Features of quantum measurement standards and special status of the second in the SI-2019

The implementation of the New SI in 2019 and the definition of base units in terms of “defining constants” has significantly changed the metrology methods and led to the introduction of a quantum approach to the reproduction of units. The paper highlights a number of features of quantum methods and measurement standards, the ultimate accuracy of which is limited only by the “quantum structure of nature”. For electrical measurements, the implementation of the New SI means the end of the dualism that has existed since 1990. The dualism was that the SI defined the ampere – base unit of electricity – by mechanical measurements and quantities, and for reference measurements in practice, it was recommended to use the quantum effects of Josephson and Hall. A number of features of quantum methods and measurement standards are considered, which made it possible to increase the accuracy of reproduction of electrical units (and even earlier – units of time and length) by several orders of magnitude compared to the “pre-quantum” era. Another feature of the SI-2019 was the fact that it linked the units of all base quantities to the second and thus paved the way for the reproduction of units in terms of defining constants (which are fixed without uncertainty in the SI-2019) and the second, which is measured with the highest accuracy. Thus, the main task of metrology is to establish the relationship between the measured value and the second. In electrical measurements and some other types of measurements, this function is performed by quantum methods, which are described in this paper. The extremely high accuracy of time measurements, the availability of its transfer via communication lines, and the system-forming nature of the second determine its special status in SI-2019. The paper presents that the success in creating frequency measurement standards in the optical range promises further improvement of the accuracy of the second, which only raises its status and may lead to official revision of its definition in terms of the frequency of another quantum transition already existing in the optical range. It is suggested that the development of quantum measurement methods, the counting nature of these methods, and the features of the second mentioned in the paper bring us closer to the creation of a new metrology that will be a logical continuation of quantum metrology and which can be conventionally called “digital”.