Long-distance distribution of atom-photon entanglement based on a cavity-free cold atomic ensemble

TL;DR

This paper demonstrates a cavity-free cold atomic ensemble as a quantum network node capable of long-distance atom-photon entanglement distribution using quantum frequency conversion, achieving high fidelity over 20 km fiber links.

Contribution

It introduces a cavity-free atomic ensemble node with high efficiency and long coherence time, combined with quantum frequency conversion for telecom wavelength transmission, advancing long-distance quantum networking.

Findings

Achieved 50% retrieval efficiency and 160 μs memory lifetime.

Demonstrated entanglement fidelity over 80% after 20 km fiber transmission.

Enabled entanglement distribution over 100 km fiber with high signal-to-noise ratio.

Abstract

Constructing a quantum memory node with the ability of long-distance atom-photon distribution is the essential task for future quantum networks, enabling distributed quantum computing, quantum cryptography and remote sensing. Here we report the demonstration of a quantum-network node with a simple cavity-free cold atomic ensemble. This node gives an initial retrieval efficiency of approximately 50\% and memory lifetime of 160 $\mu$s for atomic qubits. With the aid of a high-efficiency and polarization-independent quantum frequency conversion (QFC) module, the generated entangled photon in the node at 780-nm wavelength is converted to telecom S band at 1522 nm, enabling atom-photon distribution over long distance. We observe an entanglement fidelity between the atoms and telecom photon exceeding 80\% after photon transmission over 20-km fiber, the remaining infidelity being dominated by atomic decoherence. The low-noise QFC with an external efficiency up to 48.5\% gives a signal-to-noise-ratio of 6.9 for transmitted photons with fiber length up to 100 km, laying the cornerstone for entanglement distribution at a hundred-km level. This result provides a new platform towards the realization of a long-distance quantum network.