Atomic Coherence And Its Application In Quantum Information

Quantum coherence and quantum entanglement are two spectacular features of quantumphysics which distinguish itself from classical physics. Studies on the quantum coherenceand quantum entanglement have enhanced the development of quantum mechanics,and given birth to a new interdisciplinary field bridging quantum physics, computer science,mathematics, and information technologies, i.e., the quantum information scienceand technology. Quantum information science is, in essence, a subject that processes informationby utilizing and exploiting the quantum coherence and quantum entanglement.Among various of potential physical candidates for quantum information processing, thepromising atomic, molecular, optical (AMO) systems have attracted significant attentionfrom researchers. AMO systems have been widely applied to almost every area of quantuminformation processing, such as the generation, manipulation, storage and the transmissionof quantum states. Thus, study on atomic coherence and interference effects ofthe AMO systems and its applications in quantum information science will facilitate therealization of quantum information processor.This thesis investigates the behaviors of multi-level atomic system coupled with coherentdriving fields, noncoherent driving fields and vacuum reservoir, and studies someinteresting atomic coherence and interference effects, such as the coherent population trapping(CPT) effects, electromagnetically induced transparency (EIT) effects and spontaneouslygenerated coherence (SGC) effects , and reveals the physical mechanism underthese phenomena. By choosing atomic system with appropriate level-structure, and thedriving fields with suitable parameters, the AMO systems can bring new way of generatingthe atomic coherence, resist the decoherence effects robustly, and even enhance atomiccoherence, which helps to coherently manipulate the quantum states of AMO systems, andpave the way of realizing the quantum information processing tasks based on the AMOsystems. The detailed description about the work of this thesis can be summarized to thefollowing three parts:(1) The double EIT effects generated in a four-level diamond-shape atomic system couplingwith four coherently driving fields are investigated, and a scheme on realizinga quantum control phase gate(QCPG) based on the double EIT effects is proposed. In order to utilize the atomic coherence and interference effects of the AMO systemto manipulate quantum states coherently, a new diamond-shape structure four-levelatomic system coupling with four driving fields is studied in detail. It is found thatthe double EIT effects are smoothly generated in various of driving fields' settings,the flexibility of generating the double EIT effects indicates the feasibility of realizingthe QCPG in this system. The possibility of experimentally realizing the QCPGis also discussed.(2) The mechanism of atomic coherence generated in aΛatomic system coupled by incoherentpumping field and vacuum reservoir is revealed, which helps to overcomethe decoherence effects, and even to enhance the atomic coherence by appropriatelytreating the decaying processes. For example, the CPT state and the multi-steadystates appear in theΛsystem under the joint action of the incoherent pumping fieldsand the vacuum field. The atomic coherence generated by the dissipation processesopens door for exploring new mechanism of coherence generation, which will be ofgreat importance on manipulating quantum states.(3) Combining the flexibility of controlling the atomic system by coherent driving fieldsand the possibility of enhancing the atomic coherence through spontaneously generatedcoherence(SGC) effects, the resonance fluorescence spectrum modulated bythe SGC effects in the diamond-shape four levels atomic system is investigated.Modulated by the SGC effects, the resonance fluorescence spectrum of the atomicsystem shows many attractive characteristics, such as the enhancement of fluorescenceeffect, the enhancement of spectra-line, suppression of the spectra-line,narrowing the line-width, reducing the number of sidebands and even completelyquenching the resonance fluorescence effect. The quantum interference nature underlyingthese interesting behaviors of resonance fluorescence spectrum is explained.Finally, an experimental proposal of studying the SGC effects by using real atomicsystem is suggested.