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Das Internationale Einheitensystem (SI)

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This revolution has been a long time in the making. Behind it are metrologists working in their scientific laboratories, and scientific managers in their decision-making committees. Now the moment is approaching when many of the old quantities (specifically, the definitions behind them) will have to step down to make way for their successors, who are waiting in the wings. The quantities concerned...

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The "atomic second" was the beginning of a revolutionary era: it was born as early as 1955, when the first cesium atomic clock was put into operation. In the fall of 1967, it was included in the International System of Units. This was the beginning of a development which will, in all likelihood, come to an end in the fall of 2018 when the 20th General Conference on Weights and Measures (CGPM)...

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No one had ever come so close to the ideal laser before: theoretically, laser light has only one single color (also frequency or wavelength). In reality, however, there is always a certain linewidth. With a linewidth of only 10 mHz, the laser that researchers from the Physikalisch-Technische Bundesanstalt (PTB) have now developed together with US researchers from JILA, a joint institute of the...

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At present, the kelvin is literally based on no more than water – on the triple point of water, to be more precise. The base unit of temperature is therefore dependent on a material whose properties may vary. But this is about to change: In the fall of 2018, the kelvin, as well as all the other units of the International System of Units (SI), will be redefined to rest on a solid and invariable...

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The universe of physics is experiencing great changes: by 2018, scientists want to place all physical base units on solid, unchangeable foundation in the form of fundamental constants. The units "meter" and "second" were well ahead of schedule; now the kelvin, the kilogram, the mole and the ampere are next in line. The research activities of the Physikalisch-Technische Bundesanstalt (PTB) are...

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In the past few years, optical atomic clocks have made spectacular progress, becoming 100 times more precise than the best caesium clocks. So far, their precision has been available only locally, since frequency transfer via satellite cannot provide sufficient resolution. This has recently changed thanks to a new direct optical connection between France and Germany, established by joint work of...

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Nach fast 130 Jahren wird das internationale Ur-Kilogramm im Herbst 2018 voraussichtlich in den wohlverdienten Ruhestand geschickt. Dann kommt die Welt zu einem metrologischen Gipfeltreffen zusammen, um über die Neudefinition des Kilogramms zu entscheiden. Wie der Meter und die Sekunde soll künftig auch das Kilogramm über unveränderliche Naturkonstanten definiert werden. Dazu zählen...

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Atomic clock experts from the Physikalisch-Technische Bundesanstalt (PTB) are the first research group in the world to have built an optical single-ion clock which attains an accuracy which had only been predicted theoretically so far. As early as 1981, Hans Dehmelt, who was to be awarded a Nobel prize later, had already developed the basic notions of how to use an ion kept in a high-frequency...

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Researchers from the Physikalisch-Technische Bundesanstalt (PTB) have thoroughly analyzed the noise processes in their optical lattice clock with neutral strontium atoms. This analysis proves that their optical atomic clock has reached the best stability worldwide thanks to a newly developed laser system whose frequency is extremely stable. This allows high-precision measurements in a short time...

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