications – The article discusses the limitations of using non-representative online sampling and the strategies used to increase validity. Originality/value – The GCCRC is conducting the largest cross-national study of domestic cannabis cultivation to date. The aim is not only to better understand patterns of cannabis cultivation and how they differ between countries but also to build upon online engagement methodology with hidden populations

    The challenges and benefits of analyzing feedback comments in surveys: Lessons from a cross-national online survey of small-scale cannabis growers

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    It is common practice in survey questionnaires to include a general open and non-directive feedback question at the end, but the analysis of this type of data is rarely discussed in the methodological literature. While these open-ended comments can be useful, most researchers fail to report on this issue. The aim of this article is to illustrate and reflect upon the benefits and challenges of analyzing responses to open-ended feedback questions. The article describes the experiences of coding and analyzing data generated through a feedback question at the end of an international online survey with small-scale cannabis cultivators carried out by the Global Cannabis Cultivation Research Consortium. After describing the design and dataset of the web survey, the analytical approach and coding frame are presented. The analytical strategies chosen in this study illustrate the diversity and complexity of feedback comments which pose methodological challenges to researchers wishing to use them for data analyses. In this article, three types of feedback comments (political/policy comments, general comments of positive and negative appreciation, and methodological comments) are used to illustrate the difficulties and advantages of analyzing this type of data. The advantages of analyzing feedback comments are well known, but they seem to be rarely exploited. General feedback questions at the end of surveys are typically non-directive. If researchers want to use these data for research and analyses, they need a clear strategy. They ought to give enough thought to why they are including this type of question, and develop an analytical strategy at the design stage of the study
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