July 26, 2005
50TH ANNIVERSARY OF THE FIRST ACCURATE CESIUM ATOMIC CLOCK
Those of us who love time, frequency and clocks are celebrating the 50th anniversary of the first accurate cesium atomic clock. This clock was developed at the UK's National Physical Laboratory in 1955 by Dr Louis Essen and J.V.L. Parry. But was it really the first? Some might argue that in 1951 the Radio Propagation Laboratory at the National Bureau of Standards, which later became known as the National Institute of Standards and Technology (NIST), operated the first cesium atomic beam standard. Some might even say that it was really in the 1930s at Columbia University where I.I. Rabi invented the technique known as magnetic resonance, by which he could measure the natural resonant frequencies of atoms. Or maybe it was Rabi’s student Norman Ramsey who in 1949 suggested that making the atoms pass twice through the oscillating electromagnetic field could result in a much more accurate clock. Or perhaps we can blame it all on Einstein and his 1905 theory of relativity because physicists created these clocks in an attempt to seek answers to questions raised by Einstein about the nature of the universe.
The key word here is accurate. In May of 1955 Essen and his team developed their long beam apparatus based on the transition of the cesium-133 atom into an operable clock, and a new value for the cesium hyperfine transition frequency was measured based on the uniform time scale maintained at the Royal Greenwich Observatory. They then teamed with the U.S. Naval Observatory to measure the frequency of the cesium transition relative to Ephemeris Time. This collaboration led to the determination of a best value for the cesium hyperfine frequency (9 192 631 770 Hz) and this is how the second became defined as 9,192,631,770 periods of the cesium-133 atom.
Concurrent with this development was the work of Jerrold Zacharias of the Massachusetts Institute of Technology. In 1954 he began working for the National Company in Malden, Massachusetts, on the Atomichron, generally regarded as the world’s first commercial atomic clock. With a frequency accuracy of better than one part in 1010, the Atomichron was completed in 1956 and sold to the NRL. By 1960 the National Compay had sold approximately 50 of these clocks.
It was not until 1967 that the General Conference on Weights and Measures adopted the atomic definition for an SI second, thereby officially redefining the second and marking the end of official time-keeping based on the movements of the planets.
So what has the atomic clock brought to us?
Without atomic clocks there would be no GPS system. As the magazine Metrologia wrote in its special issue dedicated to the 50th anniversary of the cesium clock, “The atomic clock has also had consequences for navigation comparable to those brought about by Harrison's mechanical clocks almost exactly two hundred years before.”
In the IEEE paper “History of Atomic Frequency Standards: A Trip Through 20th Century Physics,” Arthur McCoubrey and Arroyo Grande wrote, “It is an overstatement to suggest that the history of atomic frequency standards is linked with 20th century physics in any complete sense. But, there is a very remarkable connection between the evolution of atomic frequency standards and the foundation of what has come to be known as modern physics.”
The accuracy and stability of atomic clocks are key determinants of the performance of command, control, communications, and intelligence for our military. Without them navigation, surveillance, electronic warfare, missile guidance and identification would be limited. The Department of Defense relies on precise time synchronization to efficiently determine the start of a code sequence in secure communications, to perform navigation, and to locate the position of signal emitters by means of time difference of arrival.
In addition to the being essential to applications in Aerospace and Defense, the atomic clock -- whether it be cesium, rubidium or hydrogen maser -- provides essential solutions to a number of key industries and organizations. In time-keeping and metrology applications, these clocks are the golden standards. Without these clocks utility companies would not be able to achieve the accurate measurement and control of energy allocation that they have grown accustomed to. Telecommunications is absolutely reliant on precise time for quality of service. And accurate time synchronization has become a critical element to secure IT systems.
The size and accuracy of the atomic clock has changed during the last 50 years. Symmetricom has been in the atomic clock business for more than 30 years. We are the world’s leading manufacturer of Active Hydrogen MASERs, Cesium frequency standards and Rubidium frequency standards. Our cesium standards are quite different in size and capability from the Atomichron. Symmetricom is among the research groups looking at miniature atomic clocks under a program funded by the US Government. These tiny atomic clocks could be made using standard semiconductor processes and slipped into cell phones, handheld computers and Global Positioning System receivers.
Here is to a Happy 50th Anniversary to the cesium atomic clock. And while we are at it, let’s also celebrate the 100th anniversary of Einstein’s Theory of Relativity.
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