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Atomic Clocks

By Alan Bruzel

Cesium, like other group I elements, has one electron orbiting in its outer shell. In an atom of cesium, either this electron and its nucleus both spin in the same direction, or each spin in opposite directions. An appropriate microwave frequency will convert one spin state to the other. In 1967, the Conférence Générale des Poids et Mesures defined the unit of time, the second, as exactly 9,192,631,770 oscillations of these two spin states in ¹³³Cs, an isotope of cesium. The purpose of a cesium atomic clock is to generate these oscillations (known as hyperfine transitions) and then to count them.

To start the process, an oven boils cesium metal into a gas. This gas has equal amounts of cesium atoms in each spin state. The magnetic field of each of the two spin states differs, permitting a magnet to direct atoms of only one spin state into a microwave cavity. Microwaves of 3.26 centimeters wavelength initiate the hyperfine transition. Those atoms successfully switched to the opposite spin state are steered by another magnet to a hot filament where their outermost electrons are stripped off. A mass spectrometer brings the ¹³³Cs ions to an electron multiplier where they are then counted. During the entire process, a feedback loop continuously adjusts the microwave frequency to maximize the number of hyperfine transitions. At this point – the resonance frequency – one second is defined by 9,192,631,770 hyperfine transitions of ¹³³Cs.

This describes a primary standard cesium clock; only a few exist in the world, and are used for research purposes. Secondary standard cesium clocks used in conjunction with hydrogen maser clocks satisfy time measurement requirements of, for example, the LORAN navigation system and the Global Positioning System. These secondary standard clocks are about ten times less accurate than the primary standard clocks, such as the NIST-7 that is accurate to 3.5 parts in 10¹⁵. Nevertheless, even a clock accurate to 1 part in 10¹⁴ will be wrong by only about one second in three million years.

What the Web Has to Say about:
Atomic Clocks

Brief History of NIST Atomic Clocks
Timeline of timekeeping discoveries. From the US National Institute of Standards and Technology.

Defining the Meter, the Second, etc.
An article from this Web site reviewing the seven base units of the Système International.

Future Clocks
Description of cesium atomic fountain clocks. From SUNY Institute of Technology at Utica/Rome.

Hydrogen Maser Clock Project
Applications of a spacecraft-borne hydrogen maser clock. From the Harvard-Smithsonian Center for Astrophysics.

In the Blink of an Atom
New Scientist article by Jeremy Webb explains atomic clocks and their successors.

Ion Plasmas Simplify Atomic Clocks
Photonics Technology News article by Susan M. Reiss describes timekeeping potential of cooled beryllium ion clouds.

Nobel Prize in Physics 1989
To Norman F. Ramsey for basic research leading to cesium atomic clocks and hydrogen masers, and to Hans G. Dehmelt and Wolfgang Paul for studies of single electrons or ions using an ion trap technique.

Nobel Prize in Physics 1997
To Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips for cooling atoms to microkelvin temperatures with lasers. One outcome may be more accurate atomic clocks.

Official US Time
Accurate to within one second (depending upon Internet connection). From the National Institute of Standards and Technology and the US Naval Observatory.

Physikalisch-Technische Bundesanstalt
Timekeeping authority – equipped with cesium clocks and hydrogen masers – for the Federal Republic of Germany.

US Naval Observatory
The civilian and military arbiter of time in the United States.

What Is an Atomic Clock And How Do They Work?
Reliability of atomic timepieces. From How Stuff Works, by Marshall Brain.

Work Done at Royal Observatory of Belgium
Basic and applied research using their cesium clocks and hydrogen masers.

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