**Atomic frequency standards and clocks:**

Atomic frequency standards and clocks – The measurement of time has two different aspects, civil and scientific. In most scientific work, it is desired to know how long an event lasts, or if dealing with an oscillator, it is desired to know its frequency of oscillation. Thus any time standard must be able to answer both the question “what time is it and the two related questions “how long does it last” or “what is its frequency”.

Any phenomena that repeats itself can be used as a measure of time, the measurement consisting of counting the repetitions. Of the many repetitive phenomena occurring in nature, the rotation of the earth on its axis which deter-mines the length of the day, has been long used as a time standard. Time de-fined in terms of rotation of the earth is called **Universal time** (**UT**),

Time defined in terms of the earth’s orbital motion is called **Ephemersis time** (**ET**). Both UT and ET are determined by astronomical observation. Since these astronomical observations extend over several weeks for UT and several years for ET, a good secondary terrestrial clock calibrated by astronomical observation is needed. A quartz crystal clock based on electrically sustained natural periodic vibrations of a quartz wafer serves as a secondary time standard. These clocks have a maximum error of 0.02 sec per year. One of the most common of time standards is the determination of frequency.

In the RF range, frequency comparisons to a quartz clock can be made electronically to a precision of at least 1 part in 10^{10}.

To meet a better time standard, Atomic frequency standards and clocks have been developed using periodic atomic vibrations as a standard. The transition between two energy levels, E_{1} and E_{2} of an atom is accompanied by the emission (or absorption) of radiation given by the following equation

Provided that the energy levels are not affected by the external conditions such as magnetic field etc.

Since frequency is the inverse of the time interval, time can be calibrated in terms of frequency.

The atomic clock is constructed on the above principle. The first atomic clock was based on the Cesium atom.

The International Committee of Weights and Measures defines the second in terms of the frequency of Cesium transitions, assigning a value of 9,192, 631,770 Hz to the hyper fine transitions of the Cesium atom unperturbed by external fields. If two Cesium clocks are operated at one precision and if there are no other sources of error, the clocks will differ by only 1 s in 5000 years.