Future quantum networks will require scalable, secure, and fully interconnected architectures that integrate diverse transmission links, such as satellite-based communication, terrestrial free-space channels, and long-distance fiber-optic links. The construction of large-scale quantum networks relies heavily on quantum repeaters, with quantum entanglement swapping (QES) serving as a core technology. However, existing approaches to QES remain insufficient for building such networks.

Here, we propose a QES scheme based on single-photon frequency conversion (SPFC). By converting photons of different frequencies to the same frequency via SPFC before performing linear-optic QES, this method eliminates the requirement for frequency-matched photons while experimentally validating its feasibility in network construction. The scheme holds promise for cross-network quantum resource sharing, dynamic channel switching, and efficient multispectral interconnections, offering a scalable solution for large-scale metropolitan quantum networks.

Figure 1. The concept of the BSM and entanglement-swappingbased FCI and DWDM network.

As illustrated in Figure 1(c), our QES design combines the advantages of existing approaches while addressing their limitations. By first applying frequency conversion to photons requiring entanglement swapping and then performing linear Bell-state measurements (BSM), we achieve efficient QES across photons of distinct frequencies.

In Figure 2, we experimentally validate the feasibility of this scheme in a quantum network. The photon frequency conversion efficiency reached 64.5%, with average visibilities of entangled photon pairs before and after SPFC measured at 95.49%±1.27% and 83.6%±1.37%, respectively, both exceeding the thresholds required for quantum communications.

Figure 2. Experiment setup

This work is published in “Zhantong Qi, Yilin Yang, Chennan Wu, Zixuan Liao, Bo Tang, Jiani Lei, Yuanhua Li, Jia Lin, Yuanlin Zheng, and Xianfeng Chen, Multiuser quantum communication network via time-bin-entanglement-based frequency conversion and Bell-state measurement, Phys. Rev. A 111, 052609 (2025).”

Link: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.111.052609