利用时间复用实现效率增强的光纤单光子源

Single photon is an essential resource in quantum information. High quality single photon source should fulfill the following requirements: high efficiency, high purity, high brightness and high indistinguishability. Additionally, easy to use, miniature size and low cost are also important factors for evaluating the practical usage of the source. The heralded single photon source, which is based on the photon pairs generated form spontaneous parametric process, is an efficient and widely-used way to generate single photons. But there is a conflict between the efficiency and purity of heralded single photon sources, due to the random nature of the emerging of photon pairs. This conflict can be reduced by temporal multiplexing of single photons. However, the current temporal multiplexing single photon sources are all based on pulse pumped spontaneous process in bulk nonlinear crystals, and have the problems of large size, low efficiency and coupling between temporal and spatial modes. This research program plans to develop a fiber source of single photons with enhanced efficiency by using temporal multiplexing. The source employs photonic crystal fiber as the nonlinear medium, and utilizes fiber laser with high repetition rate and fiber optical switch with high speed and low loss. Moreover, the program will conduct research on the influences of various factors on the multiplexing efficiency, and the results will provide useful instructions for applying temporal multiplexing to other type of single photon source.

单光子是量子信息中重要的基础性资源。高质量的单光子源需满足高效率、高纯度、高亮度和高不可区分性的要求，而易于使用和维护、小型化、低成本也是衡量其应用价值的重要因素。宣布式单光子源基于非线性介质中自发参量过程所产生的关联光子对，是一种简便有效并得到广泛应用的单光子制备途径，然而光子对产生的随机性使得此类单光子源的效率和纯度间存在矛盾。对光子对产生事件进行时间复用是解决该矛盾的有效方法，但目前的时间复用宣布式单光子源均基于块状晶体中脉冲光泵浦的参量过程，存在体积大、复用效率低以及时间和空间模式相互耦合等问题。本项目拟利用光纤系统集成度高、空间模式纯净等优点，使用光子晶体光纤作为非线性介质，并通过制备高重复频率光纤激光器以及高速率低插损全光纤光开关，研制一种利用时间复用实现效率增强的光纤宣布式单光子源。项目还将对影响复用效率的各项因素进行研究，为时间复用在各类型单光子源中的应用提供参考。

宣布式单光子源基于非线性介质中自发参量过程所产生的关联光子对，是一种简便有效并得到广泛应用的单光子制备途径。围绕如何提高基于光纤的宣布式单光子源的效率，本项目完成了如下研究内容：一、通过对关联光子对频谱进行Schmidt分解等量化分析手段，研究了关联光子对时间模式和收集效率等参数对单光子源的输出效率和纯度的影响；在此基础上，项目提出了利用多级级联结构提高关联光子对模式纯度和收集效率的方法，为制备高效率纯态宣布式单光子源提供了一种新的解决方案。二、项目研究了基于1550 nm光纤光开关的单光子时间复用装置，完成了装置的初步搭建；复用装置中的光纤环采用色散位移光纤，光开关基于铌酸锂波导，整个复用装置单次通过的实际损耗约为0.7 dB，未来将在此基础上研究实现单光子的时间复用。三、除在时间维度对单光子进行复用外，项目还研究了波长连续可调谐宣布式单光子源以及高阶空间模式光子在少模光纤中的传输，为在波长维度和空间维度对单光子源进行复用打下了基础。