单光子纠缠态并发度的直接测量
Directly measuring the concurrence of the single-photon entangled states

通过分析光学分束器对单光子态的作用关系, 提出了一个利用分束器和光子数探测器的单光子纠缠的直接测量方案. 方案中用到单光子与空间模纠缠及其两个备份, 并让它们通过一个50:50的分束器. 选用并发度为纠缠度量，其可由单光子探测器的探测概率直接获得. 此方案不需复杂的量子态层析方法，同时只用到在量子信息处理中常用的光学器件, 增强了方案在实验上实现的可行性.
By the analysis of the interaction between the single-photon and a beam-splitter, we proposed a scheme for directly measuring the entanglement of the single-photon entangled states. In our scheme, two copies of the single photon entangled state are used, and here the single photon entangled with the vacuum state. After the two photons propagate into the input ports of a 50:50 beam-splitter, the concurrence of the single-photon entangled states can be measured directly by detection of photon number of the output ports of the beam-splitter. This protocol is direct measurement of the concurrence of the entangled states only with linear-optics elements, and discards the quantum state tomography method, which increases the feasibility of this protocol in the current experimental technology.