The planned redefinition of the current seven SI base units essentially applies to the units "kilogram", "mole", "ampere" and "kelvin", with the defining constants Planck's constant h, Avogadro constant NA, elementary charge e and the Boltzmann constant k. The other three base units "second", "meter" and "candela" will merely be adapted in their linguistic phrasing which, however, does not affect its realization. Until a redefinition of the SI base units, the General Conference of the Meter Convention and the International Committee for Weights and Measures (CIPM) stipulate an even better knowledge of the defining constants, especially of h and k. It is also necessary to determine measurement values of these constants by means of at least two independent measurement methods. Consistent measurement values which will have been accepted for publication in a reviewed scientific journal until 1 July 2017 and which are available to the Committee on Data for Science and Technology (CODATA), will enter into the determination of the numerical values of h and k.
Even after the redefinition of the SI base units has been accomplished, their realization and dissemination will continue to be a priority task. As the methods employed to measure the constants according to the currently valid SI can also be directly used for the realization of the respective units according to the new definitions, the present work in this regard is of great importance for the future because the further refinement of these techniques, as well as the development of completely new methods, will allow us in future to provide industry with ever more precise realizations of the units over extended measurement ranges.
Scientists at PTB are working on the determination of the Planck constant h by means of monocrystalline ultra-pure and ultra-round 28Si spheres. These measurements are synonymous with measurements of the Avogadro constant.
Research work carried out at PTB on the measurements of sphere volumes, lattice parameters, isotopic ratio, chemical purity and quantitative chemical surface characterization will be intensively continued. The objective is to reduce the relative uncertainty of the measurement value for h or for NA, respectively, to below 1.5 × 10–8.
PTB has decided to use silicon spheres for the future realization of the kilogram and the mole. For this purpose, highly enriched and chemically ultrapure 28Si was, and will be, purchased. PTB is able to produce several spheres of 1 kg mass per year from 28Si monocrystals with a roundness deviation in the range between 10 nm and 20 nm. Thus, PTB will be able to realize the "new" kilogram as primary realization with an uncertainty of 2 × 10–8 or better, by providing and advancing all the measurement methods required for this purpose. Furthermore, PTB will further develop and provide procedures to determine the masses of much cheaper spheres with natural isotope composition with a total uncertainty of 3 × 10–8. In this way, it will be possible for PTB to offer to other metrology institutes and also to calibration laboratories its own direct realization of the kilogram. By producing spheres with a mass of 28 g, also 1 mole 28Si can be realized. Furthermore, smaller spheres offer the possibility – which is completely novel in the new SI – to represent masses at values in the gram or milligram ranges, which may be of interest, for example, to the pharmaceutical industry, directly as a primary standard which – in future – can permit higher accuracies in these measurement ranges.
At the triple point of water, PTB uses the method of dielectric-constant gas thermometry (DCGT) to determine the Boltzmann constant. As other institutes use acoustic gas thermometry for this purpose, validation via this independent technology is important. At least a relative uncertainty smaller than 3 × 10–6 is required, whereby PTB aims at achieving 2 × 10–6 by means of the DCGT measurement.
After the redefinition of the unit "kelvin", every thermometry procedure which only refers to the defining constants, can be a primary kelvin realization. A future realization of the unit kelvin at PTB will be carried out by means of dielectric-constant gas thermometry and by means of promising innovative methods of noise thermometry. In both fields, PTB has excellent preconditions and great expertise at its disposal.
The most direct realization of the ampere according to its future new definition by fixing the numerical value of the elementary charge can be carried out by means of electronic components which use individually countable electrons. PTB is already prominent here and has set itself the objective to realize a primary single-electron current source by 2018 that is superior to other realizations, for currents larger than 160 pA and a relative uncertainty smaller than 1 × 10–7. For clearly higher currents, realizations of the ampere by means of the quantum Hall effect and the Josephson effect are better suited. They, too, will be further developed at the high level achieved, especially for AC applications. In future, graphene, a carbon crystal which is only one atomic layer thick, could play an important role for quantum Hall measurements, as for this material, the quantum Hall effect is not only usable close to the absolute temperature zero point, but – in the long run – also at room temperature. Due to the – then – much easier manageability, this development offers considerable future potential for the dissemination, so that PTB will continue to strongly commit itself in this field of work.
The definition of the unit of time, the second, has already been determined via a defining constant – the frequency of the hyperfine splitting transition in the cesium ground state. Already, it is obvious, however, that in other ions and atoms there are transitions in the optical spectral range which permit clocks with relative uncertainties of only a few 10–18 and might, in future, advance to the range of 10–19. PTB's aim is to continue to drive forward the prominent research work carried out on optical clocks, thus leaving its mark on a long-term redefinition of the second in a significant way, and thus to conduct front-line research in the related application possibilities in communications technology, satellite technology, in applied geodesy and in many more fields, and to make it available to the economy.