2,466 research outputs found
Global environmental effects of impact-generated aerosols: Results from a general circulation model
Cooling and darkening at Earth's surface are expected to result from the interception of sunlight by the high altitude worldwide dust cloud generated by impact of a large asteroid or comet, according to the one-dimensional radioactive-convective atmospheric model (RCM) of Pollack et al. An analogous three-dimensional general circulation model (GCM) simulation obtains the same basic result as the RCM but there are important differences in detail. In the GCM simulation the heat capacity of the oceans, not included in the RCM, substantially mitigates land surface cooling. On the other hand, the GCM's low heat capacity surface allows surface temperatures to drop much more rapidly than reported by Pollack et al. These two differences between RCM and GCM simulations were noted previously in studies of nuclear winter; GCM results for comet/asteroid winter, however, are much more severe than for nuclear winter because the assumed aerosol amount is large enough to intercept all sunlight falling on Earth. In the simulation the global average of land surface temperature drops to the freezing point in just 4.5 days, one-tenth the time required in the Pollack et al. simulation. In addition to the standard case of Pollack et al., which represents the collision of a 10-km diameter asteroid with Earth, additional scenarios are considered ranging from the statistically more frequent impacts of smaller asteroids to the collision of Halley's comet with Earth. In the latter case the kinetic energy of impact is extremely large due to the head-on collision resulting from Halley's retrograde orbit
2000-times repeated imaging of strontium atoms in clock-magic tweezer arrays
We demonstrate single-atom resolved imaging with a survival probability of
and a fidelity of , enabling us to perform repeated
high-fidelity imaging of single atoms in tweezers for thousands of times. We
further observe lifetimes under laser cooling of more than seven minutes, an
order of magnitude longer than in previous tweezer studies. Experiments are
performed with strontium atoms in tweezer arrays, which is at
a magic wavelength for the clock transition. Tuning to this wavelength is
enabled by off-magic Sisyphus cooling on the intercombination line, which lets
us choose the tweezer wavelength almost arbitrarily. We find that a single not
retro-reflected cooling beam in the radial direction is sufficient for
mitigating recoil heating during imaging. Moreover, this cooling technique
yields temperatures below K, as measured by release and recapture.
Finally, we demonstrate clock-state resolved detection with average survival
probability of and average state detection fidelity of .
Our work paves the way for atom-by-atom assembly of large defect-free arrays of
alkaline-earth atoms, in which repeated interrogation of the clock transition
is an imminent possibility.Comment: 6 pages, 5 figures, 1 vide
Alkaline earth atoms in optical tweezers
We demonstrate single-shot imaging and narrow-line cooling of individual
alkaline earth atoms in optical tweezers; specifically, strontium-88 atoms
trapped in light. We achieve high-fidelity
single-atom-resolved imaging by detecting photons from the broad singlet
transition while cooling on the narrow intercombination line, and extend this
technique to highly uniform two-dimensional arrays of tweezers. Cooling
during imaging is based on a previously unobserved narrow-line Sisyphus
mechanism, which we predict to be applicable in a wide variety of experimental
situations. Further, we demonstrate optically resolved sideband cooling of a
single atom close to the motional ground state of a tweezer. Precise
determination of losses during imaging indicate that the branching ratio from
P to D is more than a factor of two larger than commonly
quoted, a discrepancy also predicted by our ab initio calculations. We also
measure the differential polarizability of the intercombination line in a
tweezer and achieve a magic-trapping configuration by tuning
the tweezer polarization from linear to elliptical. We present calculations, in
agreement with our results, which predict a magic crossing for linear
polarization at and a crossing independent of polarization
at 500.65(50)nm. Our results pave the way for a wide range of novel
experimental avenues based on individually controlled alkaline earth atoms in
tweezers -- from fundamental experiments in atomic physics to quantum
computing, simulation, and metrology implementations
Risk Perception of Cigarette and E-cigarette use during Pregnancy: A Qualitative Postpartum Perspective
Aim: The aim of this exploratory qualitative analysis is to assess the perceptions of risks of cigarette and e-cigarette use during pregnancy. Background: An important public health aim is a reduction of smoking at time of delivery (SATOD) from 10.6% to less than 6% by 2022 in the United Kingdom (UK). In order to successfully meet this target, we need to have a better understanding of the perceived risks associated with cigarette smoking. Additionally, the use of e-cigarettes is increasing in the general population, with pregnant women being supported to use such products if it helps them remain smoke free. However, in contrast to cigarette smoking, there is little definitive research assessing the safety of e-cigarette use during pregnancy, with most information disregarding the health of the growing fetus. E-cigarettes are of special interest, given they are an unlicensed product for use during pregnancy, yet women are being supported to use them as a method of harm reduction. A better understanding of perceived risks is essential. Method: Fourteen interviews were conducted one month postpartum with women who smoked during pregnancy and continued to smoke after the birth. Thematic analysis was conducted. Suzanne Froggatt, Nadja Reissland, Judith Covey 2 Findings: Two themes emerged for cigarette smoking; health and justifications. Six themes were identified for e-cigarette use; the unknown, experience, comparison to cigarettes, the product, advice and healthier option. A range of subthemes are discussed. Conclusion: Women provided a range of justifications for continuing to smoke during pregnancy. Women felt e-cigarettes were a riskier option than continuing to smoke
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