A Brief History of Atomic Clocks at NIST
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.
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.
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.
1993 -- NIST-7 comes on line; eventually, it achieves an uncertainty of 5 x 10-15, or 20 times more accurate than NBS-6.