Study On Decoherence Control For Quantum Information Systems Via Measurements And Quantum Gates

Currently,quantum information science and technology is developing rapidly.In practical quantum information processing tasks,decoherence occurs due to the inevitable interaction of the quantum system with its environment,which is one of the biggest bottlenecks in the realization of quantum information theories.Hence,it is of vital theoretical value and practical significance to study decoherence suppression for quantum information systems via appropriate control strategies.In this context,this dissertation focuses on the decoherence control problem of quantum information systems via measurements and quantum gates,mainly including the following three aspects.1.A novel decoherence control scheme is presented via weak measurements and environment-assisted measurement to suppress the amplitude-damping decoherence for arbitrary unknown N-qubit states.The proposed scheme consists of three stages.First,transfer the initial quantum state to a state that is robust to decoherence via pre-weak measurement and feed-forward control before the decoherence channel.Second,perform a measurement on the environment coupled to the protected quantum system during the decoherence process,so as to keep the quantum trajectory corresponding to the invertible Kraus operator.Third,recover the initial state via reversed feed-forward control and post-weak measurement after the decoherence channel.On the one hand,a total fidelity of unity can be always achieved even in heavy damping cases provided that strength of the post-weak measurement satisfy the condition of complete recovery.On the other hand,the effects of strengths of weak measurements,parameters of the initial state and the magnitude of the amplitude-damping channel on the performance of the proposed scheme are analyzed comprehensively provided that the condition of complete recovery fails.The proposed scheme is employed to enhance the concurrency of two-qubit entanglement,and its superiority is demonstrated in comparison with the unprotected scenario.2.A quantum teleportation framework is suggested via weak measurements and environment-assisted measurement to achieve high-fidelity quantum teleportation through amplitude-damping channels,where the cases of uncontrolled/controlled quantum teleportation based on a Bell/W state are taken into account.In the designed framework,environment-assisted measurement is applied during the entanglement distribution process,and then the unitary operator in the last step of the original protocol is replaced by a well-designed weak measurement operator to restore the state of the output qubit.On the one hand,the designed framework can achieve an average fidelity of unity by setting the strength of the weak measurement equal to the magnitude of the amplitude-damping channel.On the other hand,it can also strike a balance between the average fidelity and the total success probability.Particularly,for the controlled quantum teleportation based on a W state,the decoherence effect of the shared entanglement can be completely suppressed by only applying environment-assisted measurement during its entanglement distribution process;hence an average fidelity of unity can be achieved without any subsequent state recovery operations.Simulation results show that the designed framework is capable of achieving higher fidelities in comparison with the quantum teleportation protocols under no protection and a quantum teleportation protocol using only weak measurement.3.Three quantum teleportation schemes that are robust to amplitude-damping decoherence are developed via quantum gates,to overcome the obstacle that the existing decoherence control schemes in quantum teleportation are generally difficult to adapt to the needs of quantum networks and quantum computing.In the first scheme,a quantum gates control unit consisting of a Hadamard and a CNOT gate is applied at the receiver’s end to restore the state of the output qubit after the original quantum teleportation protocol is accomplished.Next,a unit fidelity quantum teleportation scheme is developed by adding environment-assisted measurement to the entanglement distribution process of the first scheme.Furthermore,the fidelity of quantum teleportation is improved from the perspective of protecting the shared entanglement,by applying a pre-and a post-quantum gates control unit during the entanglement distribution process before the original quantum teleportation protocol.The designed quantum gates assisted quantum teleportation schemes are neither resource-intensive nor time-consuming;hence they have potential application value in quantum networks and quantum computing.Simulation results indicate that the fidelities of the proposed quantum gates assisted quantum teleportation schemes outperform the quantum teleportation protocol under no protection and a quantum teleportation protocol using weak measurement.