Topic summary
Hyperfine level
Extracted from the Wikipedia article Hyperfine structure.
Use in defining the SI second and meter
The hyperfine structure transition can be used to make a microwavenotch filter with very high stability, repeatability, and Q factor, which can thus be used as a basis for very precise atomic clocks. The term transition frequency denotes the frequency of radiation corresponding to the transition between the two hyperfine levels of the atom, and is equal to f = ΔE/h, where ΔE is difference in energy between the levels and h is the Planck constant. Typically, the transition frequency of a particular isotope of caesium or rubidium atoms is used as a basis for these clocks.