Briefly trace the history of the atomic clock from 1940 until 1991.

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Explanation:

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.