Theoretical Investigation On The Coherent Feedback Control Systems Based On The Cascaded Four-wave Mixing Processes

Quantum information technology is a research upsurge of global concern.Its beautiful blueprint makes the development of quantum information technology become an important strategic direction of many countries.The nonclassical fields have great potential applications in quantum information technology,such as quantum precise measurement,quantum computation,ultra high security and ultra capacity optical information communication.Hence,it is of great significance to study the quantum properties of nonclassical fields for the development of quantum information technology.For example,highly correlated multiple beams can not only improve the communication fidelity of the quantum information protocol,but also improve the measurement precision of quantum metrology.Quantum entanglement is one of the important resources of quantum information technology.And many groups are working to improve the component or the degree of entanglement.In this paper,we combine the coherent feedback control strategy with the cascaded four-wave mixing processes to construct three kinds of coherent-feedback control systems.Through the theoretical calculation,we study the influence of feedback strength on the enhancement of tripartite quantum correlation of the three systems and quantum entanglement of one of the systems to find the optimal parameters.This paper is divided into the following three parts:1.Combining with the energy-level diagram of atomic transition,we introduce the four-wave mixing process in hot rubidium atomic vapor and describe its theoretical principle in detail.The input-output relationship is derived by using quantum mechanics method,and the degree of relative intensity squeezing of the output fields is analyzed,which paves the way for the later two chapters.2.Firstly,based on the cascaded four-wave mixing processes in hot rubidium atomic vapors,three kinds of coherent feedback control systems are constructed by implementing a linear beam splitter.Considering the absorption effect of Rb vapor cells and the loss of optical propagation in the coherent feedback loop,we calculate the input-output relation of the systems.The quantum correlation in the three structures can be characterized by the degree of relative intensity squeezing of the output fields.Finally,we find that the tripartite quantum correlation can be maximally enhanced by an optimal feedback ratio.In addition,two of the three pairwise correlations in the quantum regime can also be improved by tuning the strength of feedback,while the other pairwise correlation remains in the classical regime.3.One of the three coherent feedback control systems based on the cascaded fourwave mixing processes is choosed.By calculating the covariance matrix of the system and applying the positivity under partial transpose criterion to all possible bipartitions,we theoretically investigate the entanglement properties of the system with different feedback ratio,gains and phase.The result shows that the genuine tripartite entanglement exists in the coherent feedback control system,but the entanglement properties of the system will be destroyed by the excessive feedback of beam splitter.In addition,when the phase is 180°,we find that the tripartite entanglement can be enhanced by changing the gains and the reflectivity of the beam splitter in the range of 0.1 to 0.4.