标题： 从顺序量子测量和反馈中产生的保纠缠极限环 显示英文标题

标题： Entanglement-Preserving Limit Cycles from Sequential Quantum Measurements and Feedback

摘要： 纠缠生成和保持是量子信息处理中的关键任务，存在各种协议通过测量自发辐射来纠缠远程的量子比特。我们这里提出基于监测两个量子比特的荧光并仅使用本地π脉冲进行控制的反馈方法，以提高纠缠的两量子比特态的产量和/或寿命。具体而言，我们描述了一种基于自发辐射的光检测（即使用量子跳跃轨迹）的协议，该协议可以通过测量逆转实现纠缠保持，在贝尔态周围形成极限环。然后我们证明，可以对一种基于零差测量（即使用扩散量子轨迹）的近期反馈方案[ L. S. Martin 和 K. B. Whaley, arXiv:1912.00067 ]进行类似修改，以增加其创建的纠缠寿命。我们的方案在高测量效率时最为有效，且量化了测量效率不理想的影响。我们在此描述的方法以新颖的方式结合了经过验证的技术，补充了现有协议，并为生成和保护纠缠态提供了一条路径，以便按需用于各种应用。 显示英文摘要

摘要： Entanglement generation and preservation is a key task in quantum information processing, and a variety of protocols exist to entangle remote qubits via measurement of their spontaneous emission. We here propose feedback methods, based on monitoring the fluorescence of two qubits and using only local pi-pulses for control, to increase the yield and/or lifetime of entangled two-qubit states. Specifically, we describe a protocol based on photodetection of spontaneous emission (i.e. using quantum jump trajectories) which allows for entanglement preservation via measurement undoing, creating a limit cycle around a Bell states. We then demonstrate that a similar modification can be made to a recent feedback scheme based on homodyne measurement (i.e. using diffusive quantum trajectories), [L. S. Martin and K. B. Whaley, arXiv:1912.00067] in order to increase the lifetime of the entanglement it creates. Our schemes are most effective for high measurement efficiencies, and the impact of less-than-ideal measurement efficiency is quantified. The method we describe here combines proven techniques in a novel way, complementing existing protocols, and offering a pathway towards generating and protecting entangled states so that they may be used in various applications on demand.