Norman Ramsey, who died in November 2011 at age 96, revolutionized the measurement of time by inventing, in 1949, the separated-fields atomic interferometer. His immediate (and inspired) quest was to improve the spectral resolution of magnetic resonance using molecular beams. For that story, see Daniel Kleppner’s account on page 25 and Ramsey’s own description of the achievement on page 36. But Ramsey’s device finds its most widespread application in atomic clocks, which count the oscillations of atomic transitions with extraordinary precision.1,2 

Ramsey proposed the hydrogen maser about a decade after his invention, shortly after Kleppner joined his group, and together they demonstrated one in 1960. Atomic-beam clocks and hydrogen masers now underlie the operation of the modern global positioning system. But the GPS is but one of the many uses of atomic clocks.

In fundamental physics, atomic clocks and their ability to time electromagnetic signals are...

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