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The second 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 cesium 133 atom.

--> This definition refers to a cesium atom at rest at a temperature of 0 K.

-From what I learnt, changing in energy level of atoms means change from the ground state to the "excited" state. Does "transition between the two hyperfine levels" as quoted above has the same meaning?

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  • $\begingroup$ There is a nice description of the Cs transitions at hyperphysics.phy-astr.gsu.edu/hbase/acloc.html#c4. The definition of 'excited state' is somewhat subjective, since the atom is placed in an upper hyperfine energy by a microwave photon it is certainly 'excited' as it can loose energy again. More common is to used 'excited state' to mean that the transition occurs between levels with different principle quantum numbers, 1s-2p for example. Others may have different views on this, however. $\endgroup$
    – porphyrin
    Commented Mar 13, 2017 at 8:56

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