1,842 research outputs found
Provably Secure Experimental Quantum Bit-String Generation
Coin tossing is a cryptographic task in which two parties who do not trust
each other aim to generate a common random bit. Using classical communication
this is impossible, but non trivial coin tossing is possible using quantum
communication. Here we consider the case when the parties do not want to toss a
single coin, but many. This is called bit string generation. We report the
experimental generation of strings of coins which are provably more random than
achievable using classical communication. The experiment is based on the ``plug
and play'' scheme developed for quantum cryptography, and therefore well suited
for long distance quantum communication.Comment: 4 pages, 3 figures. Submitted to Phys. Rev. Lett. A complete security
analysis for the experiment is given in quant-ph/040812
Soil weathering rates in 21 catchments of the Canadian Shield
Soil mineral weathering represents an essential source of nutrient base cation (Ca, Mg and K) for forest growth in addition to provide a buffering power against precipitation acidity for soils and surface waters. Weathering rates of base cations were obtained for 21 catchments located within the temperate and the boreal forest of the Canadian Shield with the geochemical model PROFILE. Weathering rates ranged from 0.58 to 4.46 kmol<sub>c</sub> ha<sup>â1</sup> yr<sup>â1</sup> and their spatial variation within the studied area was mostly in agreement with spatial variations in soil mineralogy. Weathering rates of Ca and Mg were significantly correlated (<i>r</i> = 0.80 and 0.64) with their respective lake concentrations. Weathering rates of K and Na did not correlate with lake concentrations of K and Na. The modeled weathering rates for each catchment were also compared with estimations of net catchment exportations. The result show that modeled weathering rates of Ca were not significantly different than the net catchment exportations while modeled weathering rates of Mg were higher by 51%. Larger differences were observed for K and Na weathering rates that were significantly different than net catchment exportations being 6.9 and 2.2 times higher than net exportations, respectively. The results for K were expected given its high reactivity with biotic compartments and suggest that most of the K produced by weathering reactions was retained within soil catchments and/or above ground biomass. This explanation does not apply to Na, however, which is a conservative element in forest ecosystems because of the insignificant needs of Na for soil microorganisms and above ground vegetations. It raises concern about the liability of the PROFILE model to provide reliable values of Na weathering rates. Overall, we concluded that the PROFILE model is powerful enough to reproduce spatial geographical gradients in weathering rates for relatively large areas as well as adequately predict absolute weathering rates values for the sum of base cations, Ca and Mg
Thermal phase diagrams of columnar liquid crystals
In order to understand the possible sequence of transitions from the
disordered columnar phase to the helical phase in hexa(hexylthio)triphenylene
(HHTT), we study a three-dimensional planar model with octupolar interactions
inscribed on a triangular lattice of columns. We obtain thermal phase diagrams
using a mean-field approximation and Monte Carlo simulations. These two
approaches give similar results, namely, in the quasi one-dimensional regime,
as the temperature is lowered, the columns order with a linear polarization,
whereas helical phases develop at lower temperatures. The helicity patterns of
the helical phases are determined by the exact nature of the frustration in the
system, itself related to the octupolar nature of the molecules.Comment: 12 pages, 9 figures, ReVTe
Improved efficacy response attributed to diagnostic selection â Interim results of the phase 1 experience from ALKA-372-001
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Weak Lensing from Space I: Instrumentation and Survey Strategy
A wide field space-based imaging telescope is necessary to fully exploit the
technique of observing dark matter via weak gravitational lensing. This first
paper in a three part series outlines the survey strategies and relevant
instrumental parameters for such a mission. As a concrete example of hardware
design, we consider the proposed Supernova/Acceleration Probe (SNAP). Using
SNAP engineering models, we quantify the major contributions to this
telescope's Point Spread Function (PSF). These PSF contributions are relevant
to any similar wide field space telescope. We further show that the PSF of SNAP
or a similar telescope will be smaller than current ground-based PSFs, and more
isotropic and stable over time than the PSF of the Hubble Space Telescope. We
outline survey strategies for two different regimes - a ``wide'' 300 square
degree survey and a ``deep'' 15 square degree survey that will accomplish
various weak lensing goals including statistical studies and dark matter
mapping.Comment: 25 pages, 8 figures, 1 table, replaced with Published Versio
GRB 081203A: Swift UVOT captures the earliest ultraviolet spectrum of a gamma-ray burst
We present the earliest ultraviolet (UV) spectrum of a gamma-ray burst (GRB) as observed with the Swift Ultra-Violet/Optical Telescope (UVOT). The GRB 081203A spectrum was observed for 50 s with the UV-grism starting 251 s after the Swift-Burst-Alert-Telescope (BAT) trigger. During this time, the GRB was â13.4 mag (u filter) and was still rising to its peak optical brightness. In the UV-grism spectrum, we find a damped Lyα line, LyÎČ and the Lyman continuum break at a redshift z= 2.05 ± 0.01. A model fit to the Lyman absorption implies a gas column density of log NH i= 22.0 ± 0.1 cmâ2, which is typical of GRB host galaxies with damped Lyα absorbers. This observation of GRB 081203A demonstrates that for brighter GRBs (vâ 14 mag) with moderate redshift (0.5 < z < 3.5) the UVOT is able to provide redshifts, and probe for damped Lyα absorbers within 4â6 min from the time of the Swift-BAT trigger
Supernova / Acceleration Probe: A Satellite Experiment to Study the Nature of the Dark Energy
The Supernova / Acceleration Probe (SNAP) is a proposed space-based
experiment designed to study the dark energy and alternative explanations of
the acceleration of the Universe's expansion by performing a series of
complementary systematics-controlled measurements. We describe a
self-consistent reference mission design for building a Type Ia supernova
Hubble diagram and for performing a wide-area weak gravitational lensing study.
