431 research outputs found
Whispering Gallery Mode Resonator Stabilized Narrow Linewidth Fiber Loop Laser
We demonstrate a narrow line, fiber loop laser using Erbium-doped fiber as
the gain material, stabilized by using a microsphere as a transmissive
frequency selective element. Stable lasing with a linewidth of 170 kHz is
observed, limited by the experimental spectral resolution. A linear increase in
output power and a red-shift of the lasing mode were also observed with
increasing pump power. Its potential application is also discussed
Location-Quality-aware Policy Optimisation for Relay Selection in Mobile Networks
Relaying can improve the coverage and performance of wireless access
networks. In presence of a localisation system at the mobile nodes, the use of
such location estimates for relay node selection can be advantageous as such
information can be collected by access points in linear effort with respect to
number of mobile nodes (while the number of links grows quadratically).
However, the localisation error and the chosen update rate of location
information in conjunction with the mobility model affect the performance of
such location-based relay schemes; these parameters also need to be taken into
account in the design of optimal policies. This paper develops a Markov model
that can capture the joint impact of localisation errors and inaccuracies of
location information due to forwarding delays and mobility; the Markov model is
used to develop algorithms to determine optimal location-based relay policies
that take the aforementioned factors into account. The model is subsequently
used to analyse the impact of deployment parameter choices on the performance
of location-based relaying in WLAN scenarios with free-space propagation
conditions and in an measurement-based indoor office scenario.Comment: Accepted for publication in ACM/Springer Wireless Network
Static Envelope Patterns in Composite Resonances Generated by Level Crossing in Optical Toroidal Microcavities
We study level crossing in the optical whispering-gallery (WG) modes by using toroidal microcavities. Experimentally, we image the stationary envelope patterns of the composite optical modes that arise when WG modes of different wavelengths coincide in frequency. Numerically, we calculate crossings of levels that correspond with the observed degenerate modes, where our method takes into account the not perfectly transverse nature of their field polarizations. In addition, we analyze anticrossing with a large avoidance gap between modes of the same azimuthal number
Coherent conversion between microwave and optical photons -- an overview of physical implementations
Quantum information technology based on solid state qubits has created much
interest in converting quantum states from the microwave to the optical domain.
Optical photons, unlike microwave photons, can be transmitted by fiber, making
them suitable for long distance quantum communication. Moreover, the optical
domain offers access to a large set of very well developed quantum optical
tools, such as highly efficient single-photon detectors and long-lived quantum
memories. For a high fidelity microwave to optical transducer, efficient
conversion at single photon level and low added noise is needed. Currently, the
most promising approaches to build such systems are based on second order
nonlinear phenomena such as optomechanical and electro-optic interactions.
Alternative approaches, although not yet as efficient, include magneto-optical
coupling and schemes based on isolated quantum systems like atoms, ions or
quantum dots. In this Progress Report, we provide the necessary theoretical
foundations for the most important microwave-to-optical conversion experiments,
describe their implementations and discuss current limitations and future
prospects.Comment: 17 Pages, 8 Figure
Discovering the Elite Hypervolume by Leveraging Interspecies Correlation
Evolution has produced an astonishing diversity of species, each filling a
different niche. Algorithms like MAP-Elites mimic this divergent evolutionary
process to find a set of behaviorally diverse but high-performing solutions,
called the elites. Our key insight is that species in nature often share a
surprisingly large part of their genome, in spite of occupying very different
niches; similarly, the elites are likely to be concentrated in a specific
"elite hypervolume" whose shape is defined by their common features. In this
paper, we first introduce the elite hypervolume concept and propose two metrics
to characterize it: the genotypic spread and the genotypic similarity. We then
introduce a new variation operator, called "directional variation", that
exploits interspecies (or inter-elites) correlations to accelerate the
MAP-Elites algorithm. We demonstrate the effectiveness of this operator in
three problems (a toy function, a redundant robotic arm, and a hexapod robot).Comment: In GECCO 201
Electronic structure and total energy of interstitial hydrogen in iron: Tight binding models
An application of the tight binding approximation is presented for the
description of electronic structure and interatomic force in magnetic iron,
both pure and containing hydrogen impurities. We assess the simple canonical
d-band description in comparison to a non orthogonal model including s and d
bands. The transferability of our models is tested against known properties
including the segregation energies of hydrogen to vacancies and to surfaces of
iron. In many cases agreement is remarkably good, opening up the way to quantum
mechanical atomistic simulation of the effects of hydrogen on mechanical
properties
A tight binding model for water
We demonstrate for the first time a tight binding model for water
incorporating polarizable anions. A novel aspect is that we adopt a "ground up"
approach in that properties of the monomer and dimer only are fitted.
Subsequently we make predictions of the structure and properties of hexamer
clusters, ice-XI and liquid water. A particular feature, missing in current
tight binding and semiempirical hamiltonians, is that we reproduce the almost
two-fold increase in molecular dipole moment as clusters are built up towards
the limit of bulk liquid. We concentrate on properties of liquid water which
are very well rendered in comparison with experiment and published density
functional calculations. Finally we comment on the question of the contrasting
densities of water and ice which is central to an understanding of the
subtleties of the hydrogen bond
- …