Observation of a Cooperative Radiation Force in the Presence of Disorder

Cooperative scattering of light by an extended object such as an atomic ensemble or a dielectric sphere is fundamentally different from scattering from many point-like scatterers such as single atoms. Homogeneous distributions tend to scatter cooperatively, whereas fluctuations of the density distribution increase the disorder and suppress cooperativity. In an atomic cloud, the amount of disorder can be tuned via the optical thickness, and its role can be studied via the radiation force exerted by the light on the atomic cloud. Monitoring cold $^{87}\text{Rb}$ atoms released from a magneto-optical trap, we present the first experimental signatures of radiation force reduction due to cooperative scattering. The results are in agreement with an analytical expression interpolating between the disorder and the cooperativity-dominated regimes

Cavity-enhanced superradiant Rayleigh scattering with ultra-cold and Bose-Einstein condensed atoms

We report on the observation of collective atomic recoil lasing and superradiant Rayleigh scattering with ultracold and Bose-Einstein condensed atoms in an optical ring cavity. Both phenomena are based on instabilities evoked by the collective interaction of light with cold atomic gases. This publication clarifies the link between the two effects. The observation of superradiant behavior with thermal clouds as hot as several tens of $\mu\textrm{K}$ proves that the phenomena are driven by the cooperative dynamics of the atoms, which is strongly enhanced by the presence of the ring cavity.Comment: 10 pages, 10 figure

Cooperative Scattering by Cold Atoms

We have studied the interplay between disorder and cooperative scattering for single scattering limit in the presence of a driving laser. Analytical results have been derived and we have observed cooperative scattering effects in a variety of experiments, ranging from thermal atoms in an optical dipole trap, atoms released from a dark MOT and atoms in a BEC, consistent with our theoretical predictions.Comment: submitted for special issue of PQE 201

In situ characterization of mixing and sedimentation dynamics in an impinging jet ballast tank via acoustic backscatter

Impinging jets are utilized in numerous applications, including nuclear waste treatment, for both the erosion of sediment beds and maintaining particulates in suspension. Pulse-echo ultrasonic methods offer great potential for the in situ monitoring of critical mixing and settling dynamics, in concentrated dispersions. A non-active scaled version of a Highly Active Storage Tank at Sellafield, UK, was profiled with an acoustic backscatter system under various jet firing conditions. An advanced analysis technique enabled the direct quantification of dispersion concentration changes from the converted backscatter attenuation. Hence, the erosion and mixing capability of the jets, and settling kinetics were characterized. It was found that jet operation alone provided inadequate localized mixing of eroded sediment. An additional air-lift process operation was required to hinder the rapid re-settling of dispersed particulates

Automorphisms of Partially Commutative Groups II: Combinatorial Subgroups

We define several "standard" subgroups of the automorphism group Aut(G) of a partially commutative (right-angled Artin) group and use these standard subgroups to describe decompositions of Aut(G). If C is the commutation graph of G, we show how Aut(G) decomposes in terms of the connected components of C: obtaining a particularly clear decomposition theorem in the special case where C has no isolated vertices. If C has no vertices of a type we call dominated then we give a semi-direct decompostion of Aut(G) into a subgroup of locally conjugating automorphisms by the subgroup stabilising a certain lattice of "admissible subsets" of the vertices of C. We then characterise those graphs for which Aut(G) is a product (not necessarily semi-direct) of two such subgroups.Comment: 7 figures, 63 pages. Notation and definitions clarified and typos corrected. 2 new figures added. Appendix containing details of presentation and proof of a theorem adde

Validity of self-reported smoking status: comparison of patients admitted to hospital with acute coronary syndrome and the general population

Many studies rely on self-reported smoking status. We hypothesized that patients with acute coronary syndrome (ACS), a smoking-related condition, would be more prone to misclassify themselves as ex-smokers, because of pressure to quit. We compared patients admitted with ACS with a general population survey conducted in the same country at a similar time. We determined whether ACS patients who classified themselves as ex-smokers (n = 635) were more likely to have cotinine levels suggestive of smoking deception than self-reported ex-smokers in the general population (n = 289). On univariate analysis, the percentage of smoking deceivers was similar among ACS patients and the general population (11% vs. 12%, p = .530). Following adjustment for age, sex and exposure to environmental tobacco smoke, ACS patients were significantly more likely to misclassify themselves (adjusted OR = 14.06, 95% CI 2.13-93.01, p = .006). There was an interaction with age whereby the probability of misclassification fell significantly with increasing age in the ACS group (adjusted OR = 0.95, 95% CI 0.93-0.97, p<.001), but not in the general population. Overall, smoking deception was more common among ACS patients than the general population. Studies comparing patients with cardiovascular disease and healthy individuals risk introducing bias if they rely solely on self-reported smoking status. Biochemical confirmation should be undertaken in such studies

