Electromagnetically induced transparency in cold free atoms: Test of a sum rule for nonlinear optics

The resonance absorption line shape from cold $(T<\mathrm{}100\mathrm{}$ $\mu$K) atomic cesium vapor has been investigated with a probe laser after switching off the magneto-optical trapping fields. An additional nonabsorbed laser field has been used to produce quantum interference in a three-level ladder scheme. Predicted but counterintuitive modifications of the natural absorption line shape have been directly observed including inhibition of resonance absorption. The validity of a nonlinear sum rule deduced from the causality principle has been satisfactorily tested.