可用于构建空海一体化量子网络的频率转换器件

未来，全球化的量子通信网络除了地面光纤和自由空间，还会涉及覆盖了地球面积71%的海洋。将自由空间量子通信和水下量子通信相结合，可以建立卫星或者飞行器与潜艇之间的保密通信，是实现海陆空一体化量子网络的关键一环。但自由空间通信常用的810nm在海水中的消光系数比水下低损耗的532nm大两个数量级，难以在海水中远距离传输。因此本申请书提出使用1550nm作为泵浦光，基于反质子交换PPLN波导，利用非线性和频过程实现两者的波长匹配。一方面，该过程的准相位匹配周期小，约6µm，超出了液体电极可以实现的范围，需要研究使用液体电极快速简易大面积实现铌酸锂小周期极化的新方案；另一方面，为了提高整体转换效率，需要研究反质子交换波导的功能性单元以实现高效的波导耦合与传输；进而构建从810nm到532nm的频率转换器件，以连接自由空间和水下量子通信过程，为空海一体化量子网络的构建，扫清工作波长不一致的障碍。

In the future, apart from optical fiber and free space, the channels of global quantum communication will involve seawater, which covers 71% of the earth after all. The combination of free-space quantum communication and underwater quantum communication can establish the secure communication between satellite or aircraft and submarine, which is the key to realize the integration of sea, land and air quantum networks. However, there is a clear bottleneck of frequency incompatibility. Specifically, the popular wavelength for free-space communication is 810nm, whereas the optimal spectral window for underwater communication is around 532nm. The photons at 810nm cannot penetrate deep in the seawater, wherein the extinction coefficient of 810nm is two orders of magnitude larger than that of 532nm. Therefore, in this proposal, based on the reverse-proton-exchanged (RPE) periodically poled lithium niobate (PPLN) waveguides, we suggest to use the nonlinear sum frequency generation (SFG) to overcome this difficulty. On the one hand, the required quasi-phase-matching period for the SFG process of 1550nm pump and 810nm signal is about 6µm, which is so small that a new poling scheme through liquid electrode is needed. On the other hand, the functional waveguide elements also need to be developed further to minimize the total loss so that improve the conversion efficiency. Finally, an integrated RPE PPLN waveguides chip will be designed and fabricated, achieving the wavelength shifting on the single-photon level from 850nm to 532nm, serving as a quantum interface connecting free-space communication and underwater communication, paving the way towards building the sea-air quantum networks in the near future.