基于PPKTP晶体的多光子纠缠源实验研究

As the fundamental aspect of quantum information, quantum entanglement is considered to be a key resource in testing quantum mechanics problems, such as quantum teleportation and quantum computation. Typically, multiphoton sources employ spontaneous parametric down-conversion (SPDC) process in nonlinear barium borate (BBO) crystals to generate entanglement. The reported two-photon coincidence rate is usually about one thousand pairs per mW of pump power. The traditional way to generate multiphoton source with BBO is limited to state-of-the-art eight-photon entanglement. In this project, we propose a new method to generate pulsed multiphoton source utilizing periodically poled potassium titanyl phosphate（PPKTP）crystal, and we seek for ways to enhance the brightness and quality of entanglement source by investigating the relevant physical theory. By now, we have some prior experimental result which indicate the possibility of our proposal. With the project sponsor, we further plan to use a 30mm long type-II PPKTP crystal in a high-stable Sagnac structure to generate a high-brightness photon source through the SPDC nonlinear process. Our main advantage is we decrease the demand for pump laser power, we increase the coherent length of generated photons to centimeter magnitude, which would further decrease the difficulty of time synchronization in long distant quantum communication. Thus, this project is meaningful and potential in practical applications.

作为量子信息理论的基础，量子纠缠是量子计算、隐形传态、量子力学基本问题检验的核心资源。在以往多光子纠缠实验中，大都采用基于BBO晶体参量下转换的纠缠源，每毫瓦泵浦产生双光子的符合数只有千对左右，随着光子数的增加，到八光子纠缠时基本已挖掘了所有可能的潜力。本项目将开发利用PPKTP晶体研制脉冲多光子源的可能，从产生机理上探寻大幅度提高纠缠光源的亮度和品质的方法，目前我们已经在实验室取得了一些成果，通过项目资助拟采用稳定的Sagnac结构和30mm长的II型PPKTP晶体自发参量转换来产生高亮度的光子对，降低对泵浦光功率的需求，进一步通过滤波将有望使光子的相干长度达到厘米级，大大降低了真正的远距离通讯中对同步的需求，具有实际而长远的意义。

本项目旨在利用PPKTP晶体的参量下转换来产生高亮度的脉冲双光子纠缠源，采用非经典干涉的办法将两套纠缠源进行干涉，旨在构造高亮度的四光子纠缠源。通过研究本项目给出了如何提⾼PPKTP晶体源的谱亮度的办法，通过理论分析得出最优的滤波参数和泵浦选型，并实验上搭建了两套PPKTP脉冲泵浦双光光源。进一步采⽤滤波的⼿段来得到光谱的⾼纯度，实验中仍得到了⾼的光谱亮度57000 pairs/mW/nm/s，并进行双光子干涉观察到Hong-Ou-Mandel dip验证了四光子纠缠源的可行性。该纠缠源的光谱亮度相较于之前的国际上的已报道结果有明显的进步。我们得到的⾼纯度的纠缠源可以直接应⽤在多光⼦⼲涉的实验中。受限于泵浦光源和滤波器件的效率，四体符合数仍然偏低为8 pairs/hour，通过改进未来有希望将会⼤幅提⾼。该基于PPKTP晶体的四光子纠缠源，具有泵浦光功率需求、高亮度的特点，使其在小型化和集成化方面具有优势，在实⽤化的量⼦通信中有重要的应⽤潜⼒。

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