Stimulated Raman Adiabatic Passage Based On Two-photon Transitions And Controlled-NOT Gate In A Two-mode Optical Cavity

In this thesis of master degree, we discuss stimulated Raman adiabatic passage (STIRAP) in a five-level A system firstly, and then study the Controlled-NOT gate for manipulating photonic polarization states in a two-mode optical cavity in the second part.I. Stimulated Raman adiabatic passage based on two-photon transitionsStimulated Raman adiabatic passage (STIRAP) is a well developed technique that permits precise control of population transfer in a three-level atomic or molecular system. It achieves nearly complete population transfer between two quantum levels using of a partially overlapping counterintuitive time sequence which begins with the Stokes pulse and ends with the pump pulse in a three-level Raman system or in a ladder system. STIRAP has the remarkable property of placing almost no population into the intermediate level, and thus it is insensitive to any possible decay from that level. Although STIRAP attracted a great many of attentions of researchers, there are few articles related to STIRAP in systems which have more than three levels. Therefore, the first part of this thesis describes a scheme in which we apply stimulated Raman adiabatic passage in a Fig.1. Representative five-level system five-level A system. However, it is hard to deduce an analytic solution of the zero-energy eigenstate (dark state) of the present STIRAP in the five-level A system. We explain the phenomenon using approximate dark state with an approximate eigenvalue which tends toward zero. We have extended successful implementation of STIRAP in the five-level A system and achieve complete transfer of population from the initial level to the final level. Also, it should be noted that four-photon resonance is very important to this five-level stimulated Raman adiabatic passage. It guarantees that the atomic system meet the requirement of having a STIRAP, which is the existence of a dressed eigenstate with a zero-energy throughout the evolution. The two photon resonance is significant for it avoid population in either of these intermediate levels 2) and|4), respectively. In addition, the effective structure represents an actual physical process of the STIRAP in a five-level A system. Further more, based on the phenomena that the STIRAP technique occurs coherently, we discuss the generation of the maximally coherent state of the ground level and the final level.II. Controlled-NOT gate for manipulating photonic polarization states in a two-mode optical cavityA prerequisite for distributed quantum computation is the physical realization of diverse logic gates for implementing quantum operations. In fact, all quantum computers can be decomposed into a sequence of universal one-qubit unitary gates and two qubit phase gates. One intriguing method for implementing such logic gates is to utilize the strong photon-atom dynamics (QED) devices. Cavity QED is coupling in cavity quantum electro-well known as a unique architecture for quantum information processing as it allows for the coherent information exchange between computation and communication systems. Therefore, we present an alternative proposal for realizing a C-NOT gate to manipulate two Fig.2. Energy-level diagram of the intracavity photons interacting with a traveling atom Y-type atom in the Lamb—Dicke limit.We envision here a setup as depicted in Fig.2 where an atom goes across an optical cavity in a controllable time. Then transitions |1>(?)|2>,|2>(?)|3>, and |2>(?)|4> are dipole-allowed and coupled byωh photons of left circular polarization(σh+),ωv photons of right circular polarization (σv-), andωv photons of left circular polarization (σr+), respectively. The successful operation of the C-NOT gate requires a close-loop four-photon transition dominant over all one-photon and two-photon transitions. With a large single-photon detuning, all one-photon transitions are found to be well suppressed in the coherent dynamics. Applying a classical field on the two upmost levels, one may further suppress all two-photon transitions via a big dynamic Stark shift. In the strong coupling regime, our desired C-NOT operation, referring to the close-loop four-photon transition where the second photonωv is flipped betweenσv+ andσv- polarizations depending on the state of the first photonωh, may have a rather high gate fidelity and a moderate photon loss.