4,989 research outputs found
Weak lensing of the Lyman-alpha forest
The angular positions of quasars are deflected by the gravitational lensing
effect of foreground matter. The Lyman-alpha forest seen in the spectra of
these quasars is therefore also lensed. We propose that the signature of weak
gravitational lensing of the forest could be measured using similar techniques
that have been applied to the lensed Cosmic Microwave Background, and which
have also been proposed for application to spectral data from 21cm radio
telescopes. As with 21cm data, the forest has the advantage of spectral
information, potentially yielding many lensed "slices" at different redshifts.
We perform an illustrative idealized test, generating a high resolution angular
grid of quasars (of order arcminute separation), and lensing the
Lyman-alphaforest spectra at redshifts z=2-3 using a foreground density field.
We find that standard quadratic estimators can be used to reconstruct images of
the foreground mass distribution at z~1. There currently exists a wealth of Lya
forest data from quasar and galaxy spectral surveys, with smaller sightline
separations expected in the future. Lyman-alpha forest lensing is sensitive to
the foreground mass distribution at redshifts intermediate between CMB lensing
and galaxy shear, and avoids the difficulties of shape measurement associated
with the latter. With further refinement and application of mass reconstruction
techniques, weak gravitational lensing of the high redshift Lya forest may
become a useful new cosmological probe.Comment: 9 pages, 7 figures, submitted to MNRA
Noise Estimates for Measurements of Weak Lensing from the Lyman-alpha Forest
We have proposed a method for measuring weak lensing using the Lyman-alpha
forest. Here we estimate the noise expected in weak lensing maps and power
spectra for different sets of observational parameters. We find that surveys of
the size and quality of the ones being done today and ones planned for the
future will be able to measure the lensing power spectrum at a source redshift
of z~2.5 with high precision and even be able to image the distribution of
foreground matter with high fidelity on degree scales. For example, we predict
that Lyman-alpha forest lensing measurement from the Dark Energy Spectroscopic
Instrument survey should yield the mass fluctuation amplitude with statistical
errors of 1.5%. By dividing the redshift range into multiple bins some
tomographic lensing information should be accessible as well. This would allow
for cosmological lensing measurements at higher redshift than are accessible
with galaxy shear surveys and correspondingly better constraints on the
evolution of dark energy at relatively early times.Comment: 8 pages, 8 figures, submitted to MNRA
Impact of a large wildfire on water-soluble organic aerosol in a major urban area: the 2009 Station Fire in Los Angeles County
This study examines the nature of water-soluble organic aerosol measured in Pasadena, CA, under typical conditions and under the influence of a large wildfire (the 2009 Station Fire). During non-fire periods, water-soluble organic carbon (WSOC) variability was driven by photochemical production processes and sea breeze transport, resulting in an average diurnal cycle with a maximum at 15:00 local time (up to 4.9 μg C m^(−3)). During the Station Fire, primary production was a key formation mechanism for WSOC. High concentrations of WSOC (up to 41 μg C m^(−3)) in smoke plumes advected to the site in the morning hours were tightly correlated with nitrate and chloride, numerous aerosol mass spectrometer (AMS) organic mass spectral markers, and total non-refractory organic mass. Processed residual smoke was transported to the measurement site by the sea breeze later in the day, leading to higher afternoon WSOC levels than on non-fire days. Parameters representing higher degrees of oxidation of organics, including the ratios of the organic metrics m/z 44:m/z 57 and m/z 44:m/z 43, were elevated in those air masses. Intercomparisons of relative amounts of WSOC, organics, m/z 44, and m/z 43 show that the fraction of WSOC comprising acid-oxygenates increased as a function of photochemical aging owing to the conversion of aliphatic and non-acid oxygenated organics to more acid-like organics. The contribution of water-soluble organic species to the organic mass budget (10th–90th percentile values) ranged between 27 %–72 % and 27 %–68 % during fire and non-fire periods, respectively. The seasonal incidence of wildfires in the Los Angeles Basin greatly enhances the importance of water-soluble organics, which has implications for the radiative and hygroscopic properties of the regional aerosol
An Optimal Design for Universal Multiport Interferometers
Universal multiport interferometers, which can be programmed to implement any
linear transformation between multiple channels, are emerging as a powerful
tool for both classical and quantum photonics. These interferometers are
typically composed of a regular mesh of beam splitters and phase shifters,
allowing for straightforward fabrication using integrated photonic
architectures and ready scalability. The current, standard design for universal
multiport interferometers is based on work by Reck et al (Phys. Rev. Lett. 73,
58, 1994). We demonstrate a new design for universal multiport interferometers
based on an alternative arrangement of beam splitters and phase shifters, which
outperforms that by Reck et al. Our design occupies half the physical footprint
of the Reck design and is significantly more robust to optical losses.Comment: 8 pages, 4 figure
Andreev reflection in bosonic condensates
We study the bosonic analog of Andreev reflection at a normal-superfluid
interface where the superfluid is a boson condensate. We model the normal
region as a zone where nonlinear effects can be neglected. Against the
background of a decaying condensate, we identify a novel contribution to the
current of reflected atoms. The group velocity of this Andreev reflected
component differs from that of the normally reflected one. For a
three-dimensional planar or two-dimensional linear interface Andreev reflection
is neither specular nor conjugate.Comment: 5 pages, 3 figures. Text revise
Structural dichroism in the antiferromagnetic insulating phase of V_2O_3
We performed near-edge x-ray absorption spectroscopy (XANES) at V K edge in
the antiferromagnetic insulating (AFI) phase of a 2.8% Cr-doped V_2O_3 single
crystal. Linear dichroism of several percent is measured in the hexagonal plane
and found to be in good agreement with ab-initio calculations based on multiple
scattering theory. This experiment definitively proves the structural origin of
the signal and therefore solves a controversy raised by previous
interpretations of the same dichroism as non-reciprocal. It also calls for a
further investigation of the role of the magnetoelectric annealing procedure in
cooling to the AFI phase.Comment: 4 pages 3 figures. To be published in Phys. Rev. B (2005
A proposal for continuous loading of an optical dipole trap with magnetically guided ultra cold atoms
The capture of a moving atom by a non-dissipative trap, such as an optical
dipole trap, requires the removal of the excessive kinetic energy of the atom.
