1949 -- Using Rabi’s technique, NIST (then the National Bureau of Standards) announces the world’s first atomic clock using the ammonia molecule as the source of vibrations.

1952 -- NIST announces the first atomic clock using cesium atoms as the vibration source. This clock is named NBS-1.

1954 -- NBS-1 is moved to NIST’s new laboratories in Boulder, Colo.

1955 --The National Physical Laboratory in England builds the first cesium-beam clock used as a calibration source.

1958 -- Commercial cesium clocks become available, costing $20,000 each.

1960 -- NBS-2 is inaugurated in Boulder; it can run for long periods unattended and is used to calibrate secondary standards.

1963 -- The search for a clock with improved accuracy and stability results in NBS-3.

1967 -- The 13th General Conference on Weights and Measures defines the second on the basis of vibrations of the cesium atom; the world’s timekeeping system no longer has an astronomical basis.

1968 -- NBS-4, the world’s most stable cesium clock, is completed. This clock was used into the 1990s as part of the NIST time system.

1972 -- NBS-5, an advanced cesium beam device, is completed and serves as the primary standard.

1975 -- NBS-6 begins operation; an outgrowth of NBS-5, it is one of the world’s most accurate atomic clocks, neither gaining nor losing one second in 300,000 years.

1989 -- The Nobel Prize in Physics is awarded to three researchers -- Norman Ramsey of Harvard University, Hans Dehmelt of the University of Washington and Wolfgang Paul of the University of Bonn -- for their work in the development of atomic clocks. NIST’s work is cited as advancing their earlier research.

1993 -- NIST-7 comes on line; eventually, it achieves an uncertainty of 5 parts in 10 to the 15th power, or 20 times more accurate than NBS-6.