The First Atomic-Beam Clock

The remarkable advances in electronics and microwave technology made during World War II stimulated the physicists who had worked on them to imagine new applications after the war for peacetime conditions. An outstanding example is the cesium-beam frequency standard, one of several types of "atomic clock" developed in the postwar years.

This is the experimental instrument built under the supervision of Jerrold Zacharias at the Massachusetts Institute of Technology in 1954. It showed that the atomic beam principle was feasible as a technique for extremely precise timekeeping, and paved the way immediately for a commercial version closely modeled on it.

The idea on which it relied had been known for two decades. The American physicist I. I. Rabi had applied it in the late 1930s to precise measurements of the magnetic moments and "spins" of nuclei of various kinds of atoms. Rabi knew that atoms behave as tiny magnets: a beam of them, traveling in a vacuum, can be deflected slightly by passing through a non-uniform magnetic field.

Furthermore, the strength of the atomic magnet, and its direction relative to that of the magnetic field, can be altered by microwaves whose frequency exactly matches (is in resonance with) a frequency characteristic of the atoms used in the experiment. Rabi's apparatus detected the change in deflection of the atomic beam when this resonance occurred.

In 1953, Zacharias, who as a graduate student had collaborated in Rabi's prewar experiments, started vigorous work on making such an atomic-beam apparatus function as a clock. By the next summer, he and his student R. D. Haun, assisted by visiting researcher J. G. Yates, were able to make the atomic vibrations of a cesium beam control a crystal oscillator, whose frequency then became as precise as that of the cesium atoms. This oscillator frequency in turn could be used for timekeeping far more precise than any previously possible.

The device shown is the atomic beam portion, the heart of the system, which was enclosed in a tall vacuum chamber when in use. Cesium atoms boiled out of an oven near the bottom and formed a beam, which passed a deflecting magnet, and then traversed a space in which it was subjected to the oscillating microwave field. It then passed a second deflecting magnet, which served to bring the atoms to a focus, as in Rabi's method, on a detector. This determined any deviation from resonance and sent a signal to circuits which adjusted the microwave frequency accordingly.

Zacharias's apparatus is noteworthy for being designed as a prototype for an instrument intended to be sold commercially. Unlike the traditional horizontal atomic beam apparatus, this one stood compactly vertical. It used permanent magnets rather than electromagnets; had convenient connections for vacuum pump, electronics, and microwaves; and had an oven designed to run for a long time without stopping. Zacharias persuaded the National Company, a manufacturer of radio equipment in nearby Malden, Mass., to take on the task of developing a commercial version under his supervision. After overcoming many difficulties, they began delivering the "Atomichron" in the autumn of 1956, mainly to military laboratories. Despite its high cost, $50,000, it sold well to those laboratories, and the Signal Corps declared that it "performed well beyond all expectations."

Reference: Paul Forman, "'Atomichron': The Atomic Clock from Concept to Commercial Product," Proceedings of the IEEE, Vol. 73, No. 7, July 1985, pp. 1811-1204.

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