In this article we develop a mechanism to harvest ultra cold atoms from a
guided atom beam into an optical dipole trap by removing their directed kinetic
energy. We propose a continuous loading scheme where this is accomplished via
deceleration by a magnetic potential barrier followed by optical pumping to the
energetically lowest Zeeman sublevel. We theoretically investigate the
application of this scheme to the transfer of ultra cold chromium atoms from a
magnetically guided atom beam into a deep optical dipole trap. We discuss the
realization of a suitable magnetic field configuration. Based on numerical
simulations of the loading process we analyze the feasibility and efficiency of
our loading scheme.Comment: 10 pages, 5 figure
The Pasadena Aerosol Characterization Observatory (PACO): chemical and physical analysis of the Western Los Angeles basin aerosol
The Pasadena Aerosol Characterization Observatory (PACO) represents the first major aerosol characterization experiment centered in the Western/Central Los Angeles Basin. The sampling site, located on the campus of the California Institute of Technology in Pasadena, was positioned to sample a continuous afternoon influx of transported urban aerosol with a photochemical age of 1–2 h and generally free from major local contributions. Sampling spanned 5 months during the summer of 2009, which were broken into 3 regimes on the basis of distinct meteorological conditions. Regime I was characterized by a series of low pressure systems, resulting in high humidity and rainy periods with clean conditions. Regime II typified early summer meteorology, with significant morning marine layers and warm, sunny afternoons. Regime III was characterized by hot, dry conditions with little marine layer influence. Regardless of regime, organic aerosol (OA) is the most significant constituent of nonrefractory submicron Los Angeles aerosol (42, 43, and 55 % of total submicron mass in regimes I, II, and III, respectively). The overall oxidation state remains relatively constant on timescales of days to weeks (O:C = 0.44 ± 0.08, 0.55 ± 0.05, and 0.48 ± 0.08 during regimes I, II, and III, respectively), with no difference in O:C between morning and afternoon periods. Periods characterized by significant morning marine layer influence followed by photochemically favorable afternoons displayed significantly higher aerosol mass and O:C ratio, suggesting that aqueous processes may be important in the generation of secondary aerosol and oxidized organic aerosol (OOA) in Los Angeles. Online analysis of water soluble organic carbon (WSOC) indicates that water soluble organic mass (WSOM) reaches maxima near 14:00–15:00 local time (LT), but the percentage of AMS organic mass contributed by WSOM remains relatively constant throughout the day. Sulfate and nitrate reside predominantly in accumulation mode aerosol, while afternoon SOA production coincides with the appearance of a distinct fine mode dominated by organics. Particulate NH_4NO_3 and (NH_4)_2SO_4 appear to be NH_3-limited in regimes I and II, but a significant excess of particulate NH_4^+ in the hot, dry regime III suggests less SO_4^(2−) and the presence of either organic amines or NH_4^+-associated organic acids. C-ToF-AMS data were analyzed by Positive Matrix Factorization (PMF), which resolved three factors, corresponding to a hydrocarbon-like OA (HOA), semivolatile OOA (SV-OOA), and low-volatility OOA (LV-OOA). HOA appears to be a periodic plume source, while SV-OOA exhibits a strong diurnal pattern correlating with ozone. Peaks in SV-OOA concentration correspond to peaks in DMA number concentration and the appearance of a fine organic mode. LV-OOA appears to be an aged accumulation mode constituent that may be associated with aqueous-phase processing, correlating strongly with sulfate and representing the dominant background organic component. Periods characterized by high SV-OOA and LV-OOA were analyzed by filter analysis, revealing a complex mixture of species during periods dominated by SV-OOA and LV-OOA, with LV-OOA periods characterized by shorter-chain dicarboxylic acids (higher O:C ratio), as well as appreciable amounts of nitrate- and sulfate-substituted organics. Phthalic acid was ubiquitous in filter samples, suggesting that PAH photochemistry may be an important SOA pathway in Los Angeles. Aerosol composition was related to water uptake characteristics, and it is concluded that hygroscopicity is largely controlled by organic mass fraction (OMF). The hygroscopicity parameter κ averaged 0.31 ± 0.08, approaching 0.5 at low OMF and 0.1 at high OMF, with increasing OMF suppressing hygroscopic growth and increasing critical dry diameter for CCN activation (D_d). An experiment-averaged κ_(org) of 0.14 was calculated, indicating that the highly-oxidized organic fraction of aerosol in Los Angeles is appreciably more hygroscopic than previously reported in urban areas. Finally, PACO will provide context for results forthcoming from the CalNex field campaign, which involved ground sampling in Pasadena during the spring and summer of 2010
Cavity Assisted Nondestructive Laser Cooling of Atomic Qubits
We analyze two configurations for laser cooling of neutral atoms whose
internal states store qubits. The atoms are trapped in an optical lattice which
is placed inside a cavity. We show that the coupling of the atoms to the damped
cavity mode can provide a mechanism which leads to cooling of the motion
without destroying the quantum information.Comment: 12 page
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