Atomic Clock - invention - how it works?

Atomic Clocks An atomic clock is a clock that uses an electronic transition frequency in the microwave region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are among the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the frequency of television broadcasts, and in global navigation satellite systems such as GPS. The principle of operation of an atomic clock is not based on nuclear physics, but rather on the microwave signal that electrons in atoms emit when they change energy levels. Early atomic clocks were based on masers at room temperature. Currently, the most accurate atomic clocks first cool the atoms to near absolute zero temperature by slowing them with lasers and probing them in atomic fountains in a microwave-filled cavity. Astronomers and physicists use accurate time pieces to measure distances in space.
	Invention Atomic clock was invented by Louis Essen. Louis Essen was born in 1908 in a small city in England called Nottingham. His childhood was typical of the time and he pursued his education with enjoyment and dedication. At the age of 20 Louis graduated from the University of Nottingham, where he had been studying. It was at this time that his career started to take off, as he was invited to join the NPL, or National Physics Laboratory. It was during Louis's time at the NPL that he began working to develop a quartz crystal oscillator as he believed they were capable of measuring time as accurately as a pendulum based clock. Ten years after joining the NPL Louis had invented the Essen ring. This was an eponymous invention which took its name from the shape of the quartz which Louis had used in his latest clock and which was three times more accurate than the previous versions.
Louis soon moved on to newer areas of research and began to study ways to measure the speed of light. During World War II he began to work on high frequency radar and used his technical ability to develop the cavity resonance wavemeter. From 1946 it was this wavemeter which he used, along with a colleague by the name of Albert Gordon-Smith, to make his lightspeed measurements. It has been acknowledged recently that Louis's measurements were by far the most accurate to have been recorded up until that time. During the early part of the 1950's Louis began to take an interest in research which was being carried out at the National Bureau of Standards (NBS) in the United States of America. He learnt that work was being carried out to invent a clock which was more accurate than any other. The American scientists were using the idea of maintaining a clock's accuracy by using the radiation emitted or absorbed by atoms. At that time the Americans were using a molecule of ammonia but Louis felt that this was not working as well as if they were using different atoms, such as hydrogen or caesium, and so he began working on his own clock using these materials instead. 1953 saw Louis and a colleague, Jack Parry, receiving permission to develop an atomic clock at the NPL based on Louis's existing knowledge of quartz crystal oscillators and other relevant techniques he had learned from the cavity resonance wavemeter he had previously designed. Only two years later Louis's first atomic clock was running, Caesium I, designed by the UK scientists. Development in the United States had all but stopped due to political difficulties. Louis continued to work on his atomic clock and by 1964 he had managed to increase the accuracy of the atomic clock from one second in 300 years to one second every 2000 years! The continued success of Louis's work resulted in the definition of a second being changed from 1/864000 of a mean solar day to being calculated as the time it took for 9192631770 cycles of the radiation in an atomic clock. Louis Essen died in 1997 and before his death had been honoured with, amongst others, an OBE and the Tompion Gold Medal of the Clockmakers' Company. How it works Some people think that the atomic clock works by measuring the decay of some isotope. This is not the case; rather it measures oscillations of an atom, just as a pendulum clock measures the swing of a pendulum. In the atomic clock, cesium atoms are placed in a copper tube that is surrounded by laser beams (four perpendicular and at right angles to each other and one above and one below). When they are all turned on the cesium atoms gather at the very center of the tube. Then all of the lasers except the one beneath the copper tube are turned off. The one beneath is turned off and on and the cesium atoms are moved up and down in a fountain-like action. To ensure precision, the clock is shielded from the Earth's magnetic field and the temperature is set to near absolute zero (-273 degrees Centigrade). As the cesium atoms are thrown up by the laser beneath the tube, they go through a microwave-emitting cavity and then return via gravity through the same cavity. The microwaves are fired at a fluctuating rate around 9,192,631,770 Hertz - regulated by a vibrating crystal. The cesium atoms actually begin to glow or emit light. As the atoms change state, their change is measured. When the state changes are at their optimum, the number of Hertz (cycles per second) of the microwave is measured. This is used to adjust the crystal oscillator, which should now register true seconds. This all occurs at two places at once, in Colorado and in Paris. The two clocks really only register the length of a second. An average is found between the two measurements. This second is used to synchronize 200 or so other less accurate clocks stationed at other locations around the world. The average of all of these clocks is called International Atomic Time. The clock is so accurate that with the degree of possible error assigned it would take 10 million years for the clock to be off by a second. More details on WIKI , INDEPTHINFO