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I see in Wikipedia that the Bloch-Siegert shift is analogies to the rotating pot lid, could you explain that analogy?

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 is identical to the spin's transition frequency. The RWA is, however, an approximation. In 1940 Bloch and Siegert showed that the dropped parts oscillating rapidly can give rise to a shift in the true resonance frequency of the dipoles.

The pot lid is rotating around an axis along the surface of the table that is quickly rotating. This results in a secondary rotation which is perpendicular to the table. This is equivalent to the Bloch-Siegert shift and can be seen by watching the motion of the red dot.

The pot lid is rotating around an axis along the surface of the table that is quickly rotating. This results in a secondary rotation which is perpendicular to the table.This is equivalent to the Bloch-Siegert shift and can be seen by watching the motion of the red dot.

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This is very cool –  Mark Mitchison Apr 15 '13 at 9:38
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