Full Research, which consists of Type 1 Basic Research, Type 2 Basic Research and Product Realization Research, may be viewed as being created through a “fusion” of differing fields of technology. And metrology, which intrinsically consists of an aggregation of differing fields such as electromagnetics, dynamics, thermophysics, and chemistry, possesses high potential for spawning new Full Research.

AIST, in our efforts to support the diverse measurement technologies required in every aspect of society, boasts a long history of developing highly-reliable measurement technologies (since the time of our forerunner, the National Research Laboratory of Metrology) and supplying the measurement standards to society.

Now we are seeing increased activity to redefine the base units in the International System of Units (SI) using universal fundamental physical constants. I would like to discuss the true nature of Full Research, using such activities to review the fundamentals of units. History of the meter and kilogram are shown in Table 1 as typical examples. Originally, efforts to scientifically determine the units of physical values trace back to the period of the French Revolution. At the time, they were looking for a universal unit that was independent of, say, a length of a king’s arm. Later, owing to progress in science and technology, the meter prototype was deemed unnecessary, and presently, by defining c, the velocity of light in a vacuum, the unit of length is realized by measurement of optical frequency. However, of the SI units, the kilogram remained as the only SI base unit to depend upon an artificial prototype. The mass of the world’s only international prototype of the kilogram stored at the International Bureau of Weights and Measures (BIPM) in Paris is believed to have changed by roughly 5 x 10^-8 within the past 100 years due to effects of surface contamination. Even using today’s state-of-the-art technology, there has not yet been established another method for realizing mass with such high repeatability. Recently, however, two different methods are finally being proposed. The first is the X-ray crystal density method which determines mass from the Avogadro constant N_A, based upon the mass of atoms such as carbon ¹²C. The other is the watt balance method, which determines mass from measurements using electrical standards upon definition of the Planck constant, h.

Although these two principles of measurement are completely different, the Type 1 Basic Research which made these measurements possible originated from the development of X-ray interferometry technology in the 1960s (Table 2). Owing to this development, high-accuracy measurement of the lattice constant of crystals with reference to the wavelength of light was made possible, making way for measurement of the Avogadro constant by the X-ray crystal density method. Later, the discoveries of the Josephson effect and the quantum hall effect were extremely significant in terms of measurement standards, as they brought about the dramatic improvement in repeatability of voltage and electrical resistance. In recent years, progress is being made in research and development to extend the frequencies of the microwave range made by the cesium atomic clock to the optical frequency range, to enable measurements in the terahertz range. Thus, the accuracy of length measurement has been improved dramatically. Each of these research outcomes represents a major discovery in physics, thus we can say that the foundation of measurement standards lies in Type 1 Basic Research.

Next, let us consider the Type 2 Basic Research that directly contributed to redefinition of the kilogram (Table 3). In the 1970s, National Institute of Standards and Technology (NIST, or NBS at the time) in the U.S.A. became the first to succeed in measuring the Avogadro constant using the X-ray crystal density method. This measurement technology was passed on to AIST’s National Metrology Institute of Japan (NMIJ, or National Research Laboratory of Metrology at the time) and Physikalisch-Technische Bundesanstalt (PTB) of Germany, where precision was improved further. The polishing technology for silicon spheres developed in the 1980s was also an important element in Type 2 Basic Research, as it enabled the dramatic improvement in measurement accuracy of crystal density (see Photo). Mass analyses of silicon isotopes are also important elementary technologies in this area. Recently, an international project undertaken by eight research organizations including AIST is in progress to further increase accuracy of the Avogadro constant, by isotopically enriching ²⁸Si to up to 99.99%. The accurate measurement of the Avogadro constant is achieved upon a fusion of many research fields and measurement technologies, such as X-ray engineering, crystal engineering, optics, mass standards, nanometer/picometer measurements, density standards, chemical analysis, temperature standards and surface measurement. Without any one of these fields, such accuracy enhancements cannot be achieved.

The measurement of the Planck constant by the watt balance method is also made possible by a fusion of dynamics, electrical standards, optics, electromagnetics, etc. The electrical power (product of voltage and current) is determined from measurements of dynamic values (force and velocity), and, using the Josephson effect and quantum hall effect, h is determined. The research outcome which set the stage for this technology was obtained at National Physical Laboratory (NPL) in the U.K. in the 1980s, while recently, high-accuracy measurements of the Planck constant are being performed at NIST.

In this way, this Type 2 Basic Research is characteristic in that it is achieved only upon the fusion of theories and technologies of differing fields, and thus requires a relatively long time span from conception and development to reaching final outcome. Meanwhile, it is similar to other Full Researches in that it must also take on the Valley of Death, although research has successfully been continued with the cooperation of international metrology institutes.

The product obtained in these researches is information in the form of a “database” - covering roughly 300 fundamental physical constants obtained and theoretically, including the Avogadro constant N_A, the Planck constant h, and elementary charge e - possessing an extremely high propagational effect. In addition, by investigating whether or not the fundamental physical constants obtained from differing principles such as the X-ray crystal density method and the watt balance method are consistent within the range of the uncertainty of the experiment, we are able to verify the exactness of the Josephson effect and the quantum hall effect. In other words, we are able to confirm the degree of accuracy of our current physics system (outcomes of Type 1 Basic Research) through Type 2 Basic Research.

Now, the current new international trend is to define the standards of base units using these fundamental physical constants (see Table 4). By using fundamental physical constants as benchmarks, we can build “units directly linked to universality of the natural world.” It will be possible to realize the kilogram, not only at the International Bureau of Weights and Measures (BIPM) but at any research institute in the world, and our ideal from the time of the French Revolution will finally bear fruit. Then, the new definition will likely foster new measurement and experimental technologies. The development of a measurement standard across such a long time period is also an example of Full Research that is particularly and highly characteristic in that a fusion of differing fields becomes the soil to spawn new outcomes of research.