基于氢化非晶硅波导的芯片式简并关联光子对源

The degenerate correlated photon pairs is a vital resource in realizing quantum computing. In recent years, the basic component of quantum computing such as C-NOT logic gate which relays on the degenerate photon pairs has developed for miniaturization and large-scale integration. The on-chip source of degenerate correlated photon pairs is therefore become an active research focus. The traditional methods based on the parametric down-conversion (PDC) in nonlinear crystals and four wave mixing (FWM) in optical fibers are effective ways to generate the degenerate photon pairs. However, due to the poor compatibility and scalability, these sources are not suitable for the application in quantum photonic integration. The photon pairs source based on silicon on insulator (SOI) waveguide is considered as a promising candidate for large-scale integration system. In the demonstrated experiments, however, the crystalline silicon was used to generate the correlated photon pairs, which suffers from the two photon absorption and free carries absorption, resulting in low photon pair generation rates. Besides, pumped by the modulated continuous wave also requires high power intensity and affects the creation time of photons that is harmful for Hong-Ou-Mandel interference. In this project, the hydrogenated amorphous-silicon (a-Si:H) waveguide was used for generating the degenerate photon pairs through the process of FWM, which is benificial to eliminate effects of two photon absorption. In addition, the ultrashort femtosecond pulsed laser is also employed as the pump source to improve the peak power as well as determine the creation time of photon pairs. Meanwhile, the Sagnac loop is utilized to splitting the degenerate twin photons into different spatial modes. The aim of this project is developing an on-chip integrated quantum source based on the a-Si:H waveguide, which can produce the degenerate correlated photon pairs on telecommunication band with high brightness and purity.

简并关联光子对是实现量子计算的重要资源。近年来，量子计算的基本单元C-NOT门也向微型化、大规模集成化发展，因此研制芯片式简并关联光子对源成为当今的研究热点。基于晶体的参量下转换和基于光纤的四波混频是制备简并光子对的有效方法，但这两种光源的兼容性和扩展性较差，不适于在集成光学中的应用。基于硅基光波导的简并光子对源具有芯片化和可大规模集成的应用前景。目前硅基光波导制备简并光子对易受双光子吸收效应的影响，产生效率低；泵浦光源均采用连续光，功率要求高，光子对产生时间不确定，不利于在Hong-Ou-Mandel干涉中的应用。本项目利用氢化非晶硅光波导中的四波混频制备简并光子对，可降低双光子吸收的影响；采用超短脉冲作为泵浦，既提高泵浦的峰值功率又可确定光子对的产生时间；利用Sagnac环实现光子对的空间分离。项目的目标是研制一套基于氢化非晶硅波导产生通讯波段高质量简并光子对的可集成化量子光源。

近年来，随着大规模集成光量子技术的发展，简并光子对源不仅需要有良好的兼容性和扩展性，而且还需要易于集成到芯片上。氢化非晶硅光波导是一种可实现大规模集成光路的三阶非线性介质，具有尺寸小、非线性系数高、传输损耗低等优点，是既适合于集成化加工又能制备高质量频率简并光子对的理想材料。本项目中首先采用氢化非晶硅制作的纳米波导线作为产生四波混频的非线性介质，波导的非线性系数可达1320 /(w•m)，传输损耗为3.5 dB/cm，可增加光子对源的亮度和提高光子对的传输效率。实验中利用双端口Sagnac环作为产生和分离简并关联光子对的实验装置，Sagnac环由50/50分束器、1 cm非氢化硅基光波导和光纤偏振控制器组成，其中单模光纤与氢化非晶硅光波导之间的焊接损耗为3 dB。82 MHz的光纤锁模脉冲激光器的输出光经过双光栅滤波器分光色散，分别获取中心波长为分别为1539.37 nm和1552.12 nm的双泵浦脉冲光，获取中心波长为1545.72 nm的简并关联光子对。双脉冲光经过Sagnac环中的50/50分束器后，等功率的分为顺时针传输和逆时针传输的两组脉冲光，并分别产生顺时针传输和逆时针传输的简并光子对，对向传播光子对在50/50分束器处发生量子干涉，获取空间模式分离的简并光子对。实验中，通过调整入射泵浦光的偏振态以保证输出光子对为偏振纯态，从而实现对向传播光子对的偏振模式匹配与完全的量子干涉。经过单光子计数系统对于简并光子对的探测，空间模式分离的光子对的符合计数率与随机符合计数率的比值可达20:1， Hong-Ou-Mandel量子干涉的可见度为91±2%，达到在量子信息处理中使用的要求。本项目的研究成果不仅能为量子信息技术提供一种高质量的简并关联光子对源，同时也为量子信息技术应用向芯片化、大规模集成化方向发展提供技术支持。

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