Entanglement-based quantum key distribution (QKD) is an essential ingredient in quantum communication, owing to the property of source-independent security and the potential on constructing large-scale quantum communication networks. However, implementation of entanglement-based QKD over long-distance optical fiber links is still challenging, especially over deployed fibers. In this work, we report an experimental QKD using energy-time entangled photon pairs that transmit over optical fibers of 242 km (including a section of 19 km deployed fibers). The QKD is realized through the protocol of dispersive-optics QKD (DO-QKD) with high-dimensional encoding to utilize coincidence counts more efficiently. A reliable, high-accuracy time synchronization technology for long-distance entanglement-based QKD is developed based on the distribution of optical pulses in quantum channels. Our system operates continuously for more than 7 d without active polarization or phase calibration, showing the stability and robustness of the system. After error correction and privacy amplification, we ultimately generate secure keys in the asymptotic and finite-size regimes, respectively. It shows great potential for realizing entanglement-based QKD in large-scale optical fiber networks.

This work has published on Quantum Science and Technology (Quantum Sci. Technol. 9 (2024) 015003. https://doi.org/10.1088/2058-9565/acfe37) at 12^th Oct. 2023. The first author of this paper is Ph.D candidate Jingyuan Liu. The corresponding author of this paper is Prof. Wei Zhang. Quantum Science and Technology is a highly selective journal published by IOP Publishing. It is dedicated to bringing together the latest and most important results and perspectives from across the emerging field of quantum science and technology.