After the war, the U.S. National Bureau of Standards and the British National Physical Laboratory both set out to create atomic-time standards based on the atomic-resonance work of Rabi and his students. The first atomic clock
was established at the National Physical Laboratory by Louis Essen and John V.L. Parry, but this clock required a roomful of equipment. Another of Rabi's former associates, Jerrold Zacharias of MIT, managed to turn the atomic clocks into practical devices. Zacharias had plans for building what he called an atomic fountain, a visionary type of atomic clock that would be accurate enough to study the effect of gravity on time that had been predicted by Einstein. In the process, he developed an atomic clock small enough to be wheeled from one laboratory to another. In 1954, Zacharias joined with the National Company in Malden, Massachusetts, to build a commercial atomic clock based on his portable device. The company produced the Atomichron, the first commercial atomic clock, 2 years later and sold 50 within 4 years. The cesium atomic clocks used in GPS today are all descendants of the Atomichron.
Physicists have continued to experiment with novel variations on the atomic-resonance ideas of Rabi and his students and to put them to work in atomic clocks. Rather than using magnets, one technique makes use of a phenomenon known as optical pumping to select out the energy levels of the atoms that will do the timekeeping and employs a beam of light to force all the atoms in the beam into the desired state. This work led to a Nobel Prize for Alfred Kastler of the Ecole Normal Suprieure in Paris. Today, many atomic clocks use optically pumped rubidium atoms instead of cesium. The rubidium clocks are considerably less expensive and smaller than cesium clocks, but they are not quite as accurate.
Another type of atomic clock is known as the hydrogen maser. Masers originated in research on the structure of molecules by Charles Townes and his colleagues at Columbia University in 1954, work for which Townes shared the 1964 Nobel Prize in physics. The maser, which is the precursor of the laser, is a microwave device that generates its signal by direct emission of radiation from atoms or molecules. While Townes's original maser used ammonia, Ramsey and his colleagues at Harvard developed a maser in 1960 that operates with hydrogen and serves as an atomic clock of extreme precision.
By 1967, research in atomic clocks had proved so fruitful that the second was redefined in terms of the oscillations of a cesium atom. Today's atomic clocks are typically accurate to within 1 second in 100,000 years. Our nation's primary time standard is the recently inaugurated atomic clock at the National Institute of Standards and Technology, called NIST-7. Its estimated accuracy is to within 1 second in 3 million years.
Over the years, all three clocks--the cesium-beam clock, the hydrogen-maser clock, and the rubidium clock--have seen service in space, either in satellites or in ground control systems. GPS satellites ultimately rely on cesium clocks that resemble those conceptualized by Rabi 60 years ago.
In 1993, 2 decades after it was conceived in the Pentagon, GPS became fully functional with the launching of its 24th satellite. The satellites are operated by the U.S. Air Force, which monitors them from five ground stations around the world. The data gathered are analyzed at the Air Force Consolidated Space Operations Center in Colorado, which transmits daily updates to each satellite, correcting their clocks and their orbital data.
