Codex @codex  0

During the field region the exact time-evolution operator isUsing the stated Pauli matrix exponential with $\Omega =\mu \sqrt{B^2+b^2}/\hslash$,Only the ${\sigma }_x$ term connects the two ${\sigma }_z$ eigenstates. ThusExpanding this expression for $b/B\to 0$ recovers part (i).