29,572 research outputs found
Scalable photonic quantum computation through cavity-assisted interaction
We propose a scheme for scalable photonic quantum computation based on cavity
assisted interaction between single-photon pulses. The prototypical quantum
controlled phase-flip gate between the single-photon pulses is achieved by
successively reflecting them from an optical cavity with a single-trapped atom.
Our proposed protocol is shown to be robust to practical nose and experimental
imperfections in current cavity-QED setups.Comment: 5 pages, 2 figure
Volatiles beneath mid-ocean ridges: deep melting, channelised transport, focusing, and metasomatism
Deep-Earth volatile cycles couple the mantle with near-surface reservoirs.
Volatiles are emitted by volcanism and, in particular, from mid-ocean ridges,
which are the most prolific source of basaltic volcanism. Estimates of volatile
extraction from the asthenosphere beneath ridges typically rely on measurements
of undegassed lavas combined with simple petrogenetic models of the mean degree
of melting. Estimated volatile fluxes have large uncertainties; this is partly
due to a poor understanding of how volatiles are transported by magma in the
asthenosphere. Here, we assess the fate of mantle volatiles through numerical
simulations of melting and melt transport at mid-ocean ridges. Our simulations
are based on two-phase, magma/mantle dynamics theory coupled to idealised
thermodynamic model of mantle melting in the presence of water and carbon
dioxide. We combine simulation results with catalogued observations of all
ridge segments to estimate a range of likely volatile output from the global
mid-ocean ridge system. We thus predict global MOR crust production of 66-73
Gt/yr (22-24 km3/yr) and global volatile output of 52-110 Mt/yr, corresponding
to mantle volatile contents of 100--200~ppm. We find that volatile extraction
is limited: up to half of deep, volatile-rich melt is not focused to the axis
but is rather deposited along the LAB. As these distal melts crystallise and
fractionate, they metasomatise the base of the lithosphere, creating
rheological heterogeneity that could contribute to the seismic signature of the
LAB.Comment: 42 pages; 8 figures; 2 appendices (incl 1 table); 7 suppl. figures; 1
suppl. tabl
Widespread HCN maser emission in carbon-rich evolved stars
Context. HCN is a major constituent of the circumstellar envelopes of
carbon-rich evolved stars, and rotational lines from within its vibrationally
excited states probe parts of these regions closest to the stellar surface. A
number of such lines are known to show maser action. Historically, in one of
them, the 177 GHz line in the -doubled bending mode has
been found to show relatively strong maser action, with results only published
for a single object, the archetypical high-mass loss asymptotic giant branch
(AGB) star IRC+10216. Aims. To examine how common 177 GHz HCN maser emission
is, we conducted an exploratory survey for this line toward a select sample of
carbon-rich asymptotic giant branch stars that are observable from the southern
hemisphere. Methods. We used the Atacama Pathfinder Experiment 12 meter
submillimeter Telescope (APEX) equipped with a new receiver to simultaneously
observe three HCN rotational transitions, the and -doublet components, and the line from
the (0,0,0) ground state. Results. The maser line is
detected toward 11 of 13 observed sources, which all show emission in the
(0,0,0) transition. In most of the sources, the peak intensity of the
line rivals that of the (0,0,0) line; in two sources,
it is even stronger. Except for the object with the highest mass-loss rate,
IRC+10216, the line covers a smaller velocity range
than the (0,0,0) line. Conclusions. Maser emission in the 177 GHz
line of HCN appears to be common in
carbon-rich AGB stars. (Abbreviated)Comment: 12 pages (including appendix), 3 figures / Astronomy & Astrophysics
(in press
The quantum metrology triangle and the re-definition of the SI ampere and kilogram; Analysis of a reduced set of observational equations
We have developed a set of seven observational equations that include all of
the physics necessary to relate the most important of the fundamental constants
to the definitions of the SI kilogram and ampere. We have used these to
determine the influence of alternative definitions being considered for the SI
kilogram and ampere on the uncertainty of three of the fundamental constants
(h, e and mu). We have also reviewed the experimental evidence for the
exactness of the quantum metrology triangle resulting from experiments
combining the quantum Hall effect, the Josephson effects and single-electron
tunnelling.Comment: 16 pages, 3 figures & 5 table
Instrumental polarisation at the Nasmyth focus of the E-ELT
The ~39-m European Extremely Large Telescope (E-ELT) will be the largest
telescope ever built. This makes it particularly suitable for sensitive
polarimetric observations, as polarimetry is a photon-starved technique.
