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A Brief History of Atomic Clocks at NIST

1945 -- Isidor Rabi, a physics professor at Columbia University, suggests a clock could be made from a technique he developed in the 1930's called atomic beam magnetic resonance.

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 completes the first accurate measurement of the frequency of the cesium clock resonance. The apparatus for this measurement is named NBS-1.

NBS-1 Cesium Clock

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

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

1956 -- Commercial cesium clocks become available, costing $20,000 each.

1959 -- NBS-1 goes into regular service as NIST's primary frequency standard.

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.

NBS-4 Cesium Clock

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

NBS-5 Cesium Clock

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.

NBS-6 Cesium Clock

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 x 10-15, or 20 times more accurate than NBS-6.

NIST-7 Cesium Clock

1999 -- NIST-F1 begins operation with an uncertainty of 1.7 x 10-15, or accuracy to about one second in 20 million years, making it one of the most accurate clocks ever made (a distinction shared with similar standards in France and Germany).

NIST-F1 Cesium Clock

For More Information, download ...

NIST Primary Frequency Standards and the Realization of the SI Second
A detailed overview of the NIST Primary Frequency Standards. Includes historical information, a technical description of the devices, color photographs of the standards, and an extensive list of references. Published in December 2007. 17 pages.