Quantum repeaters^1,2,3,4 are essential elements for demonstrating global-scale quantum communication. Over the past few decades, tremendous efforts have been dedicated to implementing a practical quantum repeater^5,6,7,8,9,10. However, nested purification¹, the backbone of a quantum repeater, remains a challenge because the capacity for successive entanglement manipulation is still absent. Here, we propose and demonstrate an architecture of nested purification using spontaneous parametric downconversion sources¹¹. A heralded entangled photon pair with higher fidelity is successfully purified from two copies of low-fidelity pairs that experience entanglement swapping and noisy channels. By delicately designing the optical circuits, double-pair emission noise is eliminated automatically and the purified state can be used for scalable entanglement connections to extend the communication distance. Combined with a quantum memory, our approach can be applied immediately in the implemention of a practical quantum repeater.

量子中继器^1,2,3,4是演示全球规模量子通信的必要元素。在过去的几十年中，致力于实现实用的量子中继器^{5,6,7,8,9,10付出}了巨大的努力。但是，嵌套纯化¹（量子重复器的主干）仍然是一个挑战，因为仍然缺乏连续纠缠操纵的能力。在这里，我们提出并演示了使用自发参数下变频源¹¹进行嵌套纯化的体系结构。从经历纠缠交换和嘈杂通道的两对低保真度副本中成功纯化出了一个具有较高保真度的先驱纠缠光子对。通过精心设计光路，可以自动消除双对发射噪声，并且纯化后的状态可用于可伸缩的纠缠连接，以延长通信距离。结合量子存储器，我们的方法可以立即应用于实际的量子中继器的实现中。