Absorption-emission quantum repeater using diamond quantum memories
Quantum repeaters are essential for overcoming the exponential photon loss that limits entanglement generation over long distances in quantum networks. An absorption-emission-based quantum repeater exploits the fundamental light-matter interactions of a diamond nitrogen-vacancy (NV) center---photon absorption and photon emission---to transfer a quantum state from an absorbed photon to an emitted photon, offering a scalable architecture that operates without photon interference between remote nodes. Here we demonstrate an absorption-emission-based quantum repeater node using a single NV center, realizing the complete single-node operation in which heralded photon-to-memory quantum state transfer, repeat-until-success (RUS) emission of a spin-entangled photon, and quantum teleportation of the memory state onto the emitted photon constitute the essential repeater operation. By characterizing the complete repeater operation as a quantum channel from the absorbed photon to the emitted photon via quantum process tomography, we obtain a process fidelity of 78%. This demonstration establishes the absorption-emission approach as a fundamental building block for scalable quantum repeater architectures and paves the way toward practical long-distance quantum networks.