Cooperative effects in one-dimensional random atomic gases: Absence of single atom limit

We study superradiance in a one-dimensional geometry, where N>>1 atoms are randomly distributed along a line. We present an analytic calculation of the photon escape rates based on the diagonalization of the N x N coupling matrix Uij = cos xij, where xij is the dimensionless random distance between any two atoms. We show that unlike a three-dimensional geometry, for a one- dimensional atomic gas the single-atom limit is never reached and the photon is always localized within the atomic ensemble. This localization originates from long-range cooperative effects and not from disorder as expected on the basis of the theory of Anderson localization.Comment: 5 pages, 3 figure

Cooperative effects and disorder: A scaling analysis of the spectrum of the effective atomic Hamiltonian

We study numerically the spectrum of the non-Hermitian effective Hamiltonian that describes the dipolar interaction of a gas of $N\gg 1$ atoms with the radiation field. We analyze the interplay between cooperative effects and disorder for both scalar and vectorial radiation fields. We show that for dense gases, the resonance width distribution follows, both in the scalar and vectorial cases, a power law $P(\Gamma) \sim \Gamma^{-4/3}$ that originates from cooperative effects between more than two atoms. This power law is different from the $P(\Gamma) \sim \Gamma^{-1}$ behavior, which has been considered as a signature of Anderson localization of light in random systems. We show that in dilute clouds, the center of the energy distribution is described by Wigner's semicircle law in the scalar and vectorial cases. For dense gases, this law is replaced in the vectorial case by the Laplace distribution. Finally, we show that in the scalar case the degree of resonance overlap increases as a power law of the system size for dilute gases, but decays exponentially with the system size for dense clouds.Comment: 11 pages, 12 figure

From Soft Walls to Infrared Branes

Five dimensional warped spaces with soft walls are generalizations of the standard Randall-Sundrum compactifications, where instead of an infrared brane one has a curvature singularity (with vanishing warp factor) at finite proper distance in the bulk. We project the physics near the singularity onto a hypersurface located a small distance away from it in the bulk. This results in a completely equivalent description of the soft wall in terms of an effective infrared brane, hiding any singular point. We perform explicitly this calculation for two classes of soft wall backgrounds used in the literature. The procedure has several advantages. It separates in a clean way the physics of the soft wall from the physics of the five dimensional bulk, facilitating a more direct comparison with standard two-brane warped compactifications. Moreover, consistent soft walls show a sort of universal behavior near the singularity which is reflected in the effective brane Lagrangian. Thirdly, for many purposes, a good approximation is obtained by assuming the bulk background away from the singularity to be the usual Randall-Sundrum metric, thus making the soft wall backgrounds better analytically tractable. We check the validity of this procedure by calculating the spectrum of bulk fields and comparing it to the exact result, finding very good agreement.Comment: 14 pages, 2 figures, v2: subsection on IR brane potentials and appendix on fermions added, version to appear in PR

Effect of superradiance on transport of diffusing photons in cold atomic gases

We show that in atomic gases cooperative effects like superradiance and subradiance lead to a potential between two atoms that decays like $1/r$. In the case of superradiance, this potential is attractive for close enough atoms and can be interpreted as a coherent mesoscopic effect. The contribution of superradiant pairs to multiple scattering properties of a dilute gas, such as photon elastic mean free path and group velocity, is significantly different from that of independent atoms. We discuss the conditions under which these effects may be observed and compare our results to recent experiments on photon transport in cold atomic gases.Comment: 4 pages and 1 figur

Robust Header Compression (ROHC) in Next-Generation Network Processors

Robust Header Compression (ROHC) provides for more efficient use of radio links for wireless communication in a packet switched network. Due to its potential advantages in the wireless access area andthe proliferation of network processors in access infrastructure, there exists a need to understand the resource requirements and architectural implications of implementing ROHC in this environment. We presentan analysis of the primary functional blocks of ROHC and extract the architectural implications on next-generation network processor design for wireless access. The discussion focuses on memory space andbandwidth dimensioning as well as processing resource budgets. We conclude with an examination of resource consumption and potential performance gains achievable by offloading computationally intensiveROHC functions to application specific hardware assists. We explore the design tradeoffs for hardware as-sists in the form of reconfigurable hardware, Application-Specific Instruction-set Processors (ASIPs), andApplication-Specific Integrated Circuits (ASICs)

Cross-scale barriers to climate change adaptation in local government, Australia

This report documents a study aimed at identifying cross-scale barriers to planned adaptation within the context of local government in Australia, and the development of enabling actions to overcome these barriers. Many of the impacts of climate change and variability have been, or will be, experienced at the local level. As a result, local governments in Australia (and overseas) have initiated plans to adapt to these impacts. However, the pathway to planning and implementation of adaptation is not a barrier-free process. Local governments are embedded in a larger governance context that has the potential to limit the effectiveness of planned adaptation initiatives on the ground. Identifying barriers or constraints to adaptation is an important process in supporting successful adaptation planning, particularly where reworking the path-dependent institutional structures, organisational cultures and policy-making procedures is required

Roles of cooperative effects and disorder in photon localization: The case of a vector radiation field

We numerically study photon escape rates from three-dimensional atomic gases and investigate the respective roles of cooperative effects and disorder in photon localization, while taking into account the vectorial nature of light. A scaling behavior is observed for the escape rates, and photons undergo a crossover from delocalization toward localization as the optical thickness of the cloud is increased. This result indicates that light localization is dominated by cooperative effects rather than disorder. We compare our results with those obtained in the case of a scalar radiation field and find no significant differences. We conclude that the scalar model constitutes an excellent approximation when considering photon escape rates from atomic gases.Comment: 9 pages, 9 figure

Photon localization and Dicke superradiance in atomic gases

Photon propagation in a gas of N atoms is studied using an effective Hamiltonian describing photon mediated atomic dipolar interactions. The density P(\Gamma) of photon escape rates is determined from the spectrum of the N x N random matrix \Gamma_{ij} = \sin (x_{ij}) / x_{ij}, where x_{ij} is the dimensionless random distance between any two atoms. Varying disorder and system size, a scaling behavior is observed for the escape rates. It is explained using microscopic calculations and a stochastic model which emphasizes the role of cooperative effects in photon localization and provides an interesting relation with statistical properties of "small world networks".Comment: 4 pages, 5 figure

Beyond a token effort: Gender transformative climate change action in the Pacific

Gender inequality, unequal power relations and discrimination are barriers that often prevent women, girls and people of diverse sexual and gender identities from equal representation and participation in many aspects of society. Addressing these issues in climate change programming is crucial, given the ways in which climate change can amplify existing gender inequalities (CEDAW 2018). Pacific Island Countries (PICs) are already experiencing the impacts of climate change. Although the diverse cultures of the Pacific have adapted to severe weather over the millennia, the broad range and severity of climate change impacts require new interventions to ensure lives and access to basic rights are protected. All sectors and all levels of society—from local to national, rural to urban—require new ways of working to adapt to climate change. These new ways need to ensure that marginalised segments of society, including women, girls and boys, people of diverse sexual and gender identities, people with disability and indigenous people, are considered. ‘Gender transformative climate change action’ seeks to address some of these issues, by transforming underlying norms and behaviours, relations, systems and structures to ensure gender equality