Rubidium-87 clock transition
= Rubidium-87 clock transition
{title2=$|1,0\rangle\leftrightarrow|2,0\rangle$}
The ground-hyperfine magnetic-dipole transition between $F=1,m_F=0$ and $F=2,m_F=0$ has vanishing first-order Zeeman shift near zero field. It obeys $\Delta F=1,\Delta m_F=0$. Its residual quadratic shift follows from the <Breit-Rabi formula>, rather than from assigning both states permanently zero <magnetic dipole moment>.