基于光子晶体光纤的纯态单光子源及其在量子干涉中的应用

Single photons with pure quantum state are highly desirable in quantum information technologies relying on the quantum interference between photons from independent sources, such as quantum computing and quantum metrology. The heralded single photon source, which is based on the correlated photon pairs produced form spontaneous parametric processes in nonlinear media, is a convenient and effective way for single photon generation; moreover, single photons can be heralded in pure quantum state directly and efficiently when the parametric process is pumped by ultrafast pulses and certain phase matching condition is fulfilled. Among the various kinds of nonlinear media, the photonic crystal fiber is identified as one of the most suitable media for the generation of pure state single photons, because it has many unique merits, such as controllable dispersion, high nonlinearity, and excellent spatial mode. However, in the demonstrated experiments of generating pure state single photons by photonic crystal fiber, the purity of the sources are restricted by the structural inhomogeneity of the fiber, and all the sources employ Ti:sapphire lasers as the pump source, which are complicated and costly. This research program intends to eliminate the influence of fiber inhomogeneity and improve the purity of the photonic crystal fiber based sources of single photons, by measuring, analyzing, and re-arranging the segments of the inhomogeneous fibers. The program also intends to promote the miniaturization of the single photon source by applying a home-made compact Yb-doped laser as the pump source. The aims of this program are developing a high-performance, compact source of heralded single photons with pure quantum state, and further investigating the applications of the source in quantum interference.

纯态单光子在量子计算、量子测量等以独立光源间光量子干涉为基础的量子信息技术中具有重要的应用。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，是一种简便有效的单光子制备途径；而且，若自发参量过程由脉冲光泵浦且满足特定的相位匹配条件时，可直接高效率地产生宣布式纯态单光子。在诸多非线性介质中，光子晶体光纤具有色散可控、非线性高、空间传输模式好等优点，是制备纯态单光子源的理想介质之一。然而，在目前已有的利用光子晶体光纤产生纯态单光子的实验中，单光子源的态纯度受限于光纤的结构不均匀性，且其泵浦源均采用较为复杂和昂贵的钛宝石激光器。本项目拟通过对光子晶体光纤进行测量、分析和优化重组，消除其不均匀性的影响，以提高单光子源的纯度；并且拟采用自制掺镱光子晶体光纤飞秒激光器作为泵浦源，以促进单光子源的小型化。项目的目标是研制出高质量、小型化的宣布式纯态单光子源，并进一步研究其在量子干涉中的应用。

纯态单光子在量子计算、量子测量等以独立光源间光量子干涉为基础的量子信息技术中具有重要的应用。本项目以研制基于光子晶体光纤的纯态宣布式单光子源为主要目标，取得了以下成果：. 1）制备了一种基于光子晶体光纤的具有高纯度、高效率特征的宣布式超快单光子源。光源输出单光子的波长位于1.4 微米附近，宣布效率超过86%；当光子产生率为每泵浦脉冲0.002个时，输出单光子场的二阶相关函数为0.012；光场模式数为1.27，对应的独立光源间HOM 光子干涉的可见度为80%。2）首次演示了基于微纳光纤（可视为空气比为1的光子晶体光纤）的量子关联光子对源，光源输出光子对中的信号光子和闲频光子分别位于1.31微米和0.85微米波段。该研究显示了微纳光纤在微型化量子光源制备中重要的应用前景。3）研究了将超快纯态单光子源应用于HOM量子干涉时，提高干涉可见度的途径和方法；分析了脉冲泵浦光啁啾、光子传输介质色散等因素对HOM干涉可见度的影响；通过建立脉冲泵浦参量过程的多模理论模型，分析了泵浦光时间模式结构对量子光源输出的影响。4）完成了自制泵浦激光器与关联光子对产生装置的初步整合，为进一步制备小型化、全光纤化的宣布式纯态单光子源打下基础；所设计的基于光子晶体光纤的频谱可控和波长可调的关联光子对产生技术和装置已获得国家发明专利授权。. 本项目的研究成果不仅为量子信息技术提供高质量、小型化的纯态单光子源，也为超快纯态单光子应用于HOM 干涉时的参数优化提供了重要的参考依据，而且为光源的进一步小型化打下了基础。