Population relaxation
= Population relaxation
{title2=$T_1^{-1}=\gamma_{12}+\gamma_{21}$}
For a two-level <density operator>, rates $\gamma_{12}$ from level one to two and $\gamma_{21}$ in the reverse direction give $\dot p_1=-\gamma_{12}p_1+\gamma_{21}p_2$. Their sum sets the longitudinal population relaxation rate, and their ratio fixes the stationary population imbalance. If both rates vanish, populations are conserved.