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Das Internationale Einheitensystem (SI)


Silicon resonator

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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Die Boltzmann-Apparatur der PTB (Foto: PTB)

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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In der frisch gereinigten Si-Kugel spiegelt sich PTB-Mitarbeiter Volker Görlitz. Foto: PTB

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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Schema: Einfluss der thermischen Strahlung bei der Ytterbiumuhr

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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Noise contributions of the strontium lattice clock

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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Quantum logic spectroscopy of molecular ions

The interaction of thermal energy from the environment with motional degrees of freedom is well known and often referred to as Brownian motion (also thermal motion). But in the case of polar molecules, the internal degrees of freedom – in particular the rotational quantum state – are also influenced by the thermal radiation. So far, the detection of the rotational state was only possible by...

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Fundamental constants are to define the units in the International System of Units in the future (Grafik: PTB)

The second and the metre have already surged ahead. The kilogram, the ampere and the other physical base units are now trying to catch up. This is one way of describing what is currently happening in the world of metrology, in the world of measuring. The International System of Units is facing a redefinition of its foundations: fundamental constants are to serve as the defining reference values...

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