= Rabi vector
{c}
{title2=$\mathbf W$}
For a rotating-frame two-state <Hamiltonian operator> $H=\hbar\mathbf W\cdot\boldsymbol\sigma/2$, the <Bloch vector> obeys $\dot{\mathbf R}=\mathbf W\times\mathbf R$. Thus $\mathbf W$ specifies a rotation axis and angular speed. With detuning defined as drive <frequency> minus transition <frequency> and the lower state at positive $R_z$, a phase-zero transverse drive can give $\mathbf W=(\Omega,0,\delta)$. Changing basis or detuning convention requires changing the corresponding signs consistently.
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