9,507 research outputs found
Bidirectional Rendering of Vector Light Transport
On the foundations of many rendering algorithms it is the symmetry between the path traversed by light and its adjoint path starting from the camera. However, several effects, including polarization or ¿uorescence, break that symmetry, and are de¿ned only on the direction of light propagation. This reduces the applicability of bidirectional methods that exploit this symmetry for simulating effectively light transport. In this work, we focus on how to include these non-symmetric effects within a bidirectional rendering algorithm. We generalize the path integral to support the constraints imposed by non-symmetric light transport. Based on this theoretical framework, we propose modi¿cations on two bidirectional methods, namely bidirectional path tracing and photon mapping, extending them to support polarization and ¿uorescence, in both steady and transient stat
Observation of Caustics in the Trajectories of Cold Atoms in a Linear Magnetic Potential
We have studied the spatial and temporal dynamics of a cold atom cloud in the
conservative force field of a ferromagnetic guide, after laser cooling has been
switched off suddenly. We observe outgoing 'waves' that correspond to caustics
of individual trajectories of trapped atoms. This provides detailed information
on the magnetic field, the energy distribution and the spin states.Comment: 21 pages, incl. 12 figure
Joint Elastic Side-Scattering Lidar and Raman Lidar Measurements of Aerosol Optical Properties in South East Colorado
We describe an experiment, located in south-east Colorado, USA, that measured
aerosol optical depth profiles using two Lidar techniques. Two independent
detectors measured scattered light from a vertical UV laser beam. One detector,
located at the laser site, measured light via the inelastic Raman
backscattering process. This is a common method used in atmospheric science for
measuring aerosol optical depth profiles. The other detector, located
approximately 40km distant, viewed the laser beam from the side. This detector
featured a 3.5m2 mirror and measured elastically scattered light in a bistatic
Lidar configuration following the method used at the Pierre Auger cosmic ray
observatory. The goal of this experiment was to assess and improve methods to
measure atmospheric clarity, specifically aerosol optical depth profiles, for
cosmic ray UV fluorescence detectors that use the atmosphere as a giant
calorimeter. The experiment collected data from September 2010 to July 2011
under varying conditions of aerosol loading. We describe the instruments and
techniques and compare the aerosol optical depth profiles measured by the Raman
and bistatic Lidar detectors.Comment: 34 pages, 16 figure
Highly indistinguishable single photons from incoherently and coherently excited GaAs quantum dots
Semiconductor quantum dots are converging towards the demanding requirements
of photonic quantum technologies. Among different systems, quantum dots with
dimensions exceeding the free-exciton Bohr radius are appealing because of
their high oscillator strengths. While this property has received much
attention in the context of cavity quantum electrodynamics, little is known
about the degree of indistinguishability of single photons consecutively
emitted by such dots and on the proper excitation schemes to achieve high
indistinguishability. A prominent example is represented by GaAs quantum dots
obtained by local droplet etching, which recently outperformed other systems as
triggered sources of entangled photon pairs. On these dots, we compare
different single-photon excitation mechanisms, and we find (i) a "phonon
bottleneck" and poor indistinguishability for conventional excitation via
excited states and (ii) photon indistinguishablilities above 90% for both
strictly resonant and for incoherent acoustic- and optical-phonon-assisted
excitation. Among the excitation schemes, optical phonon-assisted excitation
enables straightforward laser rejection without a compromise on the source
brightness together with a high photon indistinguishability
A switchable pH-differential unitized regenerative fuel cell with high performance
Regenerative fuel cells are a potential candidate for future energy storage, but their applications are limited by the high cost and poor round-trip efficiency. Here we present a switchable pH-differential unitized regenerative fuel cell capable of addressing both the obstacles. Relying on a membraneless laminar flow-based design, pH environments in the cell are optimized independently for different electrode reactions and are switchable together with the cell process to ensure always favorable thermodynamics for each electrode reaction. Benefiting from the thermodynamic advantages of the switchable pH-differential arrangement, the cell allows water electrolysis at a voltage of 0.57 V, and a fuel cell open circuit voltage of 1.89 V, rendering round-trip efficiencies up to 74%. Under room conditions, operating the cell in fuel cell mode yields a power density of 1.3 W cm¯², which is the highest performance to date for laminar flow-based cells and is comparable to state-of-the-art polymer electrolyte membrane fuel cells
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