锶原子四波混频产生量子光源的实验研究

Quantum light, including single photon source, entangled photon pairs and multi-entengled photon soucre, is always the hot topic in the areas of quantum optics, quantum information, quantum computing, quantum communication and quantum precision measurement. In recent years, generation of quantum light in the way of cold atoms and four-wave mixing draws much attention due to the merit of long coherence time, narrow linewidth, time-resolved waveform and and modulation ability. However, such a method is only successfully demonstrated in the four-level system and three-level system, and the two-level system is hard to observe quantum feature of light caused by the strong background noise, up to now. We propose a route of four-wave mixng based on the two-level system from the singlet states of Strontium. Making use of the large natural linewidth of the exicted state and the technology of narrow bandwidth optical etalon, we are able to decrease the background noise and obtain high SNR, high generation quantum light. In addition, we can obtain the hyper-entangled photon pairs in time-frequency, polarization domain from the symmetry of geometry. Because the non-existence of hyper-structure of strontium atoms, strontium atoms own a pure two-level system. We also plan to study the on-resonance flsorescence and nonlinear gain coefficient as a function of frequncy in details, which will help us to verify and extend the current theoretical model.

量子光源，包括单光子源，纠缠光子对，多光子纠缠源等一直是量子光学，量子信息，量子计算，量子通信和量子精密测量等领域的研究热点。近年来，基于冷原子系统四波混频产生量子光源的方案以其相干时间长，窄线宽，允许时间波形分辨和调制等优点备受关注。然而，目前冷原子系统四波混频方案仅在四能级结构和三能级结构中成功实现，二能级结构一直受到背景噪声的困扰，难以观测产生光场的量子特性。本项目提出基于双电子锶原子单重态的二能级结构四波混频方案，利用激发态自然线宽大的特点，并结合窄带光学标准具技术，有望滤除背景噪声，获得高信噪比，高计数率的量子光源。进一步的，利用二能级结构激发或收集结构的对称性，产生时间频率，偏振的超纠缠光子对。由于锶原子存在没有超精细结构的纯净二能级系统，我们也计划细致研究其共振荧光和非线性增益频率函数，进一步检验和拓展现有的理论模型。

锶原子是碱土金属元素，原子序数38。锶原子拥有独特的宽线宽的单重态和窄线宽的三重态，在超冷原子物理和量子精密测量等领域具有重要地位和广泛的应用价值。我们开展了超冷锶原子的实验制备研究工作，探索了激光冷却和囚禁锶原子的新型技术方案，获得了一系列成果。这些成果包括首次实现小型化的锶原子二维磁光阱用于替代常规的塞曼减速器; 报道了高精度波长计实现激光器频率小于1MHz的稳定性; 采用新颖的单频半导体激光用于锶原子基态再泵浦; 实现了高调制频率（240 MHz）下的超稳腔压窄激光线宽技术，并以此为基础获得了温度小于uK的超冷锶原子。以上成果对实现小型化，结构简单的超冷锶原子平台有重要意义。在双光子二能级激发方面，我们观察到了原子荧光服从经典的偶极辐射分布规律。此外，我们精密测定了锶原子里德堡三重态的能谱，获得锶原子第一电离能的准确数值，测量结果误差小于10MHz，为基于锶原子的量子精密测量，量子模拟和量子计算的物理实现奠定了重要基础。