Study Of Quantum Coherence And Interference Phenomenon In Three-level Λ-type System

Quantum coherent and interference have led to many interesting and significative consequences such as coherent population transfer (CPTR), coherent population trapping (CPTP), amplification without population inversion (AWI) and superluminal, etc.In this thesis, we theoretically studied some phenomenon produced by quantum coherent and interference in three-level A-type system. Firstly, we analyzed coherent population transfer in a three-level A-type system driven by unmatched Gaussian pulses. Secondly, we consider a A-type system, in which the two ground states is coupled by a coherent magnetic field. We find that, in this scheme, amplification without population inversion can be realized. Choosing the relative phase of the probing and the pumping fields under the condition of the intensities of the probing and the external magnetic fields are much smaller than that of the pumping field, the group velocity of the probing field can be varied from subluminal to superluminal.In the first Chapter, I give an introduction of the research background and the purpose of my thesis.In Chapter 2, the semiclassical theory of laser-matter interaction is reviewed. It is the basis of almost all of my works.In Chapter 3, we consider coherent population transfer in a three-level A-type system driven by unmatched Gaussian pulses. According to the numerical simulation results of the density matrix equations, we find that the transfer efficiency is strongly dependent on the intensities and the pulse widthes of the probing and the pumping fields. Atoms can be transferred almost completely from the initial state to the target state by appropriately choosing those parameters.In Chapter 4, we investigated a new scheme. An external coherent magnetic field was applied to the two ground states. From the numerical simulation and the steady-state solution to the density matrix equations, we find that AWI can be realized by choosing theintensities of the pumping and the magnetic fields and the relative phase of the probing and the pumping fields. The conditions of producing AWI is also obtained.In Chapter 5, the model we considered is same as that in Chapter 4. When the intensities of the probing and the external magnetic fields are much smaller than that of the pumping field, the dispersion property of the probing field is very sensitive to the relative phase and the collisional dephasing rates, and it can be changed from normal dispersion to abnormal dispersion by increasing the relative phase. This lead to the group velocity of the probing field is varied from subluminal to superluminal.