5,374 research outputs found
The influence of forward-scattered light in transmission measurements of (exo)planetary atmospheres
[Abridged] The transmission of light through a planetary atmosphere can be
studied as a function of altitude and wavelength using stellar or solar
occultations, giving often unique constraints on the atmospheric composition.
For exoplanets, a transit yields a limb-integrated, wavelength-dependent
transmission spectrum of an atmosphere. When scattering haze and/or cloud
particles are present in the planetary atmosphere, the amount of transmitted
flux not only depends on the total optical thickness of the slant light path
that is probed, but also on the amount of forward-scattering by the scattering
particles. Here, we present results of calculations with a three-dimensional
Monte Carlo code that simulates the transmitted flux during occultations or
transits. For isotropically scattering particles, like gas molecules, the
transmitted flux appears to be well-described by the total atmospheric optical
thickness. Strongly forward-scattering particles, however, such as commonly
found in atmospheres of Solar System planets, can increase the transmitted flux
significantly. For exoplanets, such added flux can decrease the apparent radius
of the planet by several scale heights, which is comparable to predicted and
measured features in exoplanet transit spectra. We performed detailed
calculations for Titan's atmosphere between 2.0 and 2.8 micron and show that
haze and gas abundances will be underestimated by about 8% if
forward-scattering is ignored in the retrievals. At shorter wavelengths, errors
in the gas and haze abundances and in the spectral slope of the haze particles
can be several tens of percent, also for other Solar System planetary
atmospheres. We also find that the contribution of forward-scattering can be
fairly well described by modelling the atmosphere as a plane-parallel slab.Comment: Icarus, accepted for publicatio
Information gap for classical and quantum communication in a Schwarzschild spacetime
Communication between a free-falling observer and an observer hovering above
the Schwarzschild horizon of a black hole suffers from Unruh-Hawking noise,
which degrades communication channels. Ignoring time dilation, which affects
all channels equally, we show that for bosonic communication using single and
dual rail encoding the classical channel capacity reaches a finite value and
the quantum coherent information tends to zero. We conclude that classical
correlations still exist at infinite acceleration, whereas the quantum
coherence is fully removed.Comment: 5 pages, 4 figure
Transmission of pillar-based photonic crystal waveguides in InP technology
Waveguides based on line defects in pillar photonic crystals have been fabricated in InP/InGaAsP/InP technology. Transmission measurements of different line defects are reported. The results can be explained by comparison with two-dimensional band diagram simulations. The losses increase substantially at mode crossings and in the slow light regime. The agreement with the band diagrams implies a good control on the dimensions of the fabricated features, which is an important step in the actual application of these devices in photonic integrated circuit
Triangle Diagram with Off-Shell Coulomb T-Matrix for (In-)Elastic Atomic and Nuclear Three-Body Processes
The driving terms in three-body theories of elastic and inelastic scattering
of a charged particle off a bound state of two other charged particles contain
the fully off-shell two-body Coulomb T-matrix describing the intermediate-state
Coulomb scattering of the projectile with each of the charged target particles.
Up to now the latter is usually replaced by the Coulomb potential, either when
using the multiple-scattering approach or when solving three-body integral
equations. General properties of the exact and the approximate on-shell driving
terms are discussed, and the accuracy of this approximation is investigated
numerically, both for atomic and nuclear processes including bound-state
excitation, for energies below and above the corresponding three-body
dissociation threshold, over the whole range of scattering angles.Comment: 22 pages, 11 figures, figures can be obtained upon request from the
Authors, revte
From Linear Optical Quantum Computing to Heisenberg-Limited Interferometry
The working principles of linear optical quantum computing are based on
photodetection, namely, projective measurements. The use of photodetection can
provide efficient nonlinear interactions between photons at the single-photon
level, which is technically problematic otherwise. We report an application of
such a technique to prepare quantum correlations as an important resource for
Heisenberg-limited optical interferometry, where the sensitivity of phase
measurements can be improved beyond the usual shot-noise limit. Furthermore,
using such nonlinearities, optical quantum nondemolition measurements can now
be carried out at the single-photon level.Comment: 10 pages, 5 figures; Submitted to a Special Issue of J. Opt. B on
"Fluctuations and Noise in Photonics and Quantum Optics" (Herman Haus
Memorial Issue); v2: minor change
Heralded Two-Photon Entanglement from Probabilistic Quantum Logic Operations on Multiple Parametric Down-Conversion Sources
An ideal controlled-NOT gate followed by projective measurements can be used
to identify specific Bell states of its two input qubits. When the input qubits
are each members of independent Bell states, these projective measurements can
be used to swap the post-selected entanglement onto the remaining two qubits.
