Effect of Closely-Spaced Excited States on Electromagnetically Induced Transparency

Electromagnetically induced transparency (EIT) is a well-known phenomenon due in part to its applicability to quantum memories. While EIT is commonly modeled with a three-level lambda system, this simplified model does not capture all the physics of EIT experiments with real atoms. We present a theoretical study of the effect of two closely spaced excited states on EIT and off-resonance Raman transitions after Doppler broadening. We find that EIT transmission can be enhanced when the separation of the two excited states is smaller than their Doppler broadened linewidth. However, unequal dipole moments of the transitions to the excited states cause a shift of the two-photon resonance frequency that limits the maximum EIT transparency even under ideal conditions of no decoherence. As a result, complete transparency cannot be achieved in a vapor cell for transitions with unequal dipole strengths. To verify our result, we present experimental EIT measurements in the D1 lines of $^{85}$Rb and $^{87}$Rb that agree with the theoretical predictions when the interaction of the fields with the four levels is taken into account. Furthermore, we provide a model based on a dressed state picture that captures the underlying physics that explains the different behaviors of EIT in the presence of two closely spaced excited states for all possible transitions of the Rb D1 lines.