Abstract
The quantum interference phenomenon of a narrow and high-contrast absorptionpeak arising in the middle of the transparent window of a standard electromagneticallyinduced transparency (EIT) spectrum has been attributed to the interference ofinteracting dark resonances (IDR) in a four-level system. This phenomenon have beenpredicted in Phys. Rev. A 60,3225[2]. Because this quantum interference involvesa three-level electromagnetically induced transparency (EIT) scheme coupled with anextra atomic state driven by an external coherent electromagnetic field, in order toget more physics insight and simple calculation, I using three alternative methods toexplain the IDR system, the first one is the optical Bloch equation, the second one isusing dressed-state picture, and the last but not least one is the double-sided Feynmandiagram. These three methods can provide complete information of atomic system inthe quantum optics field.The other main part of this thesis is the steady-state gain mechanism of EIT andIDR, this gain mechanism is not from population inversion, but the quantum coherence.Adding an incoherence pump into above systems as same as the probe transitions,this incoherence pump will change the magnitudes of the zero-order density matrix elementsand to enhance the gain. I discuss the relation between the gain mechanism ofabove systems and the fraction of spontaneous emission rate to specific ground state.By systematically varying the spontaneous decay rate, the numerical calculations havebeen performed in EIT and IDR systems.