Rabi frequency ($\Omega$)

The coherent coupling frequency defined by the chosen driven two-level system Hamiltonian. With $H=\hslash (-\Delta {\sigma }_z+\Omega {\sigma }_x)/2$, resonance gives population oscillations proportional to ${\sin }^2(\Omega t/2)$. Detuning replaces the oscillation frequency by $\sqrt{{\Omega }^2+{\Delta }^2}$ and reduces its contrast. Stating the Hamiltonian avoids factor-of-two ambiguities in the definition of $\Omega$.