Quantum Decoherence And Quantum Repeaters In Quantum Communication

With the development of information technique and quantum mechanics, a new subject based on the law of quantum mechanics-quantum information theory comes into being.The quantum information theory mainly consist of quantum communication and quantum computing,it is the quantum coherence which lies at the heart of quantum information that makes the manipulation of information-carrier qubits possible,and the occurrence of the information coding,storing and transferring in a unconventionality way.A crucial obstruction to the transferring of information is decoherence which is caused by the unwanted interaction of the system with its environment,i.e.,the quantum channel is noisy quantum channel.Therefore,the study of how to improve the decoherence sensing of quantum channels and the influence of decoherence on the course of quantum communication are necessary.This thesis investigates the decoherence sensing of quantum channels and the influence of decoherence on the quantum repeater,present a quantum repeater scheme.Our mainly work is as follows:1.Unambiguous quantum state filtering is applied to evaluation of the decoherence sensing of a continuous variable linear loss channel and three typical discrete channel through probing by an entangled input.To the continuous variable linear loss channel,it is found that quantum entanglement can improve the sensing performance,and the larger the entanglement of the probe field is,the better the sensing performance is;the larger the mode number is,the less the sensing performance is.To discrete channels,entangled input can improve the sensing performance of depolarizing and phase damping channels, but can't always improve the sensing performance of amplitude damping channel. 2.The influence of three typical types of damping channels,i.e.,the depolarizing channel,the phase-damping channel and the amplitude-damping channel, on the information-disturbance trade-off of the single-qubit and single-user quantum repeaters based on CNOT gate are investigated by using the Krausoperator representations of decoherence.It is found that decoherence may lead to the appearance of three subspaces,i.e.,the normal subspace,the anomalous subspace and the decoherence-free subspace(DFS).It is shown that in different subspaces of the probe qubit,the transmission fidelity and the estimation fidelity of the quantum repeaters exhibit different characteristics.In the normal subspace the presence of decoherence decreases the transmission fidelity and the estimation fidelity while these fidelities may be enhanced in anomalous subspace.However,in the DFS,these fidelities do not change.Hence,in the DFS decoherence does not affect the information-disturbance trade-off of quantum repeaters.The concept of the quality factor is introduced to evaluate the quality of the quantum repeater.It is indicated that the quality factor can be efficiently controlled and manipulated by changing the amplitudes and the phase of the probe qubit.3.An optimal quantum repeater for an unknown two-qubit maximally entangle state in a transmission line is suggested.By the 'optimal',the maximum estimation fidelities can arrive at 1/2 proved by quantum mechanics through properly tuning the probes states.The performances of optimal single-user schemes in extracting information by sequential measurements in a N-user transmission line are also investigated.It is found that the estimation fidelity G does not depend on the number of users if they prepare the probes in the same state,but the transmission fidelity F is decreased with N.The resulting trade-off degrades with increasing N.It is also found that the trade-off for N=1 can be restored by an effective preparation of the probes states and present explicitly calculations for the 2-user case.