Some Studies On Quantum Communication

Quantum information technology,including mainly quantum communication(QC),quantum computing and quantum sensing,is the subject where quantum mechanics would achieve application first,while QC has become fundamentally practical.There would be two major tasks to develop QC in the next stage,one is for scientists to achieve new methods,new signal channels and new records,the other one is to fulfill practical objects such as large QC networks,highly reliable instruments,and applications combining traditional cryptography.In this thesis,the author who have been studying QC technology for quite a long time,introduced the critical elements to enhance the capability of QC and to design a QC network,and discussed in a manner of math-physics about several problems refer to high dimensions or multiple objects systems,which one may come across in design key components of QC or a large-scale QC network.The topics are listed below.First,we studied the spin evolution and the scheme to achieve high degree of polarization for a spin-nuclei system.Semiconductor quantum dot has advantages such as long coherence time and good manipulation(of the carrier electron in conduction bands),which make it a promising candidate for artificial atom,single photon source and quantum memory,which are useful for quantum communication and quantum computer.To promote the quality of a quantum dot,especially the consistence of individual quantum dots,and longer coherence time,we should suppress the dissipative effect that nuclear spins exert on carrier electrons.In earlier studies on spin evolution of spin-nuclei system,we found that the calculations were limited in the region of perturbation or short time approximation due to the extremely high dimensions.However,one can roughly deduce that the effective way to change a spin polarization should be in condition where harmonic oscillations happen,which is not located in the perturbation region.To investigate the spin evolution in oscillating region,we developed a method to block-diagonalize an extremely large Hamiltonian with group theory and used an approximation to take the continuous wave function of a carrier electron as a coarse-grained one,thus we simplified the Hamiltonian significantly so that calculating the spin evolution with numerical method become possible.Through calculation and analysis,we confirmed the deducing about optimal condition for changing spin,and showed that an extremely high degree of nuclei polarization(also means a more purified spin environment)is achievable.Second,we studied the organization and traffic engineering of a QC network.To develop QC from point-to-point quantum key distribution to a multi-component network,the structure of a network and the strategy of key exchanging are fundamental,while the traffic quality is a critical factor for designing a large scale network.According to the recent technology of exchanging fora QC network,we designed several structures and corresponding traffic engineering schemes for various application,and evaluated the accessing ability of these networks.The result has been applied in several QC networks with dozens of accessing terminals and different topologies,and their performance so far is good.Third,we studied the function design,failure and security model of a QC network.A QC network with completed function contains not only the quantum key distribution equipments from the bottom layer,but also(trusted)relay equipments,encrypting/decrypting terminals,routing system,and network management system with corresponding strategies etc.,with the models to estimate reliability,security and capability to expand.Referring to the hierarchical structure,security design in system and evolution trend of classical networks,we described the failure models and their relationship,the security frame and security elements in methodology way,we also designed the hierarchical structure and function frame of upper layers of a QC network,and carried out several secure schemes including certificate authority,core data protection and signal protection using quantum keys.These results have been applied to protect network communication for certain significant activities.