5,536 research outputs found
Detecting the Dusty Debris of Terrestrial Planet Formation
We use a multiannulus accretion code to investigate debris disks in the
terrestrial zone, at 0.7-1.3 AU around a 1 solar mass star. Terrestrial planet
formation produces a bright dusty ring of debris with a lifetime of at least 1
Myr. The early phases of terrestrial planet formation are observable with
current facilities; the late stages require more advanced instruments with
adaptive optics.Comment: 11 pages of text, 3 figures, accepted for ApJ Letters, additional
info at http://cfa-www.harvard.edu/~kenyon/pf/terra/td
Grothendieck's constant and local models for noisy entangled quantum states
We relate the nonlocal properties of noisy entangled states to Grothendieck's
constant, a mathematical constant appearing in Banach space theory. For
two-qubit Werner states \rho^W_p=p \proj{\psi^-}+(1-p){\one}/{4}, we show
that there is a local model for projective measurements if and only if , where is Grothendieck's constant of order 3. Known bounds
on prove the existence of this model at least for ,
quite close to the current region of Bell violation, . We
generalize this result to arbitrary quantum states.Comment: 6 pages, 1 figur
Calibrated in-vacuum quantum efficiency system for metallic and III-V thin-film photocathodes
The construction and calibration of a high vacuum system for thin film growth and in situ quantum efficiency (QE) measurement are described. Surface cleaning by in situ argon ion sputtering and annealing is supported. The QE measurement is based on an external 265 nm LED and in situ positively biased collector grid. The system is applied to two metallic and two semiconducting photocathodes: polycrystalline silver and copper, and single crystal InP and InSb. Surface cleaning protocols are shown to have a dramatic effect on the QE for all of these materials. The maximum QE values achieved for clean InSb and InP are around 8 × 10−5, for Cu 9 × 10−5 and for Ag 2 × 10−4
Stable carbon isotope analysis of Cedrus atlantica pollen as an indicator of moisture availability
Stable carbon isotope analysis of pollen provides potential for reconstruction of past moisture availability in the environment on longer time-scales compared to isotope analysis of plant tissue. Here we show that the carbon isotopic compositions (δ13C) of pollen, sporopollenin, leaf and stem tissues of Cedrus atlantica are strongly related. Untreated pollen δ13C has a significant linear relationship with sporopollenin δ13C (r2 = 0.97, p < 0.0001) which is relatively depleted in 13C by an average 1.5‰. Carbon isotope discrimination (Δ13C) by sporopollenin (derived from pollen δ13C values) is related to mean annual (r2 = 0.54, p < 0.001) and summer precipitation (r2 = 0.63, p < 0.0001). A 100 mm increase in mean annual precipitation results in sporopollenin Δ13C increasing by 0.52‰, or by 1.4‰ per 100 mm summer precipitation. There is a stronger relationship between sporopollenin Δ13C and long-term annual scPDSI (r2 = 0.86, p < 0.0001) and summer scPDSI (r2 = 0.86, p < 0.001) aridity indexes, with reduced Δ13C as aridity increases. These relationships suggest that stable carbon isotope analysis of C. atlantica fossil pollen could be used as a quantitative proxy for the reconstruction of summer moisture availability in Northwest Africa
Snow Cover and Precipitation Impacts on Dry Season Streamflow in the Lower Mekong Basin
Climate change impacts on dry season streamflow in the Mekong River are relatively understudied, despite the fact that water availability during this time is critically important for agricultural and ecological systems. Analyses of two gauging stations (Vientiane and Kratie) in the Lower Mekong Basin (LMB) show significant positive correlations between dry season (March through May, MAM) discharge and upper basin snow cover and local precipitation. Using snow cover, precipitation, and upstream discharge as predictors, we develop skillful regression models for MAM streamflow at Vientiane and Kratie, and force these models with output from a suite of general circulation model (GCM) experiments for the twentieth and twenty-first centuries. The GCM simulations predict divergent trends in snow cover (decreasing) and precipitation (increasing) over the twenty-first century, driving overall negligible long-term trends in dry season streamflow. Our study demonstrates how future changes in dry season streamflow in the LMB will depend on changes in snow cover and precipitation, factors that will need to be considered when assessing the full basin response to other climatic and non-climatic drivers
Synthetic photonic lattice for single-shot reconstruction of frequency combs
We formulate theoretically and demonstrate experimentally an all-optical
method for reconstruction of the amplitude, phase and coherence of frequency
combs from a single-shot measurement of the spectral intensity. Our approach
exploits synthetic frequency lattices with pump-induced spectral short- and
long-range couplings between different signal components across a broad
bandwidth of of hundreds GHz in a single nonlinear fiber. When combined with
ultra-fast signal conversion techniques, this approach has the potential to
provide real-time measurement of pulse-to-pulse variations in the spectral
phase and coherence properties of exotic light sources.Comment: 15 pages, 4 figure
Multidimensional synthetic chiral-tube lattices via nonlinear frequency conversion
Geometrical dimensionality plays a fundamentally important role in the
topological effects arising in discrete lattices. While direct experiments are
limited by three spatial dimensions, the research topic of synthetic dimensions
implemented by the frequency degree of freedom in photonics is rapidly
advancing. The manipulation of light in such artificial lattices is typically
realized through electro-optic modulation, yet their operating bandwidth
imposes practical constraints on the range of interactions between different
frequency components. Here we propose and experimentally realize all-optical
synthetic dimensions involving specially tailored simultaneous short- and
long-range interactions between discrete spectral lines mediated by frequency
conversion in a nonlinear waveguide. We realize triangular chiral-tube lattices
in three-dimensional space and explore their four-dimensional generalization.
We implement a synthetic gauge field with nonzero magnetic flux and observe the
associated multidimensional dynamics of frequency combs, all within one
physical spatial port. We anticipate that our method will provide a new means
for the fundamental study of high-dimensional physics and act as an important
step towards using topological effects in optical devices operating in the time
and frequency domains.Comment: 20 pages, 6 figure
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