Transfer from Arbitrary Pure State to Target Mixed State for Quantum Systems

Considering two-level quantum systems, this paper proposes a control strategy that drives an arbitrary pure state to target mixed state via three-step controls. The first step is the preparation of eigenstate: the state of system controlled is driven to eigenstate from an arbitrary pure state. The second one is to drive the eigenstate prepared to the mixed state in which not all of the off-diagonal elements in the density matrices are zeros. In this step, one needs to use an auxiliary system and achieves the transfer through the interaction between controlled system and auxiliary system. If the target state is the mixed state in which all of the off-diagonal elements in the density matrices are zeros, then one needs to perform the third step: to drive the mixed state obtained in the second step to the target mixed state. Numerical simulation experiments and results analysis are given. The control strategy proposed in this paper is useful in the application of quantum computation.