48 research outputs found
Evaluation of Doppler Shifts to Improve the Accuracy of Primary Atomic Fountain Clocks
We demonstrate agreement between measurements and ab initio calculations of
the frequency shifts caused by distributed cavity phase variations in the
microwave cavity of a primary atomic fountain clock. Experimental verification
of the finite element models of the cavities gives the first quantitative
evaluation of this leading uncertainty and allows it to be reduced to delta nu
/ nu = 8.4\times10^-17. Applying these experimental techniques to clocks with
improved microwave cavities will yield negligible distributed cavity phase
uncertainties, less than \pm1\times10^-17.Comment: To appear in PR
Colloquium: Comparison of Astrophysical and Terrestrial Frequency Standards
We have re-analyzed the stability of pulse arrival times from pulsars and
white dwarfs using several analysis tools for measuring the noise
characteristics of sampled time and frequency data. We show that the best
terrestrial artificial clocks substantially exceed the performance of
astronomical sources as time-keepers in terms of accuracy (as defined by cesium
primary frequency standards) and stability. This superiority in stability can
be directly demonstrated over time periods up to two years, where there is high
quality data for both. Beyond 2 years there is a deficiency of data for
clock/clock comparisons and both terrestrial and astronomical clocks show equal
performance being equally limited by the quality of the reference timescales
used to make the comparisons. Nonetheless, we show that detailed accuracy
evaluations of modern terrestrial clocks imply that these new clocks are likely
to have a stability better than any astronomical source up to comparison times
of at least hundreds of years. This article is intended to provide a correct
appreciation of the relative merits of natural and artificial clocks. The use
of natural clocks as tests of physics under the most extreme conditions is
entirely appropriate; however, the contention that these natural clocks,
particularly white dwarfs, can compete as timekeepers against devices
constructed by mankind is shown to be doubtful.Comment: 9 pages, 2 figures; presented at the International Frequency Control
Symposium, Newport Beach, Calif., June, 2010; presented at Pulsar Conference
2010, October 12th, Sardinia; accepted 13th September 2010 for publication in
Reviews of Modern Physic
An Optical Lattice Clock with Spin-polarized 87Sr Atoms
We present a new evaluation of an 87Sr optical lattice clock using spin
polarized atoms. The frequency of the 1S0-3P0 clock transition is found to be
429 228 004 229 873.6 Hz with a fractional accuracy of 2.6 10^{-15}, a value
that is comparable to the frequency difference between the various primary
standards throughout the world. This measurement is in excellent agreement with
a previous one of similar accuracy
Evaluating and Minimizing Distributed Cavity Phase Errors in Atomic Clocks
We perform 3D finite element calculations of the fields in microwave cavities
and analyze the distributed cavity phase errors of atomic clocks that they
produce. The fields of cylindrical cavities are treated as an azimuthal Fourier
series. Each of the lowest components produces clock errors with unique
characteristics that must be assessed to establish a clock's accuracy. We
describe the errors and how to evaluate them. We prove that sharp structures in
the cavity do not produce large frequency errors, even at moderately high
powers, provided the atomic density varies slowly. We model the amplitude and
phase imbalances of the feeds. For larger couplings, these can lead to
increased phase errors. We show that phase imbalances produce a novel
distributed cavity phase error that depends on the cavity detuning. We also
design improved cavities by optimizing the geometry and tuning the mode
spectrum so that there are negligible phase variations, allowing this source of
systematic error to be dramatically reduced.Comment: To appear in Metrologi
F. Chapelet (illustrateur 18..-19..) : signature [1904]
F. Chapelet (illustrateur 18..-19..) : signature [1904