Atomic Clocks

According to the encyclopedia, Atomic Clock is a type of clock that uses an atomic resonance frequency standart as its counter. It is most precise timekeeping device in the world are used by government bodies to define the absolute time. This accurate time is then transmitted from radio stations in North America and Europe and it is this radio signal that Atomic Radio Clocks use to ensure they have the correct time and date.

Atomic Radio Clocks are available for
• America, using the W.W.V.B. time signal transmitter, located in Colorado.
• Europe (Central) using the German DCF time signal transmitter.
• Europe (Britain, Eire and Portugal) using the British MSF time signal transmitter.

A little History . . . . .

A long time ago scientists reached a conclusion that atoms and molecules have resonances and that each chemical element and compound absorbs and emits electromagnetic radiation at its own characteristic frequencies. These resonances were inherently stable even over “Time and Space”.

That is why atoms constitute a potential “pendulum” with a reproducible rate that can form the basis for one highly accurate clock. The development of radar and the subsequent experimentation with high frequency radio communications in the period of 1930s and 1940s made possible to be create a vast amount of knowledge regarding 'electromagnetic waves', also known as 'microwaves', which interact with the atoms. Investigations focused firstly at developing on microwave resonances in the chemical Ammonia and its molecules.

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. In 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. But the first accurate atomic clock was built in 1955 by Dr Louis Essen at the National Physics Laboratory in the UK and originally worked with the chemical Ammonia.

Within a few years the atomic clock was based on the Cesium-133 atom and in 1967 the second was defined as 9,192,631,770 oscillations of the Cesium atoms resonant frequency. The cesium standard had been refined enough to be incorporated into the official timekeeping system of NIST and standards of this kind were also developed at a number of other national standards laboratories, leading to wide acceptance of this new timekeeping technology.

In August 2004, NIST scientist demonstrated a chip-scaled atomic clock. According to the researchers, the clock was believed to be one hundredth the size of any other. It was also claimed that it requires just 75 mW, making suitable for battery-driven application.

How the Atomic Clocks Work?

The atomic clock was made possible through the brilliant theoretical and experimental work of a number of scientists, several of whom received Nobel prizes, but the clock itself is very simple.

Every clock must contain or be connected to some apparatus that oscillates at a uniform rate to control the rate of movement of its hands or the rate of change of its digits. In the old pendulum clocks, a weight oscillated at a fairly constant frequency, so the clockmaker simply had to invent a mechanism to count the swings and drive the clock's hands. But in an atomic clock, the oscillations occur in an electromagnetic field that causes transitions between two quantum-mechanical conditions of atoms. In the commonly used cesium 133 atoms, these occur at about 9.19 billion times per second.

The part of an atomic clock which is responsible for keeping time is actually a quartz crystal oscillator. In most quartz clocks, the oscillator is tuned accurately when the clock is made but its frequency is never checked again. Over time, its frequency changes slightly but unpredictably, making the clock fast or slow. The purpose of the complicated apparatus in an atomic clock is to check the frequency of the quartz oscillator continually, giving the clock its great accuracy.

The core of the atomic clock is a microwave cavity containing ionized gas, a tunable microwave radio oscillator, and a feedback loop which is used to adjust the oscillator to the exact frequency of the absorption characteristic defined by the behavior of the individual atoms.

The microwave oscillator fills the chamber with a standing wave of radio waves. When the radio frequency matches the hyperfine transition frequency of caesium, elections in the caesium atoms are able to absorb the radio waves and move between two energy states. Only those atoms that have changed states are allowed to impinge on a detector. When the incidence of detected atoms decreases because the frequency of the microwave oscillator has drifted from the true resonance frequency, the frequency of the oscillator is corrected.

The US Government owns and operates an "atomic clock" which is located in Colorado. This atomic clock keeps precise time by dropping atoms. It is the most accurate clock in the world and is considered the official US time. The clock is hooked up to a huge radio antenna which sends out a strong radio signal across the entire contiguous US. Our clocks tune into that radio frequency, decode the signal, and automatically set their time to the US atomic clock. The clocks automatically search for the signal at least once a day in order to keep precise time.

Kinds of atomic clocks

Today, though there are different types of atomic clock, the principle behind all of them remains the same. The major difference is associated with the element used and the means of detecting when the energy level changes. The various types of atomic clock include: Cesium clocks, Rubidium clocks and Hydrogen Clocks. Most commercially available accurate time sources utilize a radio or GPS time signal that is linked to an atomic clock time reference.

Inside a cesium clock, cesium-133 atoms are heated to a gas in an oven. Atoms from the gas leave the oven in a high-velocity beam that travels toward a pair of magnets. The magnets separate the atoms according to whether they are available to absorb or release energy. The atoms that can absorb energy are directed through a microwave cavity where they are exposed to radiation with a frequency very close to 9,192,631,770 cycles per second, which is the frequency of the radiation emitted or absorbed by a cesium-133 atom as it shifts from one energy state to another. Some of the atoms absorb energy from the microwaves. These atoms are then pushed by another set of magnets toward a detector. A servomechanism monitors a feedback loop between the detector and an oscillator. This feedback tunes the microwave frequency until it exactly matches the radiation frequency of the cesium atoms, maximizing the number of atoms that reach the detector. Once the microwave frequency is locked into the cesium atoms' frequency, it is then divided down to a frequency that can be used to mark time accurately to a few billionths of a second.

The Rubidium clock, the simplest and most compact of all, uses the transition of the rubidium-87 atom between two hyperfine energy states. It employs the same basic principle as the cesium-atom clock. The rubidium atoms, however, are first forced to change their hyperfine energy state and are then subjected to microwave radiation to return them to their original state. When many atoms return to their original state, the correct transition frequency has been reached and the period of the wave can be used to measure time. Rubidium clocks are not as stable or as accurate as cesium-atom clocks, but they are more compact and less expensive.

The Hydrogen clock and the ammonia clock rely on the maser principle. In a hydrogen clock, a focused magnetic field selects hydrogen atoms in a specific hyperfine energy state. These atoms are forced to change to a lower energy state. When many atoms make the transition, they begin to oscillate between the two states, emitting energy in the form of an electromagnetic wave. The period of this emitted wave is used to measure time. The hydrogen clock is very stable for several hours at a time.

Do we need of such accuracy?

Most of us may not understand the need for such accuracy, but it turns out that the innumerable communication, scientific and navigation systems rely on it. Timing is critical for synchronizing signals between computers. In astronomy, fractional-second errors could sabotage long-baseline radio telescopes, a nifty way to fuse distant radio telescopes into one gargantuan receiver.

An atomic clock keeps time better than any other clock. They even keep time better than the rotation of the Earth and the movement of the stars. Without the atomic clock, GPS navigation would be impossible, the Internet would not synchronized, and the position of the planets would not be known with enough accuracy for space probes and landers to be launched and monitored.

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