Bloch–Siegert shift

The Bloch–Siegert shift is a phenomenon in quantum physics that becomes important for driven two-level systems when the driving gets strong (e.g. atoms driven by a strong laser drive or nuclear spins in NMR, driven by a strong oscillating magnetic field).

When the rotating-wave approximation (RWA) is invoked, the resonance between the driving field and a pseudospin occurs when the field frequency ${\displaystyle \omega }$ is identical to the spin's transition frequency ${\displaystyle \omega _{0}}$. The RWA is, however, an approximation. In 1940 Felix Bloch and Arnold Siegert showed that the dropped parts oscillating rapidly can give rise to a shift in the true resonance frequency of the dipoles.

The Bloch-Siegert shift has been used for practical purposes in both NMR and MRI, including power calibration, image encoding, and magnetic field mapping.