1,280 research outputs found
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 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 that originates
from cooperative effects between more than two atoms. This power law is
different from the 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 . 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
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