Long-distance distribution of atom-photon entanglement at telecom wavelength

TL;DR

This paper demonstrates the generation and long-distance distribution of atom-photon entanglement at telecom wavelengths over 20 km fiber, crucial for scalable quantum networks, using efficient quantum frequency conversion.

Contribution

It reports the first high-efficiency quantum frequency conversion of atom-photon entanglement to telecom wavelengths over long distances, with a fidelity exceeding 78%.

Findings

Achieved 57% external conversion efficiency.

Demonstrated entanglement fidelity ≥78.5% over 20 km fiber.

Showed potential for large-scale quantum communication.

Abstract

Entanglement between stationary quantum memories and photonic channels is the essential resource for future quantum networks. Together with entanglement distillation it will enable for efficient distribution of quantum states. Here we report on the generation and observation of entanglement between a Rb-87 atom and a photon at telecom wavelength over 20 km optical fiber. For this purpose, we use polarization-preserving quantum frequency conversion to transform the wavelength of a photon entangled with the atomic spin state from 780 nm to the telecom S-band at 1522 nm. We achieve an unprecedented external device conversion efficiency of 57% and observe an entanglement fidelity between the atom and telecom photon of $\geqslant$78.5$\pm$0.9% over 20 km optical fiber, mainly limited by decoherence of the atomic state. This result is an important milestone on the road to distribute quantum information on a large scale.