Here we apply this strategy to produce heralded two-photon polarization
entanglement using Bell states that originate from independent parametric
down-conversion sources, and a particular probabilistic controlled-NOT gate
that is constructed from linear optical elements. The resulting implementation
is closely related to an earlier proposal by Sliwa and Banaszek
[quant-ph/0207117], and can be intuitively understood in terms of familiar
quantum information protocols. The possibility of producing a ``pseudo-demand''
source of two-photon entanglement by storing and releasing these heralded pairs
from independent cyclical quantum memory devices is also discussed.Comment: 5 pages, 4 figures; submitted to IEEE Journal of Selected Topics in
Quantum Electronics, special issue on "Quantum Internet Technologies
How does land use affect the relative abundance of two mesopredators in the, Eastern Cape, South Africa?
Moderator: Wouter van Hoven.Presented at the 8th international congress for wildlife and livelihoods on private and communal lands: livestock, tourism, and spirit, that was held on September 7-12, 2014 in Estes Park, Colorado.Presenter: Armand Kok.Livestock pastoralism and game ranching are the two dominant land use types in the Eastern Cape, South Africa and conflict between humans and medium sized carnivores is widespread. In this study, we used 12 spatially explicit (3 x 3) trail camera grids (3600ha), to assess the relative abundance indices (RAI) of two common predators; black-backed jackals (Canis mesomelas) and caracals (Caracal caracal). Camera grids were equally distributed across the two land use types. Over 19121 trap nights, 726 photographs of black-backed jackals and 81 photographs of caracals were taken. The RAI of jackals was significantly higher on game ranches than livestock farms (U = 109; df = 1; p 0.05). While the two mesopredators are actively removed by managers on both land-use types, removal rates are higher on livestock farms than game ranches. Thus, monogamous, pair-bonded black-backed jackals may be more sensitive to the effects of predator control than solitary caracals. The merits of predator removal as a conflict mitigation strategy are discussed
Efficient high-fidelity quantum computation using matter qubits and linear optics
We propose a practical, scalable, and efficient scheme for quantum
computation using spatially separated matter qubits and single photon
interference effects. The qubit systems can be NV-centers in diamond,
Pauli-blockade quantum dots with an excess electron or trapped ions with
optical transitions, which are each placed in a cavity and subsequently
entangled using a double-heralded single-photon detection scheme. The fidelity
of the resulting entanglement is extremely robust against the most important
errors such as detector loss, spontaneous emission, and mismatch of cavity
parameters. We demonstrate how this entangling operation can be used to
efficiently generate cluster states of many qubits, which, together with single
qubit operations and readout, can be used to implement universal quantum
computation. Existing experimental parameters indicate that high fidelity
clusters can be generated with a moderate constant overhead.Comment: 5 pages, 3 figures, broader introduction and improved scalability of
cluster state generatio
Conditional linear-optical measurement schemes generate effective photon nonlinearities
We provide a general approach for the analysis of optical state evolution
under conditional measurement schemes, and identify the necessary and
sufficient conditions for such schemes to simulate unitary evolution on the
freely propagating modes. If such unitary evolution holds, an effective photon
nonlinearity can be identified. Our analysis extends to conditional measurement
schemes more general than those based solely on linear optics.Comment: 16 pages, 2 figure
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