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Abstract

Entangled states are crucial to quantum computation and quantum communication, and are usually treated as the target states to be accessed by quantum control methods. While most of the researches focus on the generation of the desired entangled state at the terminal state , this paper considers the time-varying entanglement of the transient state | throughout the qubit transfer process. It is found that the degree of entanglement of | determines how fast and accurately the terminal state can be achieved. Four quantitative indices of entanglement are employed here to evaluate the degree of entanglement of | and to estimate the qubit control performance resulting from different control gains in the Lyapunov control law. Our results show that increasing the degree of entanglement during the qubit transfer process is helpful to improve the convergence to the target state; however, increasing control gain tends to destroy the entanglement and attenuate the multi-qubit transfer efficiency. The lack of sufficient quantum correlation between some initial state | and terminal state is the main reason for unavailable qubit transfer between them. For these states, the insertion of an intermediate entangled state | can effectively increase the degree of entanglement and help to realize the qubit transfer | via the transition processs | | .Keywords: Qubit quantum control, entangled state, index of quantum entanglement.