利用少模光纤产生空间纠缠光子对的理论和实验研究

利用光子的多个模式和维度携带信息是增加信息容量和提升信息系统性能的重要手段。由于高维空间模式纠缠态在拓展量子态维度方面具有不可比拟的优点，近年来备受瞩目。目前产生空间模式纠缠光子对的一个主流方法是利用二阶非线性晶体中的光学参量过程，但其模式与低损耗传输光纤很难完全匹配，在光纤中分发时会不可避免地引入比较大的耦合损耗。本项目拟利用少模光纤三阶非线性效应，通过其中的模间四波混频产生通信波段的空间纠缠光子对，并将纠缠的信号光子和闲置光子将分别沿10公里少模光纤传输，演示空间模式纠缠光子对的远距离分发。由于纠缠光子对的空间模式与光纤一致，因此，可以和光纤通信空分复用系统的传输介质实现高效率的耦合。本项目的研究内容和研究结果，将揭示少模光纤的非线性，作为一种新型物理平台，在制备多模纠缠和实现多模量子信息处理的方面，有什么样的特点和前景。

The capacity and performance of communication system can be increased and improved by encoding information on the multiple degree of freedom of photons. Recently, the higher-dimensional quantum systems realized by exploiting the spatial modes of photon states have attracted a lot of attentions. So far, the optical parametric processes in the second order nonlinear crystals are the workhorse of generating spatially- or orbital-angular-momentum-entangled photons, which are the key resource for implementing various kinds of high dimension quantum information processing tasks. However, when the entangled photons are coupled into fibers for long-distance distribution, there exists significant coupling loss since there is mode mismatch between the modes of entangled photons generated in crystal platforms and those of fibers in which higher-order spatial modes are supported. If the spatially entangled photon pairs can be generated from the few mode fibers itself, the mode mismatch between the entangled sources and transmission fibers can then be dispelled. Therefore, we propose to generate spatially entangled photon pairs in 1550 nm telecom band by using the inter-modal four wave mixing in few mode fibers. Moreover, using low-loss few mode fibers as the transmission media, we will experimentally demonstrate the distribution of spatially entangled photons pairs over 10 km fibers. Our investigation will show that the nonlinearity of few mode fibers is a possible alternative to crystal platform for studying the higher-dimensional quantum systems.