This special feature is intended to present a comprehensive review of the present state and novel trends in the field of quantum measurement standards. Most of the present metrological research is concentrated on establishing and strengthening the links between the units and fundamental constants. This will be demonstrated in the nine articles in this feature.
The first four articles are devoted to time, frequency and length metrology. They describe quantum standards that are used or intended to be used for the realization of the SI base units of time and length, the second and the metre. The two units are related to each other by an adopted fixed value of the speed of light in vacuum and the second is at present defined by the energy difference (frequency) of a hyperfine transition in the ground state of caesium. The special feature starts with the discussion of caesium atomic clocks as direct
realizations of the second and as a basis for other standards (e.g. the Josephson voltage quantum standard). Whereas Cs atomic clocks still provide us with the most accurate realization of the second, optical frequency standards based on cold trapped ions or cold atoms may eventually lead us to even lower uncertainty levels and may replace the present definition of the second by an optical transition. This situation is described in the contribution on optical frequency standards based on trapped single ions. Since optical frequency standards are also needed as wavelength standards in length metrology, the third contribution reviews the definition of the metre. It describes the different methods of realization, in particular by optical frequency standards including standards based on cold atoms. The use of optical frequency standards in time and length metrology requires the precise knowledge of their frequencies. Methods of optical frequency measurements based on various methods—including frequency comb generators—are discussed in the fourth article.
Turning to the electric units, the discovery of two macroscopic quantum effects, the Josephson effect—discovered in the early sixties—and the quantum Hall effect—discovered in 1980—allowed the linking of electric units to fundamental constants. By use of the Josephson effect quantized voltage values are realized as multiples of the product of a certain frequency and of the superconducting flux quantum (h/2e). Quantized resistance values are realized with the quantum Hall effect as submultiples of h/e2, where this fundamental constant can also be interpreted as the quotient of the flux quantum of a normal conductor and the elementary charge. The metrological application became much easier by making use of the possibility of designing and manufacturing appropriate samples with microelectronic techniques. Since 1990 all calibrations of voltages or resistances worldwide have been based on these two quantum effects. Relative uncertainties of the order of 10-9 are obtained. International comparisons have proved that the calibration results in different laboratories also agree within relative uncertainties of a few parts in 109. Great attempts have also been made to realize a current quantum standard by counting single electrons. The Josephson effect and its application are described by Kohlmann and co-workers. The quantum Hall effect is only briefly described in this issue of Measurement Science and Technology
since a comprehensive review article by Jeckelmann and Jeanneret appeared recently [1].
The unit of mass, the kilogram, is the only base unit in the SI that is derived from an artefact. Therefore one major task of today's metrological research is the linking of the unit of mass to a fundamental constant. There are two rival attempts: the comparison of mechanical and electrical power using the watt-balance
and the counting of a large number of identical particles, like atoms or ions. The watt balance was pioneered by B Kibble. This experiment consists of two parts. A balance in equilibrium is opposed to a gravitational force and to a force on a coil fed by a current and opposed to a magnetic field. The latter force depends on geometrical dimensions. They can be determined electrically in the second part of
the experiment, where the coil is moved through the magnetic field and the occurring inductive voltage is measured. This experiment links the kilogram with Planck's constant h. Counting of atoms or ions is described by Becker and Glaeser. Silicon atoms are the most suitable for growing large single crystals, in this case a sphere. The number of atoms is determined by precise measurements of the volume of an elementary cell and of the volume of the total sphere. One of the
experimental problems is the appearance of voids in the crystal, another is the existence of various silicon isotopes with different masses. The latter aspect is avoided in the experiment where isotopic ions of gold or bismuth are accumulated. The number of ions is determined by integrating the ion current with time, while the current itself is measured using the Josephson and the quantum Hall effects. This experiment links the kilogram with a number of identical particles. On the basis of this experiment the definition of the unit of mass would be given through a definition of the Avogadro constant.
Reference
[1] Jeckelmann B and Jeanneret B 2001 The quantum Hall effect as an electrical resistance standard Rep. Prog. Phys.64 1603–55 (IOP Article)