A 2-m wide-field telescope feeds a focal plane consisting of a 0.7
square-degree imager tiled with equal areas of optical CCDs and near infrared
sensors, and a high-efficiency low-resolution integral field spectrograph. The
SNAP mission will obtain high-signal-to-noise calibrated light-curves and
spectra for several thousand supernovae at redshifts between z=0.1 and 1.7. A
wide-field survey covering one thousand square degrees resolves ~100 galaxies
per square arcminute. If we assume we live in a cosmological-constant-dominated
Universe, the matter density, dark energy density, and flatness of space can
all be measured with SNAP supernova and weak-lensing measurements to a
systematics-limited accuracy of 1%. For a flat universe, the
density-to-pressure ratio of dark energy can be similarly measured to 5% for
the present value w0 and ~0.1 for the time variation w'. The large survey area,
depth, spatial resolution, time-sampling, and nine-band optical to NIR
photometry will support additional independent and/or complementary dark-energy
measurement approaches as well as a broad range of auxiliary science programs.
(Abridged)Comment: 40 pages, 18 figures, submitted to PASP, http://snap.lbl.go
Feasibility of trial procedures for a randomised controlled trial of a community based group exercise intervention for falls prevention for visually impaired older people: the VIOLET study
Background Visually impaired older people (VIOP) have a higher risk of falling than their sighted peers, and are likely to avoid physical activity. The aim was to adapt the existing Falls Management Exercise (FaME) programme for VIOP, delivered in the community, and to investigate the feasibility of conducting a definitive randomised controlled trial (RCT) of this adapted intervention. Methods Two-centre randomised mixed methods pilot trial and economic evaluation of the adapted group-based FaME programme for VIOP versus usual care. A one hour exercise programme ran weekly over 12 weeks at the study sites (Newcastle and Glasgow), delivered by third sector (voluntary and community) organisations. Participants were advised to exercise at home for an additional two hours over the week. Those randomised to the usual activities group received no intervention. Outcome measures were completed at baseline, 12 and 24 weeks. The potential primary outcome was the Short Form Falls Efficacy Scale â International (SFES-I). Participantsâ adherence was assessed by reviewing attendance records and self-reported compliance to the home exercises. Adherence with the course content (fidelity) by instructors was assessed by a researcher. Adverse events were collected in a weekly phone call. Results Eighteen participants, drawn from community-living VIOP were screened; 68 met the inclusion criteria; 64 participants were randomised with 33 allocated to the intervention and 31 to the usual activities arm. 94% of participants provided data at the 12 week visit and 92% at 24 weeks. Adherence was high. The intervention was found to be safe with 76% attending nine or more classes. Median time for home exercise was 50 min per week. There was little or no evidence that fear of falling, balance and falls risk, physical activity, emotional, attitudinal or quality of life outcomes differed between trial arms at follow-up. Conclusions The intervention, FaME, was implemented successfully for VIOP and all progression criteria for a main trial were met. The lack of difference between groups on fear of falling was unsurprising given it was a pilot study but there may have been other contributory factors including suboptimal exercise dose and apparent low risk of falls in participants. These issues need addressing for a future trial
Quantum memory for entangled two-mode squeezed states
A quantum memory for light is a key element for the realization of future
quantum information networks. Requirements for a good quantum memory are (i)
versatility (allowing a wide range of inputs) and (ii) true quantum coherence
(preserving quantum information). Here we demonstrate such a quantum memory for
states possessing Einstein-Podolsky-Rosen (EPR) entanglement. These
multi-photon states are two-mode squeezed by 6.0 dB with a variable orientation
of squeezing and displaced by a few vacuum units. This range encompasses
typical input alphabets for a continuous variable quantum information protocol.
The memory consists of two cells, one for each mode, filled with cesium atoms
at room temperature with a memory time of about 1msec. The preservation of
quantum coherence is rigorously proven by showing that the experimental memory
fidelity 0.52(2) significantly exceeds the benchmark of 0.45 for the best
possible classical memory for a range of displacements.Comment: main text 5 pages, supplementary information 3 page
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