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.
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.
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)
The smallest particle of an element that can exist either alone or
in combination; the atom is considered to be a source of vast
Cesium 133 -
An isotope of cesium used especially in atomic clocks and one of
whose atomic transitions is used as a scientific time standard
(atomic second) - The
interval of time taken to complete 9,192,631,770 oscillations of the
cesium 133 atom exposed to a suitable excitation
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
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
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
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.