62,035 research outputs found
The use of perfluoroether lubricants in unprotected space environments
A series of ball bearing tests in simulated space environment are described which determine durability of perfluoroether lubricants. The results of the examination of the test bearings for each stage are described and experimental techniques designed to overcome lubricant degradation are outlined
Condensate splitting in an asymmetric double well for atom chip based sensors
We report on the adiabatic splitting of a BEC of Rb atoms by an
asymmetric double-well potential located above the edge of a perpendicularly
magnetized TbGdFeCo film atom chip. By controlling the barrier height and
double-well asymmetry the sensitivity of the axial splitting process is
investigated through observation of the fractional atom distribution between
the left and right wells. This process constitutes a novel sensor for which we
infer a single shot sensitivity to gravity fields of . From a simple analytic model we propose improvements
to chip-based gravity detectors using this demonstrated methodology.Comment: 4 pages, 5 figure
Precision measurements of s-wave scattering lengths in a two-component Bose-Einstein condensate
We use collective oscillations of a two-component Bose-Einstein condensate
(2CBEC) of \Rb atoms prepared in the internal states and for the precision measurement of
the interspecies scattering length with a relative uncertainty of
. We show that in a cigar-shaped trap the three-dimensional
(3D) dynamics of a component with a small relative population can be
conveniently described by a one-dimensional (1D) Schr\"{o}dinger equation for
an effective harmonic oscillator. The frequency of the collective oscillations
is defined by the axial trap frequency and the ratio , where
is the intra-species scattering length of a highly populated component
1, and is largely decoupled from the scattering length , the total atom
number and loss terms. By fitting numerical simulations of the coupled
Gross-Pitaevskii equations to the recorded temporal evolution of the axial
width we obtain the value , where is the Bohr
radius. Our reported value is in a reasonable agreement with the theoretical
prediction but deviates significantly from the
previously measured value \cite{Mertes07} which is commonly
used in the characterisation of spin dynamics in degenerate \Rb atoms. Using
Ramsey interferometry of the 2CBEC we measure the scattering length
which also deviates from the previously reported value
\cite{Mertes07}. We characterise two-body losses for the
component 2 and obtain the loss coefficients
and
.Comment: 11 pages, 8 figure
Three applications for mobile epidemic algorithms
This paper presents a framework for the pervasive sharing of data using wireless networks. 'FarCry' uses the mobility of users to carry files between separated networks. Through a mix of ad-hoc and infrastructure-based wireless networking, files are transferred between users without their direct involvement. As users move to different locations, files are then transmitted on to other users, spreading and sharing information. We examine three applications of this framework. Each of these exploits the physically proximate nature of social gatherings. As people group together in, for example, business meetings and cafés, this can be taken as an indication of similar interests, e.g. in the same presentation or in a type of music. MediaNet affords sharing of media files between strangers or friends, MeetingNet shares business documents in meetings, and NewsNet shares RSS feeds between mobile users. NewsNet also develops the use of pre-emptive caching: collecting information from others not for oneself, but for the predicted later sharing with others. We offer observations on developing this system for a mobile, multi-user, multi-device environment
Quasars in the 2MASS Second Incremental Data Release
Using the 2MASS Second Incremental Data Release, we have searched for near
infrared counterparts to 13214 quasars from the Veron-Cetty & Veron(2000)
catalog. We have detected counterparts within 4 arcsec for 2277 of the
approximately 6320 quasars within the area covered by the 2MASS Second
Incremental Data Release. Only 1.6% of these are expected to be chance
coincidences. Though this sample is heterogeneous, we find that known
radio-loud quasars are more likely to have large near-infrared-to-optical
luminosity ratios than radio-quiet quasars are, at a statistically significant
level. This is consistent with dust-reddened quasars being more common in
radio-selected samples than in optically-selected samples, due to stronger
selection effects against dust-reddened quasars in the latter. We also find a
statistically significant dearth of optically luminous quasars with large
near-infrared-to-optical luminosity ratios. This can be explained in a dust
obscuration model but not in a model where synchrotron emission extends from
the radio into the near-infrared and creates such large ratios. We also find
that selection of quasar candidates from the B-J/J-K color-color diagram,
modelled on the V-J/J-K selection method of Warren, Hewett & Foltz (2000), is
likely to be more sensitive to dust-obscured quasars than selection using only
infrared-infrared colors.Comment: To be published in May issue of Astronomical Journal (26 pages, 8
figures, 2 tables) Replaced Figure 6 and
Resampling-based confidence regions and multiple tests for a correlated random vector
We derive non-asymptotic confidence regions for the mean of a random vector
whose coordinates have an unknown dependence structure. The random vector is
supposed to be either Gaussian or to have a symmetric bounded distribution, and
we observe i.i.d copies of it. The confidence regions are built using a
data-dependent threshold based on a weighted bootstrap procedure. We consider
two approaches, the first based on a concentration approach and the second on a
direct boostrapped quantile approach. The first one allows to deal with a very
large class of resampling weights while our results for the second are
restricted to Rademacher weights. However, the second method seems more
accurate in practice. Our results are motivated by multiple testing problems,
and we show on simulations that our procedures are better than the Bonferroni
procedure (union bound) as soon as the observed vector has sufficiently
correlated coordinates.Comment: submitted to COL
A Bohmian approach to quantum fractals
A quantum fractal is a wavefunction with a real and an imaginary part
continuous everywhere, but differentiable nowhere. This lack of
differentiability has been used as an argument to deny the general validity of
Bohmian mechanics (and other trajectory--based approaches) in providing a
complete interpretation of quantum mechanics. Here, this assertion is overcome
by means of a formal extension of Bohmian mechanics based on a limiting
approach. Within this novel formulation, the particle dynamics is always
satisfactorily described by a well defined equation of motion. In particular,
in the case of guidance under quantum fractals, the corresponding trajectories
will also be fractal.Comment: 19 pages, 3 figures (revised version
Advanced software techniques for data management systems. Volume 3: Programming language characteristics and comparison reference
A comparative evaluation was made of eight higher order languages of general interest in the aerospace field: PL/1; HAL; JOVIAL/J3; SPL/J6; CLASP; ALGOL 60; FORTRAN 4; and MAC360. A summary of the functional requirements for a language for general use in manned aerodynamic applications is presented. The evaluation supplies background material to be used in assessing the worth of each language for some particular application
Aerodynamics of lift fan V/STOL aircraft
Aerodynamic characteristics of lift fan installation for direct lift V/STOL aircraf
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