Short answer: Erbium ions emit light in the telecom band used by optical fibres, which makes them candidates for spin qubits that link to photons. A 2024 Nature Communications paper reports optical single-shot readout of a spin in silicon using erbium in a nanophotonic resonator, a 2025 npj Quantum Information paper reports Hahn echo decay constants of 0.8 ms and 1.2 ms in nuclear-spin-free silicon, and a 2025 Nature Communications paper reports spin coherence above 10 ms in an epitaxial thin film. These are laboratory results.
Why erbium
The 2025 Nature Communications abstract says erbium dopants emerge as an attractive candidate for quantum interconnects because they emit in the telecom C band and have shielded 4f intra-shell spin and optical transitions. The 2025 npj abstract adds that spins in silicon accessible through a telecom-compatible optical transition could use the established silicon nanofabrication industry, and that long coherence times on the optical and spin transitions are key to interfacing photonic and spin qubits.
2024: a spin-photon interface in silicon
The 2024 abstract notes that small registers of spin qubits in silicon can show hour-long coherence times and exceed error-correction thresholds, but that connecting them to larger processors is an outstanding challenge. Spin qubits with optical interfaces could minimize heat load and offer modular architectures with large connectivity. The authors implement an efficient spin-photon interface based on erbium dopants in a nanophotonic resonator. They demonstrate optical single-shot readout of a spin in silicon whose coherence exceeds the Purcell-enhanced optical lifetime, which they say paves the way for entangling remote spins through photon interference. They add that tens of such qubits could be spectrally multiplexed in each resonator, which is their projection and was not shown.
2025: long coherence in pure silicon
The npj paper reports telecom-compatible erbium sites with long optical and electron spin coherence in a nuclear-spin-free silicon crystal, with less than 0.01% silicon-29, measured by optical detection. For the two sites studied, the authors find optical inhomogeneous linewidths of 0.1 GHz and homogeneous linewidths below 70 kHz. Using optically detected magnetic resonance, they observe Hahn echo decay constants of 0.8 ms and 1.2 ms at about 11 mT, and call these properties an important milestone.
2025: a thin-film platform
The second 2025 Nature Communications paper says top-down architectures for rare-earth qubits had not yet achieved simultaneously long optical and spin coherence, which long-distance networks need. The authors demonstrate dual erbium telecom spin-photon interfaces in two distinct lattice symmetry sites within an epitaxial thin-film platform. They report a kilohertz-level optical linewidth in a strongly symmetry-protected site and erbium spin coherence times exceeding 10 milliseconds. They also report single-shot readout and microwave coherent control of erbium qubits in a fiber-integrated package. The abstract does not name the host material for this film, so it should not be assumed to be silicon.
Limits
The three papers use different hosts and measure different quantities, so the coherence numbers are not a single progression. None of the abstracts reports entanglement between two remote spins, which the 2024 paper names as a future step. Claims about scalability and rapid deployment are the authors' own outlook.
FAQ
Why is the telecom band important?
Photons there suffer minimal loss in optical fibres, per the 2024 abstract.
What was read out in 2024?
A single erbium spin in silicon, in one shot, using a nanophotonic resonator.
How long did the spin stay coherent?
Hahn echo decays of 0.8 ms and 1.2 ms in silicon in 2025, and above 10 ms in a thin film.
Has remote entanglement been shown?
Not in these abstracts. The 2024 paper says it paves the way.
Is this a quantum network?
No. These are single-device spin-photon interface demonstrations.
All field notes