Long-Distance Entanglement Purification for Quantum Communication

TLDR

High‑quality long‑distance entanglement is essential for quantum communication and scalable networks, yet existing purification experiments require two low‑quality entangled pairs and are limited to tabletop setups. This work proposes a high‑efficiency, long‑distance entanglement purification protocol that uses only a single pair of hyperentangled states. The protocol distributes a single polarization–spatial‑mode hyperentangled pair over 11 km of multicore fiber, applies a deterministic CNOT gate, and achieves a purification efficiency 6.6 × 10³ times that of two‑pair SPDC‑based schemes. After purification, polarization‑entanglement fidelity rises from 0.771 to 0.887, the CHSH value from 1.829 to 2.128, and the entanglement‑based QKD key rate increases to 0.332, demonstrating the method’s suitability for quantum repeaters and large‑scale networks.

Abstract

High-quality long-distance entanglement is essential for both quantum communication and scalable quantum networks. Entanglement purification is to distill high-quality entanglement from low-quality entanglement in a noisy environment and it plays a key role in quantum repeaters. The previous significant entanglement purification experiments require two pairs of low-quality entangled states and were demonstrated in table-top. Here we propose and report a high-efficiency and long-distance entanglement purification using only one pair of hyperentangled states. We also demonstrate its practical application in entanglement-based quantum key distribution (QKD). One pair of polarization spatial-mode hyperentanglement was distributed over 11 km multicore fiber (noisy channel). After purification, the fidelity of polarization entanglement arises from 0.771 to 0.887 and the effective key rate in entanglement-based QKD increases from 0 to 0.332. The values of Clauser-Horne-Shimony-Holt (CHSH) inequality of polarization entanglement arises from 1.829 to 2.128. Moreover, by using one pair of hyperentanglement and deterministic controlled-NOT gate, the total purification efficiency can be estimated as 6.6x10^3 times than the experiment using two pairs of entangled states with spontaneous parametric down-conversion (SPDC) sources. Our results offer the potential to be implemented as part of a full quantum repeater and large scale quantum network.