The future of the SI

• The great challenges of fundamental metrology
• Which possible paths for a new SI ?
• The SI and French metrology

The great challenges of fundamental metrology: the international system of units (SI) and the fundamental constants

Since several dozens of years, many discoveries associated with the new technologies mainly in quantum physics, have initiated great changes in the area of metrology, and more particularly for the more fundamental metrology.

The SI is currently made up of seven base units: the metre (m), the kilogram (kg), the second (s), the ampere (A), the kelvin (K), the candela (cd) and the mole (mol). However, even if the seven units are present, some of the quantities matching these units are not fully independent. And so length is directly linked to time by one fundamental constant, the speed of light, c, constant set at 299 792 458 m/s, during the last definition of the metre in 1983.

The evolutions of modern physics, the experiments made for the determination of these quantities together with the experience gained should led to a transformation of the SI over the coming years.

The current orientation would be to directly link the SI units to some fundamental constants. Different proposals are currently been discussed. In any case, a new system of units, whatever, should form a consistent whole, and include putting into practice each definition to allow transfer towards the users and ensure traceability of all measurements to the SI.

Which possible paths for a new SI ?

The metre is the first unit for which a direct link has been established with one fundamental constant, the speed of light, c, which therefore has an accurate value now. This is put into practice by the realization of stabilized laser sources, which wavelength is measured in relation to the time unit. The list of the sources recommended for the “Mise en pratique du mètre” is regularly reviewed and updated by the CGPM.


The kilogram is the only unit defined on the basis of one artefact, prototype of the mass in platinum-iridium alloy approved by the CGPM of 1889 and maintained at the BIPM.
The mass prototype, invariable by definition, has in practice certainly derived by several dozen microgrammes. The replacement of the current definition by a more universal definition seems to be indispensable.

Different paths are under review, of which the most serious will consist in linking the kilogram either to h, the Planck constant, or to N_A the Avogrado’s number.
The first path is based on the so-called experiments of the Watt balance, comparing one electrical watt to a mechanical watt, and thus links the mass to the Planck constant, h.
The second path starts from one silicon sphere with a set of known physical characteristics, so as to link the mass of one elementary particle to one macroscopic mass with sufficient precision. The mass will then be defined via the number of N_A.
Note that this last experiment will also involve a change in the definition of the mol.


Several methods are under study for improving significantly the uncertainties on the value of the Boltzmann constant, k_B, and thus allow redefining the unit of thermodynamic temperature : the kelvin, currently defined on the basis of the realization of the triple point of water.

Detection of the acoustic resonances of a gas in a cavity, determination of the dielectric constant of one gas or spectroscopic methods offer the possibility to determine Boltzmann constant with sufficient uncertainty and thus contribute to a redefinition of the kelvin.


Over these last years, the electrical units have benefited from progress made in solid-state physics notably with the implementation of the Josephson effect, for voltage measurements, and the Quantum Hall effect for the resistance measurements which allows linking these two quantities to frequency, via the K_J and R_K. However, even if these phenomena are very reproducible, the constants that are attached to them are not known and determined with sufficient uncertainties.

Another quantum effect that is currently been studied, is the single electron tunneling (SET), that should allow linking the current intensity to the frequency; one quantum ohm’s law should be realized. And thus the constants implemented could be revalued, if the accuracy is sufficiently good.

This will allow linking directly the ampere to the frequency, and therefore to the time unit, the second.


The candela, related to the sensitivity of the human eye, brings back to the energy flux; it can be considered as a practical unit. Photon-counting experiments could led to redefine the candela as the number of photons.

The SI and French metrology

The International Committee of Weights and Measures (CIPM) and some Consultative Committees (CCs) of the CIPM (CC) are strongly encouraging the National Metrology Institutes (NMIs) to direct their efforts towards an improvement in the uncertainty of the value of the fundamental constants and to implement experiments of realisation of units, more particularly for the units related to mass, electricity, temperature and the amount of substance.

The LNE and French metrology have an important role to play in the international arena and can contribute to new definitions and for several units.

Experiments are currently under way on the following themes :

• The kilogram : a "watt balance" project was launched in 2002. A first experimental device should be set up by the end of 2007. The objective is to determine in SI units the value of the Planck constant h and to determine the stability of the mass unit by reference to the quantities supposed to be invariable such as the constant mentioned above. The uncertainty anticipated at longer term is in the order of 1.10^-8 in relative value.
• The ampere : two projects are under development to contribute to a redefinition of the ampere :
  - The metrology triangle, that will allow to verify the ohm’s law at the level of the quantum voltage standards (Josephson effect), of current (single electron effect) and resistance (quantum Hall effect), one of the possible ways for the redefinition of the ampere through the elementary charge e ;
  - The calculable capacitor that is directly linking the resistors to the units of length and time, another means to establish a link between the electrical units and the α fine structure constant.
• The second : the second is defined on the basis of caesium atom clocks, that operate at microwave frequencies (around 9 GHz). A new stage in the realization of the time unit is the improvement of the accuracy through the development of clocks in the area of optical frequency (clocks with neutral atoms of Sr, Ag and of Hg). Such experiments can improve by more than one order of magnitude the definition of the second.
• The kelvin : The determination the Boltzmann constant,k_B, with a spectrometric method and an acoustic thermometer is the most advanced challenge for a new definition of the temperature scale; some results can be used for the definition of the temperature scale to values below 1 K. Such an experiment is under development in our laboratories in collaboration with the Université Paris-Nord.

These different works, multidisciplinary and initiated since a few years, are of primary importance for the international metrology community and French research as well.