N G Basov's legacy: from the first masers to optical frequency standards

The pioneering ideas of N G Basov, the centennial of whose birth was solemnly celebrated in 2022, laid the foundation for a number of modern research and technology areas in the field of quantum electronics and laser physics: from precision laser experiments aimed at testing the fundamental laws of physics to laser ignition of thermonuclear targets, from highspeed data transmission lines to laser welding and material processing. In this review paper, which was presented at the Basov centennial Scientific Session of the Physical Division of the Russian Academy of Sciences, we discuss the development and current state of the `firstborn' of a series of scientific victoriesDthe hydrogen maser (H-maser). The performance of H-masers is continuously being improved, and they are involved in solving a wide range of problems: time and frequency metrology, satellite navigation, and space radio astronomy. Russia is the recognized world leader in the development of maser technology, which is a brilliant example of the successful implementation of Basov's ideas. The natural development of this field is the advent of optical frequency standards, whose development prospects are discussed in the final part of the review.