Long-distance matter-matter entanglement is pivotal for scalable quantum communication, distributed quantum computing, and sensing. In this Letter, we report entanglement between two atomic ensemble quantum memories over 420 km. We employ the Duan-Lukin-Cirac-Zoller (DLCZ) scheme for remote entanglement generation and convert photons emitted from the memories to telecom S band that enable us to harness ultralow transmission loss in fiber. We stabilize a photon’s relative phase between the two memories using full-time far-off-resonant locking to reduce high-frequency noise and intermittent dual-band locking to compensate low-frequency drift jointly. Furthermore, we demonstrate that the memory-memory entangling probability beats the repeaterless channel capacity for direct entanglement distribution. Our experiment provides a test bed of studying quantum network applications beyond metropolitan scale.

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• Quantum engineering
• Quantum memories
• Quantum networks
• Quantum optics
• Quantum repeaters