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The experimental atomic clocks of the National Institute of Standardization and Technology (NIST) have recorded three new performance records, now accurate enough to allow not only to improve the timing and navigation, but also to detect weak signals of gravity, the primitive universe and perhaps even dark matter.
The clocks each trap a thousand ytterbium atoms in optical gratings, grids made up of laser beams. Atoms work by vibrating or rocking between two levels of energy. Comparing two independent clocks, NIST physicists achieved record performance in three important ways: systematic uncertainty, stability, and reproducibility.
Posted online November 28 in the newspaper Nature, the new NIST clock records are:
- Systematic uncertainty: quality with which the clock represents natural vibrations, or frequency, atoms. NIST researchers found that each clock clocked at a natural frequency with a possible error of only 1.4 parts out of 10.18 – about a billionth of a billionth.
- Stability: the evolution of clock frequency over a specified time interval, measured at a level of 3.2 out of 10 parts19 (or 0.00000000000000000032) on a day.
- Reproducibility: The distance between the two clocks at the same frequency is illustrated by 10 comparisons of the clock pair, giving a frequency difference of less than 10-18 level (again, less than a billionth of a billionth).
"Systematic uncertainty, stability and reproducibility can be viewed as the" royal flush "of these clocks' performance," said project leader Andrew Ludlow. "The agreement of the two clocks at this unprecedented level, which we call reproducibility, is perhaps the most important result because it essentially requires and corroborates the other two results."
"This is especially true because the demonstrated reproducibility shows that the total error of the clocks is less than our general ability to take into account the effect of gravity on time here on Earth. Therefore, as we plan to use clocks of this type in the world or in the world, their relative performance would, for the first time, be limited by the effects of earth gravitation. "
Einstein's theory of relativity predicts that the ticking of an atomic clock, that is the frequency of atomic vibrations, is reduced – shifted to the red part of the electromagnetic spectrum – when it is operated under a stronger gravity. That is, time pbades more slowly at low altitude.
Although these "red shifts" degrade the clock indication, this same sensitivity can be returned to accurately measure gravity. Super-sensitive clocks can map the gravitational distortion of space-time more accurately than ever. Applications include relativistic geodesy, which measures the gravitational shape of the Earth, and detects signals from the beginning of the universe, such as gravitational waves and perhaps even unexplained "dark matter".
The NIST ytterbium clocks now exceed the conventional ability to measure the geoid, or shape of the Earth, based on sea level tide gauge readings. Comparisons of such clocks distant from each other, as on different continents , would solve the geodesic measurements to one centimeter, better than the current state of the art, which is several centimeters.
Over the last decade, NIST and other laboratories around the world have announced new clock performance records. This last document presents a reproducibility of high level, indicated the researchers. In addition, the comparison of two clocks is the traditional method of performance evaluation.
Among the improvements made to the latest NIST ytterbium clocks is the inclusion of thermal and electrical shielding, which surrounds atoms to protect them from stray electric fields and allows researchers to better characterize and correct frequency shifts caused by thermal rays.
The ytterbium atom is among the potential candidates for the future redefinition of the second – the international unit of time – in terms of optical frequencies. The new NIST clock recordings meet one of the international requirements of the redefinition roadmap, which is 100 times more accurate than the accuracy validated against the best clocks based on the current standard, the Atom cesium, which vibrates at lower microwave frequencies.
NIST is building a state-of-the-art ytterbium lattice lattice clock that can be transported to other labs around the world for clock comparison purposes and to other sites for exploring relativistic geodesy techniques.
The work is supported by NIST, the National Aeronautics and Space Administration and the Defense Advanced Research Projects Agency.
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