摘要
Coherent two-field spectroscopy calculations with different choices of the quantization (z) axis are compared. In a system driven by a strong coupling field and a weak probe field, alignment of the z axis along either the polarization direction of a linearly polarized coupling field or the propagation direction of a circularly polarized coupling field is shown to simplify the calculations and facilitate interpretation of the results. The advantages of a suitable choice of the z axis are highlighted with an example of a degenerate three-level system of electromagnetically induced transparency.