Water security in South Africa: perceptions on public expectations and municipal obligations, governance and water re-use

South Africa is a water-scarce country with increasing pressure on its water resources. Urgent interventions are needed to protect water security within this rapidly developing country. This paper reports on an interdisciplinary Water Security Colloquium, convened by the South African Young Academy of Science in 2014. A selected group of water professionals from academia, civil society and local government was brought together to discuss water security under three focus themes: ‘public expectations and municipal obligations’, ‘water security and governance: challenges and advances’, and ‘water re-use: health and infrastructural considerations’. Participant perceptions were generated using a focus group methodology, combined with participatory data collection methods. Under each theme, inputs were categorised as ‘challenges’, ‘gaps in knowledge’, and ‘solutions/recommendations’ and these inputs were thereafter ranked in order of importance via a ‘voting’ process. Major challenges perceived included a lack of both skills and political will in government, a need to restore citizen trust in government intention and capability to deliver water-related services, and a failure to up-scale existing water re-use technology. Participants identified understanding of the process and implications of the Green and Blue Drop Programmes, knowledge transfer to the public, and the role of educators as major knowledge gaps. The top suggestions proposed included creating public awareness around and buy-in to initiatives to improve water security, accessible and user-friendly conversion of research results to implementation, and ensuring an active role for educators in creating awareness around water security. In view of the concerns identified, participants suggested as potential solutions: improving government and public understanding around water issues, incentivising water re-use and conservation, introducing rising block tariffs and improving human capacity development in the water sector. Developing the ecological infrastructure that protects both quantity and quality of water and building strong partnerships among all stakeholders were also recognised as key.Keywords: capacity development, education, governance, water security, water re-use, water conservation, local governmen

Complication rates in managing hepatic trauma: a cross-sectional study stratifying their outcomes

Background: Liver trauma is the most commonly observed injured organ in abdominal trauma. The objectives of this study was to determine and evaluate the rates of complication in the management of liver traumaMethods: This cross-sectional observational study using non-probability convenient sampling technique was done at surgical unit of Liaquat University of Medical and Health Sciences, Jamshoro, for 06 months. After ethical approval from Institute’s Institutional Review Board (IRB), patients presenting to surgical emergency of the hospital between ages 16 to 60 years having blunt or penetrating liver trauma within 04 hours of incident, either road traffic accident, sustaining a fall, sporting injury, knife or stab wound were include while patients of liver trauma conservatively managed or had severe co-morbid, not fit for anesthesia, with multiple organs lesions (polytrauma) and all hepatic injury patients that were hemo-dynamically stable were excluded. SPSS version 23 was used for data analysis keeping p-value <0.05 as significant.Results: Among 136 patients with mean age 32.33±11.23 years, 120(88.2%) were males. 122(89.7%) of the patients were admitted due to liver trauma of blunt variety while 14(10%) with penetrating liver injury. Overall mean duration of hospital stay was 13.1±4.58 days. 41(30%) patients reported intra-abdominal sepsis, followed by recurrent hemorrhage in 33(24%) of patients while in 22(16%) of patients, biliary leakage was observed. An insignificant difference persisted in either surgical intervention in terms of the complication rates.Conclusions: Higher complication rates were observed in patients with peri-hepatic packing, however outcome of both surgical techniques in terms of complication rates were found to be insignificant. Further studies are needed to shed light upon the findings or this study

Chest imaging features of patients afflicted with Influenza A (H1N1) in a Malaysian tertiary referral centre

This is a retrospective descriptive study of the chest imaging findings of 118 patients with confirmed A(H1N1) in a tertiary referral centre. About 42% of the patients had positive initial chest radiographic (CXR) findings. The common findings were bi-basal air-space opacities and perihilar reticular and alveolar infiltrates. In select cases, high-resolution computed tomography (CT) imaging showed ground-glass change with some widespread reticular changes and atelectasis

Modification of radiation pressure due to cooperative scattering of light

Cooperative spontaneous emission of a single photon from a cloud of N atoms modifies substantially the radiation pressure exerted by a far-detuned laser beam exciting the atoms. On one hand, the force induced by photon absorption depends on the collective decay rate of the excited atomic state. On the other hand, directional spontaneous emission counteracts the recoil induced by the absorption. We derive an analytical expression for the radiation pressure in steady-state. For a smooth extended atomic distribution we show that the radiation pressure depends on the atom number via cooperative scattering and that, for certain atom numbers, it can be suppressed or enhanced.Comment: 8 pages, 2 Figure