Transverse relaxation ($T_2^{-1}=\Gamma =T_1^{-1}/2+{\gamma }_{\varphi }$)

Decay of the off-diagonal coherence of a two-level density operator. Population relaxation contributes half its total rate; additional dephasing contributes ${\gamma }_{\varphi }\ge 0$ in the usual completely positive two-level model. Thus transverse relaxation should not automatically be identified with pure dephasing alone.