Cross-polarization (CP) if a fundamental RF pulse sequence and a building-block in many solid-state NMR. It is typically used to enhance the signal of a dilute nuclei with a low gyromagnetic ratio (e.g. , ) by magnetization transfer from an abundant nuclei with a high gyromagnetic ratio (e.g. ), or as a spectral editing method to get through space information (e.g. directed → CP in protein spectroscopy).
To establish magnetization transfer, RF pulses ("contact pulses") are simultaneously applied on both frequency channels to produce  fields whose strength fulfil the Hartmann–Hahn condition:
where  are the gyromagnetic ratios,  is the spinning rate, and  is an integer. In practice, the pulse power, as well as the length of the contact pulse are experimentally optimised. The power of one contact pulse is typically ramped to achieve a more broadband and efficient magnetisation transfer.