a generator of electromagnetic waves that uses the phenomenon of stimulated emission. Quantum generators in the superhigh-frequency (SHF) region, like quantum mechanical amplifiers in that region, are often called masers. The first such devices in the SHF range were made in 1955 simultaneously in the USSR by N. G. Basov and A. M. Prokhorov and in the USA by C. Townes. A beam of ammonia molecules was used as the active medium, and consequently the devices where called molecular generators. Subsequently a quantum SHF generator was designed using a beam of hydrogen atoms. An important feature of such masers was the great stability of the generated frequency—up to 10−13—as a result of which they are used as quantum frequency standards.
Optical quantum generators, or lasers, were developed in 1960. They operate over a broad wavelength range, from the ultraviolet to the submillimeter regions of the spectrum, in both pulse and continuous modes. There are lasers using crystals and glasses, gases, fluids, and semiconductors. Unlike other light sources, lasers emit highly coherent monochromatic light waves having all their energy concentrated within a very narrow solid angle.