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