One of the most fascinating aspects of quantum networks is their capability to distribute entanglement as a nonlocal communication resource¹. In a first step, this requires network-ready devices that can generate and store entangled states². Another crucial step, however, is to develop measurement techniques that allow for entanglement detection. Demonstrations for different platforms^{3,4,5,6,7,8,9,10,11,12,13} suffer from being not complete, destructive or local. Here, we demonstrate a complete and nondestructive measurement scheme^14,15,16 that always projects any initial state of two spatially separated network nodes onto a maximally entangled state. Each node consists of an atom trapped inside an optical resonator from which two photons are successively reflected. Polarization measurements on the photons discriminate between the four maximally entangled states. Remarkably, such states are not destroyed by our measurement. In the future, our technique might serve to probe the decay of entanglement and to stabilize it against dephasing via repeated measurements^17,18.

量子网络最令人着迷的方面之一是它们能够将纠缠作为非本地通信资源¹进行分发。第一步，这需要能够生成和存储纠缠态²的网络就绪设备。然而，另一个关键步骤是开发允许纠缠检测的测量技术。^{针对不同平台3、4、5、6、7、8、9、10、11、12、13}的演示存在不完整、破坏性或局部性的问题。在这里，我们展示了一个完整的无损测量方案^14,15,16总是将两个空间分离的网络节点的任何初始状态投影到最大纠缠状态。每个节点由一个被困在光学谐振器内的原子组成，两个光子从该光学谐振器连续反射。光子的偏振测量区分四种最大纠缠态。值得注意的是，我们的测量并未破坏这些状态。将来，我们的技术可能会用于探测纠缠的衰减，并通过重复测量^17,18来稳定它以防止失相。