756 research outputs found
Optical diode based on the chirality of guided photons
Photons are nonchiral particles: their handedness can be both left and right.
However, when light is transversely confined, it can locally exhibit a
transverse spin whose orientation is fixed by the propagation direction of the
photons. Confined photons thus have chiral character. Here, we employ this to
demonstrate nonreciprocal transmission of light at the single-photon level
through a silica nanofibre in two experimental schemes. We either use an
ensemble of spin-polarised atoms that is weakly coupled to the nanofibre-guided
mode or a single spin-polarised atom strongly coupled to the nanofibre via a
whispering-gallery-mode resonator. We simultaneously achieve high optical
isolation and high forward transmission. Both are controlled by the internal
atomic state. The resulting optical diode is the first example of a new class
of nonreciprocal nanophotonic devices which exploit the chirality of confined
photons and which are, in principle, suitable for quantum information
processing and future quantum optical networks
A New Simulation Metric to Determine Safe Environments and Controllers for Systems with Unknown Dynamics
We consider the problem of extracting safe environments and controllers for
reach-avoid objectives for systems with known state and control spaces, but
unknown dynamics. In a given environment, a common approach is to synthesize a
controller from an abstraction or a model of the system (potentially learned
from data). However, in many situations, the relationship between the dynamics
of the model and the \textit{actual system} is not known; and hence it is
difficult to provide safety guarantees for the system. In such cases, the
Standard Simulation Metric (SSM), defined as the worst-case norm distance
between the model and the system output trajectories, can be used to modify a
reach-avoid specification for the system into a more stringent specification
for the abstraction. Nevertheless, the obtained distance, and hence the
modified specification, can be quite conservative. This limits the set of
environments for which a safe controller can be obtained. We propose SPEC, a
specification-centric simulation metric, which overcomes these limitations by
computing the distance using only the trajectories that violate the
specification for the system. We show that modifying a reach-avoid
specification with SPEC allows us to synthesize a safe controller for a larger
set of environments compared to SSM. We also propose a probabilistic method to
compute SPEC for a general class of systems. Case studies using simulators for
quadrotors and autonomous cars illustrate the advantages of the proposed metric
for determining safe environment sets and controllers.Comment: 22nd ACM International Conference on Hybrid Systems: Computation and
Control (2019
Soil and water bioengineering: practice and research needs for reconciling natural hazard control and ecological restoration
Soil and water bioengineering is a technology that encourages scientists and practitioners to combine their knowledge and skills in the management of ecosystems with a common goal to maximize benefits to both man and the natural environment. It involves techniques that use plants as living building materials, for: (i) natural hazard control (e.g., soil erosion, torrential floods and landslides) and (ii) ecological restoration or nature-based re-introduction of species on degraded lands, river embankments, and disturbed environments. For a bioengineering project to be successful, engineers are required to highlight all the potential benefits and ecosystem services by documenting the technical, ecological, economic and social values. The novel approaches used by bioengineers raise questions for researchers and necessitate innovation from practitioners to design bioengineering concepts and techniques. Our objective in this paper, therefore, is to highlight the practice and research needs in soil and water bioengineering for reconciling natural hazard control and ecological restoration. Firstly, we review the definition and development of bioengineering technology, while stressing issues concerning the design, implementation, and monitoring of bioengineering actions. Secondly, we highlight the need to reconcile natural hazard control and ecological restoration by posing novel practice and research questions
A Nanofiber-Based Optical Conveyor Belt for Cold Atoms
We demonstrate optical transport of cold cesium atoms over millimeter-scale
distances along an optical nanofiber. The atoms are trapped in a
one-dimensional optical lattice formed by a two-color evanescent field
surrounding the nanofiber, far red- and blue-detuned with respect to the atomic
transition. The blue-detuned field is a propagating nanofiber-guided mode while
the red-detuned field is a standing-wave mode which leads to the periodic axial
confinement of the atoms. Here, this standing wave is used for transporting the
atoms along the nanofiber by mutually detuning the two counter-propagating
fields which form the standing wave. The performance and limitations of the
nanofiber-based transport are evaluated and possible applications are
discussed
Integrated time-lapse geoelectrical imaging of wetland hydrological processes
Wetlands provide crucial habitats, are critical in the global carbon cycle, and act as key biogeochemical and hydrological buffers. The effectiveness of these services is mainly controlled by hydrological processes, which can be highly variable both spatially and temporally due to structural complexity and seasonality. Spatial analysis of 2D geoelectrical monitoring data integrated into the interpretation of conventional hydrological data has been implemented to provide a detailed understanding of hydrological processes in a riparian wetland. This study shows that a combination of processes can define the resistivity signature of the shallow subsurface, highlighting the seasonality of these processes and its corresponding effect on biogeochemical processesthe wetland hydrology. Groundwater exchange between peat and the underlying river terrace deposits, spatially and temporally defined by geoelectrical imaging and verified by point sensor data, highlighted the groundwater dependent nature of the wetland. A 30 % increase in peat resistivity was shown to be caused by a nearly entire exchange of the saturating groundwater. For the first time, we showed that automated interpretation of geoelectrical data can be used to quantify shrink-swell of expandable soils, affecting hydrological parameters, such as, porosity, water storage capacity, and permeability. This study shows that an integrated interpretation of hydrological and geophysical data can significantly improve the understanding of wetland hydrological processes. Potentially, this approach can provide the basis for the evaluation of ecosystem services and may aid in the optimization of wetland management strategies
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