22,184 research outputs found
An atomic clock with instability
Atomic clocks have been transformational in science and technology, leading
to innovations such as global positioning, advanced communications, and tests
of fundamental constant variation. Next-generation optical atomic clocks can
extend the capability of these timekeepers, where researchers have long aspired
toward measurement precision at 1 part in . This milestone will
enable a second revolution of new timing applications such as relativistic
geodesy, enhanced Earth- and space-based navigation and telescopy, and new
tests on physics beyond the Standard Model. Here, we describe the development
and operation of two optical lattice clocks, both utilizing spin-polarized,
ultracold atomic ytterbium. A measurement comparing these systems demonstrates
an unprecedented atomic clock instability of after
only hours of averaging
Speech rhythm: a metaphor?
Is speech rhythmic? In the absence of evidence for a traditional view that languages strive to coordinate either syllables or stress-feet with regular time intervals, we consider the alternative that languages exhibit contrastive rhythm subsisting merely in the alternation of stronger and weaker elements. This is initially plausible, particularly for languages with a steep âprominence gradientâ, i.e. a large disparity between stronger and weaker elements; but we point out that alternation is poorly achieved even by a âstress-timedâ language such as English, and, historically, languages have conspicuously failed to adopt simple phonological remedies that would ensure alternation. Languages seem more concerned to allow âsyntagmatic contrastâ between successive units and to use durational effects to support linguistic functions than to facilitate rhythm. Furthermore, some languages (e.g. Tamil, Korean) lack the lexical prominence which would most straightforwardly underpin prominence alternation. We conclude that speech is not incontestibly rhythmic, and may even be antirhythmic. However, its linguistic structure and patterning allow the metaphorical extension of rhythm in varying degrees and in different ways depending on the language, and that it is this analogical process which allows speech to be matched to external rhythms
Demonstration of a Transportable 1 Hz-Linewidth Laser
We present the setup and test of a transportable clock laser at 698 nm for a
strontium lattice clock. A master-slave diode laser system is stabilized to a
rigidly mounted optical reference cavity. The setup was transported by truck
over 400 km from Braunschweig to D\"usseldorf, where the cavity-stabilized
laser was compared to a stationary clock laser for the interrogation of
ytterbium (578 nm). Only minor realignments were necessary after the transport.
The lasers were compared by a Ti:Sapphire frequency comb used as a transfer
oscillator. The thus generated virtual beat showed a combined linewidth below 1
Hz (at 1156 nm). The transport back to Braunschweig did not degrade the laser
performance, as was shown by interrogating the strontium clock transition.Comment: 3 pages, 4 figure
Development of a strontium optical lattice clock for the SOC mission on the ISS
Ultra-precise optical clocks in space will allow new studies in fundamental
physics and astronomy. Within an European Space Agency (ESA) program, the Space
Optical Clocks (SOC) project aims to install and to operate an optical lattice
clock on the International Space Station (ISS) towards the end of this decade.
It would be a natural follow-on to the ACES mission, improving its performance
by at least one order of magnitude. The payload is planned to include an
optical lattice clock, as well as a frequency comb, a microwave link, and an
optical link for comparisons of the ISS clock with ground clocks located in
several countries and continents. Within the EU-FP7-SPACE-2010-1 project no.
263500, during the years 2011-2015 a compact, modular and robust strontium
lattice optical clock demonstrator has been developed. Goal performance is a
fractional frequency instability below 1x10^{-15}, tau^{-1/2} and a fractional
inaccuracy below 5x10^{-17}. Here we describe the current status of the
apparatus' development, including the laser subsystems. Robust preparation of
cold {88}^Sr atoms in a second stage magneto-optical trap (MOT) is achieved.Comment: 27 Pages, 15 figures, Comptes Rendus Physique 201
Progress in Atomic Fountains at LNE-SYRTE
We give an overview of the work done with the Laboratoire National de
M\'etrologie et d'Essais-Syst\`emes de R\'ef\'erence Temps-Espace (LNE-SYRTE)
fountain ensemble during the last five years. After a description of the clock
ensemble, comprising three fountains, FO1, FO2, and FOM, and the newest
developments, we review recent studies of several systematic frequency shifts.
This includes the distributed cavity phase shift, which we evaluate for the FO1
and FOM fountains, applying the techniques of our recent work on FO2. We also
report calculations of the microwave lensing frequency shift for the three
fountains, review the status of the blackbody radiation shift, and summarize
recent experimental work to control microwave leakage and spurious phase
perturbations. We give current accuracy budgets. We also describe several
applications in time and frequency metrology: fountain comparisons,
calibrations of the international atomic time, secondary representation of the
SI second based on the 87Rb hyperfine frequency, absolute measurements of
optical frequencies, tests of the T2L2 satellite laser link, and review
fundamental physics applications of the LNE-SYRTE fountain ensemble. Finally,
we give a summary of the tests of the PHARAO cold atom space clock performed
using the FOM transportable fountain.Comment: 19 pages, 12 figures, 5 tables, 126 reference
GPS Carrier Tracking Loop Performance in the presence of Ionospheric Scintillations
The performance of several GPS carrier tracking loops
is evaluated using wideband GPS data recorded during
strong ionospheric scintillations. The aim of this study is
to determine the loop structures and parameters that enable
good phase tracking during the power fades and phase
dynamics induced by scintillations. Constant-bandwidth
and variable-bandwidth loops are studied using theoretical
models, simulation, and tests with actual GPS signals.
Constant-bandwidth loops with loop bandwidths near 15
Hz are shown to lose phase lock during scintillations. Use
of the decision-directed discriminator reduces the carrier
lock threshold by âŒ1 dB relative to the arctangent and conventional Costas discriminators. A proposed variablebandwidth
loop based on a Kalman filter reduces the carrier
lock threshold by more than 7 dB compared to a 15-Hz
constant-bandwidth loop. The Kalman filter-based strategy
employs a soft-decision discriminator, explicitly models
the effects of receiver clock noise, and optimally adapts
the loop bandwidth to the carrier-to-noise ratio. In extensive
simulation and in tests using actual wideband GPS
data, the Kalman filter PLL demonstrates improved cycle
slip immunity relative to constant bandwidth PLLs.Aerospace Engineering and Engineering Mechanic
Performance of a 229 Thorium solid-state nuclear clock
The 7.8 eV nuclear isomer transition in 229 Thorium has been suggested as an
etalon transition in a new type of optical frequency standard. Here we discuss
the construction of a "solid-state nuclear clock" from Thorium nuclei implanted
into single crystals transparent in the vacuum ultraviolet range. We
investigate crystal-induced line shifts and broadening effects for the specific
system of Calcium fluoride. At liquid Nitrogen temperatures, the clock
performance will be limited by decoherence due to magnetic coupling of the
Thorium nucleus to neighboring nuclear moments, ruling out the commonly used
Rabi or Ramsey interrogation schemes. We propose a clock stabilization based on
counting of flourescence photons and present optimized operation parameters.
Taking advantage of the high number of quantum oscillators under continuous
interrogation, a fractional instability level of 10^{-19} might be reached
within the solid-state approach.Comment: 28 pages, 9 figure
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