Coupled Spin Star System And Its Application In Quantun Information Processing

With the development of information technique and quantum mechanics, a new subject based on the law of quantum mechanics--Quantum Information Science comes into being.It is an emerging field with the potential to cause revolutionary advances in fields of computation,communication,precision measurement and holds the promise to dramatically improve the acquisition, transmission security and processing speed of information.The enormous theoretical potential of quantum information processing is driving the pursuit for its practical realization by various physical systems.Electron and nuclear spins are natural qubits.The properties of the long coherence times,the ability of high efficiency spin-specific controllability and measurement make spins as ideal candidates for physical implementation of quantum information processing. Recently,the investigation of the properties of spin system,especially of multi-particle strongly coupled spin systems is one of the hottest areas in the field of quantum information processing.This thesis investigates coupled spin star system and its application in quantun information processing.We solve some coupled spin star system and study the implemetation of quantum information processing,such as quanum state transfer,quantum phase transition,quantun cloning,quantum decoherence, quantum measurement,quantum cryptography in these systems.Our mainly work is as follow:1,We consider a general spin star system,with a central spin-(1/2)particle surrounded by a ring of N spin-(1/2)particles.Using the symmetries of the system Hamiltonian,we solve the system analytically and obtain the general expressions of eigenstates and the corresponding eigenenergies.We invesgate the properties of ground state quantum phase transitions in some limiting cases.Moreover,we study the generation the some desired and usefull quantum states,i.e,GHZ states,in the process of quantum phase transitions.We further analyze the properties of quantum state transfer and entanglement dynamics in this model.In the time evolution,some simple and interesting results were discovered concerning transfer fidelity,as well as entanglements created.2,The configuration can also be modeled as quantum cloning model.We analyse two types of quantum initial states preparation in the implementation of quantum cloning in spin networks,we show that by properly choosing the initial state of spin ring and the control parameters,the model is capable of implementing optimal 1→N phase covariant cloning in this model.3,We study properties of quantum decoherence in spin systems.When we only concern about the central spin of the model,the spin ring is modelled as decoherence enviromnent.We analyse the dynamical property in the presence of spin enviromnents and the fidelity of the single spin system.Meanwhile we extend the single spin system to a spin chain,with each spin in this chain coupled a spin ring environment.We study the influence of spin environments on quantum state transfer fidelity in this spin chain.4,We present an efficient one-step scheme for a single spin measurement in spin networks based on nuclear magnetic resonance(NMR)techniques.We solve the system analytically using the effective Hamiltonian method,i.e.,by considering only the secular part of the system Hamiltonian.We discover that only one step is required to accomplish single spin measurement process. As opposed to the spin chain and the cubic lattice crystal proposals,here considerable speedup is achieved.We also prove that this model implement state-dependent cloning when single spin measurement is achieved.5,Finaly,we propose a long-distance quantum cryptography scheme in spin networks with qubits stored in electron spins of quantum dots.In this scheme,the spin-photon interaction time can be precisely controlled through placing each quantum dot in individual hight-Q microcavity.Using conditional Faraday rotation,quantum state transfer and quantum memory,we can implement a long-distance quantum cryptography.