NIST-F1, source of the official time of the United States Boulder, Colorado


How Does a Practical Cesium Atomic Clock Work?

Atoms have characteristic oscillation frequencies. Perhaps the most familiar frequency is the orange glow from the sodium in table salt if it is sprinkled on a flame. An atom will have many frequencies, some at radio wavelength, some in the visible spectrum, and some in between the two. Cesium 133 is the element most commonly chosen for atomic clocks.


Some Definitions

Atomic Clock - A precision clock that depends for its operation on an electrical oscillator regulated by the natural vibration frequencies of an atomic system (as a beam of cesium atoms)

Atom - The smallest particle of an element that can exist either alone or in combination; the atom is considered to be a source of vast potential energy

Cesium 133 - An isotope of cesium used especially in atomic clocks and one of whose atomic transitions is used as a scientific time standard

SI Second (atomic second) - The interval of time taken to complete 9,192,631,770 oscillations of the cesium 133 atom exposed to a suitable excitation

Source: Merriam-Webster Online

To turn the cesium atomic resonance into an atomic clock, it is necessary to measure one of its transition or resonant frequencies accurately. This is normally done by locking a crystal oscillator to the principal microwave resonance of the cesium atom. This signal is in the microwave range of the radio spectrum, and just happens to be at the same sort of frequency as direct broadcast satellite signals. Engineers understand how to build equipment in this area of the spectrum in great detail.

To create a clock, cesium is first heated so that atoms boil off and pass down a tube maintained at a high vacuum. First they pass through a magnetic field that selects atoms of the right energy state; then they pass through an intense microwave field. The frequency of the microwave energy sweeps backward and forward within a narrow range of frequencies, so that at some point in each cycle it crosses the frequency of exactly 9,192,631,770 Hertz (Hz, or cycles per second). The range of the microwave generator is already close to this exact frequency, as it comes from an accurate crystal oscillator. When a cesium atom receives microwave energy at exactly the right frequency, it changes its energy state.

At the far end of the tube, another magnetic field separates out the atoms that have changed their energy state if the microwave field was at exactly the correct frequency. A detector at the end of the tube gives an output proportional to the number of cesium atoms striking it, and therefore peaks in output when the microwave frequency is exactly correct. This peak is then used to make the slight correction necessary to bring the crystal oscillator and hence the microwave field exactly on frequency. This locked frequency is then divided by 9,192,631,770 to give the familiar one pulse per second required by the real world.

When Was The Atomic Clock Invented?

In 1945, Columbia University physics professor Isidor Rabi suggested that a clock could be made from a technique he developed in the 1930s called atomic beam magnetic resonance. By 1949, the National Bureau of Standards (NBS, now the National Institute of Standards and Technology, NIST) announced the world’s first atomic clock using the ammonia molecule as the source of vibrations, and by 1952 it announced the first atomic clock using cesium atoms as the vibration source, NBS-1.

In 1955, the National Physical Laboratory in England built the first cesium-beam clock used as a calibration source. Over the next decade, more advanced forms of the clocks were created. In 1967, the 13th General Conference on Weights and Measures defined the SI second on the basis of vibrations of the cesium atom; the world’s time keeping system no longer had an astronomical basis at that point! NBS-4, the world’s most stable cesium clock, was completed in 1968, and was used into the 1990s as part of the NIST time system.

In 1999, NIST-F1 began operation with an uncertainty of 1.7 parts in 10 to the 15th power, or accuracy to about one second in 20 million years, making it the most accurate clock ever made (a distinction shared with a similar standard in Paris).

How Is Atomic Time Measured?

The correct frequency for the particular cesium resonance is now defined by international agreement as 9,192,631,770 Hz so that when divided by this number the output is exactly 1 Hz, or 1 cycle per second.

The long-term accuracy achievable by modern cesium atomic clocks (the most common type) is better than one second per one million years. Hydrogen atomic clocks show a better short-term (one week) accuracy, approximately 10 times the accuracy of cesium atomic clocks. Therefore, the atomic clocks have increased the accuracy of time measurement about one million times in comparison with the measurements carried out by means of astronomical techniques.

The National Company in Massachusetts produced the first commercial atomic clocks using cesium. Today, they are produced by various manufacturers, including Hewlett Packard, Frequency Electronics, and FTS. New technology continues to improve performance. The most accurate laboratory cesium atomic clocks are thousands of times better than commercially produced units.

About the Author
Douglas Dwyer is the founder of Frequency Precision Ltd. in the UK. He provides consulting and design services to the world-wide electronics industry.