We propose a scheme for creating atomic coherent superposition states via the technique of stimulated Raman adiabatic passage in a $\ensuremath{\Lambda}$-type system where the final state has twofold levels. With the application of a control field, it is found that the presence of double dark states leads to two arbitrary coherent superposition states with equal amplitude but inverse relative phases, even though the condition of multiphoton resonance is not met. In particular, two orthogonal maximal coherent superposition states are created when the control field is resonant with the transition of the twofold levels. This scheme can also be extended to manifold $\ensuremath{\Lambda}$-type systems.
We propose a scheme for creating atomic coherent superpositions via stimulated Raman adiabatic passage in a Lambda-type system where the final state has twofold levels. In the employ of a control field, the presence of double dark states leads to two arbitrary coherent superposition states with equal amplitude but inverse relative phase without the condition of multi-photon resonance. In particular, two orthogonal maximal coherent superposition states are created when the control field is resonant with the transition of the twofold levels. This scheme can also be extended to manifold Lambda-type systems.
In this thesis, quantum coherent control of dynamic properties of double-dark-resonance atomic systems is pursued in detail. First we show our scheme for creating atomic coherent superposition states via the technique of stimulated Raman adiabatic passage in a Lambda-type four-level system. With the application of a control field, it is found that the presence of double dark states leads to two arbitrary coherent superposition states with equal amplitude but inverse relative phases, even though the condition of multiphoton resonance is not met. The suitable manipulation of the control field and detunings enables the proposed scheme to create any coherent superposition states one desired. Then, in the above mentioned double-dark-resonant system, a scheme for giant enhancement of the Kerr nonlinearity is put forward. Compared with that generated in a single-dark-resonance system, the Kerr nonlinearity can be enhanced by several orders of magnitude with vanishing linear absorption. We attribute this dramatic enhancement to the interaction of dark resonances. As another possible means for enhancement of Kerr nonlinearity, spontaneously generated coherence is proposed. Meanwhile, a theoretical investigation is carried out in a five-level tripod system for obtaining a high efficiency double-channel four-wave-mixing process. By manipulating the intensity of the two coupling fields, the conversion efficiency of four-wave-mixing can be controlled.
A technique for creation of well-defined preselected coherent superpositions of multiple quantum states is proposed. It is based on an extension of the technique of stimulated Raman adiabatic passage (STIRAP) to degenerate levels. As an example, the nine-state system composed of the magnetic sublevels of three levels with angular momenta
Jg=0,
Je=1, and
Jf=2 is studied in detail. Starting from the $\ensuremath{\mid}{J}_{g}=0,{M}_{g}=0⟩$ state, STIRAP can create an arbitrary preselected coherent superposition between the five
Mf sublevels $({M}_{f}=\ensuremath{-}2,\ensuremath{-}1,0,+1,+2)$ of the
Jf=2 level with 100% efficiency in the adiabatic limit. The populations and the phases of the
Mf states in this superposition are determined entirely by the polarizations of the two laser fields. It is shown that this technique allows one to create any superposition of the five
Mf states, that is, to reach any point in the Hilbert space of the
Jf=2 manifold, and the corresponding recipes for choosing the laser polarizations are presented.