Hybrid satellite-fiber quantum network

Quantum networks hold promise for key distribution, private and distributed computing, and quantum sensing, among other applications. The scale of such networks for ground users is currently limited by one’s ability to distribute entanglement between distant locations. This can in principle be carried out by transmitting entangled photons through optical fibers or satellites. The former is limited by fiber-optic attenuation while the latter is limited by atmospheric extinction and diffraction. Here, we propose a hybrid network and protocol that outperform both ground- and satellite-based designs and lead to high-fidelity entanglement at a continental or even global scale.

💡 Research Summary

The paper proposes a hybrid quantum networking architecture that combines terrestrial optical‑fiber links with a single medium‑Earth‑orbit (MEO) satellite to achieve high‑rate, high‑fidelity entanglement distribution over continental and even global distances. The authors first review the limitations of pure fiber‑based quantum repeaters (exponential photon loss with distance, requiring many repeaters) and pure satellite‑based schemes (diffraction loss, atmospheric extinction, limited ground‑track visibility for low‑Earth‑orbit satellites, and severe loss for geostationary satellites). They argue that a MEO satellite at ~10 000 km altitude offers a compromise: longer visibility windows than LEO, and much lower diffraction loss than GEO.

In the fiber segment, the authors adopt a chain of trapped‑ion quantum repeaters interleaved with photon‑repeater nodes. Each trapped‑ion repeater stores qubits in two ions, emits ion‑photon entangled pairs, and performs deterministic Bell‑state measurements (DBSM) for entanglement swapping. Photon repeaters perform probabilistic Bell‑state analysis on incoming photons. An analytical expression for the average entanglement‑generation time (T_f(L,n_p)) (Eq. 1) incorporates the fiber length (L), number of photon repeaters (n_p), photon‑detector efficiency (P), and fiber transmission (\eta(L)=10^{-\gamma L/(2n_p)}) with attenuation (\gamma=0.0173\ \text{km}^{-1}). By minimizing (T_f) with respect to (n_p), the authors find that for distances up to a few thousand kilometres the optimal number of repeaters grows roughly linearly (e.g., 7 repeaters for 2 000 km). The corresponding entanglement‑distribution rate (R_f=1/T_f) exceeds the rate without repeaters by up to two orders of magnitude at long distances.

Fidelity analysis starts from an initial ion‑photon mixed state with fidelity (F_0). After each photon‑photon Bell measurement the ion‑ion state fidelity becomes (F_{ii}=