Next we consider the effect of a light field on the energy levels of
a simple two level atom. The light field will shift the energy levels
via the A.C. Stark shift, also known as the "light shift". If the
detuning of the light field with respect to the transition frequency is
positive (δ >0, 'red detuned'), then the excited and ground states
will shift up and down, respectively, by equal amounts (Figure 2).
In the case of δ < 0 (blue detuning), then the excited and ground
states shift down and up, respectively, in energy.
The magnitude of the light shift depends on the detuning (δ), light
intensity (here, given by the
Rabi frequency Ω), and on the strength of the transition between
the two levels, which is characterized by the relevant Clebsch-Gordon coefficient (C).
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