Short answer: A trapped-ion network node holds quantum information in ions and links to remote nodes through photons entangled with the ions. The limiting factor is that entanglement over tens of kilometres is slowed by light travel time, so experiments try many attempts in parallel or in quick succession. Papers from 2020, 2024 and 2025 report ion-photon entanglement over fibre up to 101 km and multiplexing that raises the entangling rate. These are laboratory experiments with small nodes.
Why multiplexing
A 2024 PRX Quantum paper says the rate of establishing entanglement between systems tens of kilometres apart or more is strongly limited by the time light takes to travel between them. A proposed fix is to make many attempts in parallel or in close succession using many quantum systems at each site. A 2025 Nature Communications paper adds that in a functional network, the heralded remote entanglement rate must be higher than the decoherence rate of each local node.
A 2020 ion-ion link
A 2020 Physical Review Letters paper demonstrates remote entanglement of trapped-ion qubits through an optical fibre link with fidelity and rate approaching those of local operations. Two qubits are entangled through the polarization of two spontaneously emitted 422 nm photons, coupled into single-mode fibres by high-numerical-aperture lenses and interfered on a beam splitter. A novel geometry gives high-efficiency photon collection while keeping unit fidelity for ion-photon entanglement. The authors generate heralded Bell pairs with a fidelity of 94 percent.
A three-ion node over 101 km
The 2024 paper presents a three-qubit node of trapped atomic ions, three co-trapped ions at the focus of an optical cavity for efficient photon collection. It demonstrates entanglement between each qubit in the node and a separate photon that has travelled over 101 km of optical fibre. Sending those photons through the fibre in close succession gives a remote entanglement rate greater than with a single qubit in the node. The authors say that extended to more qubits, this multimode approach could boost entanglement-distribution rates in future long-distance networks.
A multiplexed node in 2025
The 2025 paper realises a multiplexed node based on a chain of 40 calcium ions. A hybrid multiplexing scheme generates up to 44 time-bin modes sent through a long fibre. It produces heralded ion-photon entanglement at 4.28 per second over 12 km of fibre. The authors test three fibre lengths, 3 m, 1 km and 12 km, and report entanglement rate improvements of 3-fold, 5-fold and 16-fold over single-mode operation. A dual-type framework protects stored quantum information from the entangling attempts, and a memory coherence time of 366 ms exceeds the entanglement generation time.
What this does not tell you
Each result uses a specific apparatus and distance, so the rates and fidelities do not transfer to other settings. These are single nodes or pairs, not operating networks.
FAQ
What limits entanglement over long distances?
Light travel time, which slows how quickly attempts can succeed.
How does multiplexing help?
It makes many attempts in parallel or close succession, raising the entangling rate.
How far have ion-photon links reached?
A 2024 paper reports photons travelling over 101 km of fibre.
What rate was reported in 2025?
4.28 per second over 12 km, with up to 44 time-bin modes.
Is there a working network?
These papers describe laboratory nodes, not a deployed network.
All field notes