# Frequency Standards

## Realization of the units of time and frequency

The SI unit of time, the second (s), was defined 1967 by the 13^{th }General Conference on Weights and Measures as follows:

The second (s) is the unit of time. It is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

The hertz (Hz) is the unit of frequency (number of periodic events per second), where 1 Hz = 1 s^{-1}.

Since 20 Mai 2019 by the redefinition of the base units at 26^{th} meeting of the General Conference on Weights and Measures the second has been defined as:

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency Δ*ν*_{Cs}, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s^{-1}.

The second is realized by a time and frequency standard, which is an atomic clock.

Besides different types of caesium frequency standards as high precision standards also rubidium frequency standards and hydrogen masers have been developed.

These caesium frequency standards measure the frequency of the quantum mechanical transition, which is fixed by the definition of the unit second. In a thermal atomic beam a transition of the atoms into an excited state is induced by micro wave excitation. The number of excited atoms is determined by a detector. The frequency of the internal quartz oscillator is stabilized at a maximum number of the excited atoms at the detector. This process can be controlled so exactly, that such atomic clocks reach a very low frequency deviation (relative frequency deviation approx. 5 x 10^{-13}) and a very high accuracy (1 second deviation in 1 million years).

The developments of so called caesium-fountains give rise to very low relative frequency deviations in the range of 10^{-15}. For high precision short time stability especially hydrogen masers are suitable. The most recent developments in research lead towards optical frequency standards and let hope for a further improvement of the frequency standards, since instead of the microwave transition in the range of Gigahertz the thousand times higher frequencies in the optical frequency range would be used. By that a new definition of the second might be necessary.

References for further reading:

*The Measurement of Time*, Claude Audoin, Bernard Guinot, Cambridge University Press, 2001*Frequency Standards*, Fritz Riehle, Wiley-VCH, 2004