It is known that maximally entangled Bell state and three-qubit W-type states are very useful in various quantum information processing task. Thus the problem of preparation of these type of states is very important in quantum information theory. But the factor which prohibit the generation of the above mentioned pure states shared between two and three distant partners is decoherence. When we send one qubit, from a two qubit state, through decoherence channel like amplitude damping channel, the purity of the qubit is lost and it ends up with a mixed state. Therefore it is very difficult to keep the pure maximally entangled state in a maximally entangled pure state or in an entangled state with high entanglement. In this work we have provided a method by which one can generate experimentally a maximally entangled Bell states shared between distant parties with a non-zero probability when a qubit, from a two qubit general state, passes through decoherence channel. Therefore, despite of the fact that qubit is interacting with the noisy channel, we are able to generate Bell state shared between two distant partners. Further, we have shown that it is possible to generate pure three-qubit W-type states shared between three distant partners using economical quantum cloning machine and weak-measurement based feed-forward control scheme, even though the second and third qubit is interacting with the noisy channel. Lastly, we have shown that the generated three-qubit W-type states can be used in teleporting one of the two non-orthogonal states.
Optimality of feedback control strategies for qubit purification
最近两篇论文[K. Jacobs，Phys. Rev. A { \bf 67}，030301（R）（2003）; H. M. Wiseman和J.F. Ralph，New J. Physics { \bf 8}，90（2006）]已经导出了用于快速纯化量子位的控制策略，其针对各种目标进行了优化。在前一篇论文中，最优性的证明在数学上并不严格，而后者只给出了最优性的启发式参数。在本文中，我们提供了严格的最优性证明在所有情况下，应用简单的概念，从最优控制理论，包括贝尔曼方程和验证定理。
Near-Optimal Bell Nonlocality with GKP and Entangled Cat States
University of Technology Sydney（悉尼科技大学）；Centre for Quantum Software and Information, University of Technology Sydney（悉尼科技大学量子软件与信息研究中心）；Centre of Excellence for Quantum Computation and Communication Technology, School of Mathematics and Physics, University of Queensland（昆士兰大学数学与物理学院量子计算与通信技术卓越中心）；University of Queensland（昆士兰大学）
