= Caesium standard
{tag=Caesium}
{title2=1967}
{title2=3.26 cm}
{wiki}
Uses the <frequency> of the <hyperfine structure> of <caesium-133> ground state, i.e spin up vs spin down of its valence electron $6s^1$, to define the <second>.
<International System of Units> definition of the second since 1967, because this is what <atomic clocks> use.
TODO why does this have more energy than the hyperfine split of the <hydrogen line> given that it is further from the nucleus?
Why <caesium> <hyperfine structure> is used:
* https://physics.stackexchange.com/questions/191871/why-do-atomic-clocks-only-use-caesium