PTB-Experiment

Optical atomic clocks are the next generation of atomic clocks. Currently, they are still at the development stage. In the case of today's atomic clocks, the clock frequency lies in the microwave range, and cesium atoms are usually used as a reference. In the case of optical clocks, the clock frequency is more than 10 000 times higher (100 THz - 1000 THz) and lies, thus, in the optical spectral range. Due to this fact, their accuracy – which can be achieved after clearly shorter averaging times – is approx. a 100 times higher. There are different variants, with different reference atoms or reference ions, for which different technologies have been used and which, therefore, also have different advantages and disadvantages. However, no special type has so far established itself. But the race to find the best clock of the future has already started. At PTB, several possible variants are being investigated.

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The Avogadro experiment and the watt balance are the pillars of the redefinition of the kilogram. They represent two independent possibilities of realizing the redefinition. Today, the unit "kilogram" is defined as the mass of the international prototype of the kilogram which is kept in a safe in Sèvres near Paris. International efforts are aimed at relating all base units, such as the second, the meter, the kilogram and the ampere, in future only to fundamental constants in order to create a system of units which is independent of artifacts. For this purpose, it is determined in the Avogadro experiment how many atoms are contained in almost perfect silicon spheres. The next possibility for a redefinition of the kilogram will present itself on the occasion of the General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM) in 2018. At the same time, the results of the Avogadro experiment must beconsistent with those of the watt balance. It is only then that a safe and exact redefinition of the unit of mass is guaranteed. The realization of the Avogadro experiment is based on the contributions of different working groups of PTB and of national and international partners.

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The Boltzmann constant is the key factor when it comes to redefining the base unit "kelvin" by relating it to a fundamental constant. At the international level, the aim is therefore to determine the numerical value of the Boltzmann constant and to replace the current definition via the triple point of water. Then, also the base unit "kelvin" would no longer depend on a special material property. This, however, requires the Boltzmann constant to be known with a very low uncertainty – so low that it equals the uncertainty of the realization of the triple point of water. To achieve this, scientists worldwide are working on determining the Boltzmann constant in different ways. At PTB, a dielectric-constant gas thermometer is used for this purpose in Department 7.4: Temperature (Berlin site).

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One ampere corresponds to the flux of approx. 6 × 10¹⁸ electrons per second. By counting the electrons, the ampere can be traced to the second. The precondition for this is that the charge of an electron is exactly known or fixed, as planned in the new SI. For this purpose, single-electron pumps are developed in semiconductor structures in order to realize a precise electrical current by electron counting.

Single-electron pumps in semiconductor structures
Working Group 2.53: Low-dimensional Electron Systems

SET, Current and Charge
Working Group 2.61: SET, Current and Charge

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