308 research outputs found
Rotational dynamics of a soft filament: wrapping transition and propulsive forces
We analyze experimentally the shape of a long elastic filament rotating in a
viscous liquid. We identify a continuous but sharp transition from a straight
to an helical shape, resulting from the competition between viscous stresses
and elastic forces. This induced helicity generates a propulsive force along
the axis of rotation. In addition, we show that the shape transition is
associated with an unstable branch in the force-torque relation. A linearized
model of the fluid-structure interaction is proposed to account for all the
features of the non-linear filament dynamics
Ultra-Low Noise Microwave Extraction from Fiber-Based Optical Frequency Comb
In this letter, we report on all-optical fiber approach to the generation of
ultra-low noise microwave signals. We make use of two erbium fiber mode-locked
lasers phase locked to a common ultra-stable laser source to generate an 11.55
GHz signal with an unprecedented relative phase noise of -111 dBc/Hz at 1 Hz
from the carrier.The residual frequency instability of the microwave signals
derived from the two optical frequency combs is below 2.3 10^(-16) at 1s and
about 4 10^(-19) at 6.5 10^(4)s (in 5 Hz bandwidth, three days continuous
operation).Comment: 12 pages, 3 figure
Generic flow profiles induced by a beating cilium
We describe a multipole expansion for the low Reynolds number fluid flows
generated by a localized source embedded in a plane with a no-slip boundary
condition. It contains 3 independent terms that fall quadratically with the
distance and 6 terms that fall with the third power. Within this framework we
discuss the flows induced by a beating cilium described in different ways: a
small particle circling on an elliptical trajectory, a thin rod and a general
ciliary beating pattern. We identify the flow modes present based on the
symmetry properties of the ciliary beat.Comment: 12 pages, 6 figures, to appear in EPJ
Religious faith in education: enemy or asset?
In this article I hope to cast some light on the relationship between religious faith and education by a preliminary mapping of the field. There are three parts to the article. First, I lay out the assumptions from which the rest of the article builds. Second, I seek to identify possible links between religion and education. As a sub-set of this, I explore a range of ways that theology might relate to education. Third, as a step towards a more healthy relationship between education and religious faith, I offer reasons why the church needs the academy and the academy needs the church. In the light of a convergence of the concerns that I show are shared by religious believers and educators, it is suggested that religious faith in the context of education should be considered an asset rather than an enemy
A clock network for geodesy and fundamental science
Leveraging the unrivaled performance of optical clocks in applications in
fundamental physics beyond the standard model, in geo-sciences, and in
astronomy requires comparing the frequency of distant optical clocks
truthfully. Meeting this requirement, we report on the first comparison and
agreement of fully independent optical clocks separated by 700 km being only
limited by the uncertainties of the clocks themselves. This is achieved by a
phase-coherent optical frequency transfer via a 1415 km long telecom fiber link
that enables substantially better precision than classical means of frequency
transfer. The fractional precision in comparing the optical clocks of three
parts in was reached after only 1000 s averaging time, which is
already 10 times better and more than four orders of magnitude faster than with
any other existing frequency transfer method. The capability of performing high
resolution international clock comparisons paves the way for a redefinition of
the unit of time and an all-optical dissemination of the SI-second.Comment: 14 pages, 3 figures, 1 tabl
A two-state Raman coupler for coherent atom optics
We present results on a Raman laser-system that resonantly drives a closed
two-photon transition between two levels in different hyperfine ground states
of 87Rb. The coupler is based on a novel optical design for producing two
phase-coherent optical beams to drive a Raman transition. Operated as an
outcoupler, it produces an atom laser in a single internal atomic state, with
the lower divergence and increased brightness typical of a Raman outcoupler.
Due to the optical nature of the outcoupling, the two-state outcoupler is an
ideal candidate for transferring photon correlations onto atom-laser beams. As
our laser system couples just two hyperfine ground states, it has also been
used as an internal state beamsplitter, taking the next major step towards free
space Ramsey interferometry with an atom laser.Comment: 7 Pages, 4 figures: Revised and published in Optics Expres
Sr lattice clock at 1x10^{-16} fractional uncertainty by remote optical evaluation with a Ca clock
Optical atomic clocks promise timekeeping at the highest precision and
accuracy, owing to their high operating frequencies. Rigorous evaluations of
these clocks require direct comparisons between them. We have realized a
high-performance remote comparison of optical clocks over km-scale urban
distances, a key step for development, dissemination, and application of these
optical standards. Through this remote comparison and a proper design of
lattice-confined neutral atoms for clock operation, we evaluate the uncertainty
of a strontium (Sr) optical lattice clock at the 1x10-16 fractional level,
surpassing the best current evaluations of cesium (Cs) primary standards. We
also report on the observation of density-dependent effects in the
spin-polarized fermionic sample and discuss the current limiting effect of
blackbody radiation-induced frequency shifts.Comment: To be published in Science, 200
Atom laser coherence and its control via feedback
We present a quantum-mechanical treatment of the coherence properties of a
single-mode atom laser. Specifically, we focus on the quantum phase noise of
the atomic field as expressed by the first-order coherence function, for which
we derive analytical expressions in various regimes. The decay of this function
is characterized by the coherence time, or its reciprocal, the linewidth. A
crucial contributor to the linewidth is the collisional interaction of the
atoms. We find four distinct regimes for the linewidth with increasing
interaction strength. These range from the standard laser linewidth, through
quadratic and linear regimes, to another constant regime due to quantum
revivals of the coherence function. The laser output is only coherent (Bose
degenerate) up to the linear regime. However, we show that application of a
quantum nondemolition measurement and feedback scheme will increase, by many
orders of magnitude, the range of interaction strengths for which it remains
coherent.Comment: 15 pages, 6 figures, revtex
A coinductive semantics of the Unlimited Register Machine
We exploit (co)inductive specifications and proofs to approach the evaluation of low-level programs for the Unlimited Register Machine (URM) within the Coq system, a proof assistant based on the Calculus of (Co)Inductive Constructions type theory. Our formalization allows us to certify the implementation of partial functions, thus it can be regarded as a first step towards the development of a workbench for the formal analysis and verification of both converging and diverging computations
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