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.

1958 — 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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