Fifty years of atomic time-keeping at VNIIFTRI

Time metrology in Russia in the second half of the twentieth century has been marked, as in other advanced countries, by the rapid development of time and frequency quantum standards and the beginning of atomic time-keeping. This brief review presents the main developments and studies in time and frequency measurement, and the improvement of accuracy and atomic time-keeping at the VNIIFTRI--the National Metrology Institute keeping primary time and frequency standards and ensuring unification of measurement. The milestones along the way have been the ammonia and hydrogen masers, primary caesium beam and fountain standards and laser frequency standards. For many years, VNIIFTRI was the only world laboratory that applied hydrogen-maser clock ensembles for time-keeping. VNIIFTRI's work on international laser standard frequency comparisons and absolute frequency measurements contributed greatly to the adoption by the CIPM of a highly accurate value for the He-Ne/CH4 laser frequency. VNIIFTRI and the VNIIM were the first to establish a united time, frequency and length standard. [PUBLICATION ABSTRACT]