However, the telescope mirrors may severely limit the polarimetric accuracy of
instruments on the Nasmyth platforms by creating instrumental polarisation
and/or modifying the polarisation signal of the object. In this paper we
characterise the polarisation effects of the two currently considered designs
for the E-ELT Nasmyth ports as well as the effect of ageing of the mirrors. By
means of the Mueller matrix formalism, we compute the response matrices of each
mirror arrangement for a range of zenith angles and wavelengths. We then
present two techniques to correct for these effects that require the addition
of a modulating device at the polarisation-free intermediate focus that acts
either as a switch or as a part of a two-stage modulator. We find that the
values of instrumental polarisation, Stokes transmission reduction and cross-
talk vary significantly with wavelength, and with pointing, for the lateral
Nasmyth case, often exceeding the accuracy requirements for proposed
polarimetric instruments. Realistic ageing effects of the mirrors after perfect
calibration of these effects may cause polarimetric errors beyond the
requirements. We show that the modulation approach with a polarimetric element
located in the intermediate focus reduces the instrumental polarisation effects
down to tolerable values, or even removes them altogether. The E-ELT will be
suitable for sensitive and accurate polarimetry, provided frequent calibrations
are carried out, or a dedicated polarimetric element is installed at the
intermediate focus.Comment: Accepted for publication in A&
A mixed approach and a distribution free multiple imputation technique for the estimation of multivariate probit models with missing values
In the present paper a mixed generalized estimating/pseudoÂscore equations (GEPSE) approach together with a distribution free multiple imputation technique is proposed for the estimation of regression and correlation structure parameters of multivariate probit models with missing values for an ordered categorical time invariant variable. Furthermore, a generalization of the squared trace correlation (R_T^2) for multivariate probit models, denoted as pseudo R_T^2, is proposed. A simulation study was conducted, simulating a probit model with an equicorrelation structure in the errors of an underlying regression model and using two different missing mechanisms. For a low `true' correlation the difference between the GEPSE, a generalized estimating equations (GEE) and a maximum likelihood (ML) estimator were negligible. For a high `true' correlation the GEPSE estimator turned out to be more efficient than the GEE and very efficient relative to the ML estimator. Furthermore, the pseudo R_T^2 was close to R_T^2 of the underlying linear model. The mixed approach is illustrated using a psychiatric data set of depressive inpatients. The results of this analysis suggest, that the depression score at discharge from a psychiatric hospital and the occurence of stressful life events seem to increase the probability of having an episode of major depression within a oneÂyear interval after discharge. Furthermore, the correlation structure points to shortÂtime effects on having or not having a depressive episode, not accounted for in the systematic part of the regression model
Carbohydrate Metabolism in Drought-Stressed Leaves of Pigeonpea (Cajanus cajan)
Pigeonpea is a tropical grain-legume, which is highly dehydration tolerant. The effect of drought stress on the carbohydrate metabolism in mature pigeonpea leaves was investigated by withholding water from plants grown in very large pots (50 kg of soil). The most striking feature of drought-stressed plants was the pronounced accumulation of D-pinitol (1D-3-methyl-chiro-inositol), which increased from 14 to 85 mg g−1 dry weight during a 27 d stress period. Concomitantly, the levels of starch, sucrose and the pinitol precursors myo-inositol and ononitol all decreased rapidly to zero or near-zero in response to drought. The levels of glucose and fructose increased moderately. Drought stress induced a pronounced increase of the activities of enzymes hydrolysing soluble starch (amylases) and sucrose (invertase and sucrose synthase). The two anabolic enzymes sucrose phosphate synthase (sucrose synthetic pathway) and myo-inositol methyl transferase (pinitol synthetic pathway) also showed an increase of activity during stress. These results indicate that pinitol accumulated in pigeonpea leaves, because the carbon flux was diverted from starch and sucrose into polyol
An analytical analysis of vesicle tumbling under a shear flow
Vesicles under a shear flow exhibit a tank-treading motion of their membrane,
while their long axis points with an angle < 45 degrees with respect to the
shear stress if the viscosity contrast between the interior and the exterior is
not large enough. Above a certain viscosity contrast, the vesicle undergoes a
tumbling bifurcation, a bifurcation which is known for red blood cells. We have
recently presented the full numerical analysis of this transition. In this
paper, we introduce an analytical model that has the advantage of being both
simple enough and capturing the essential features found numerically. The model
is based on general considerations and does not resort to the explicit
computation of the full hydrodynamic field inside and outside the vesicle.Comment: 19 pages, 9 figures, to be published in Phys. Rev.
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