Short answer: Rare-earth ions in solids can emit and absorb light at telecom wavelengths, but each ion is faint. Three papers report ways to see single ions: a 2018 Physical Review Letters paper with neodymium ions in a photonic crystal cavity, a 2020 Nature Communications paper with single-shot spin readout of erbium ions at 94.6% fidelity, and a publication listed on an exa.ai library page with erbium-doped nanoparticles in a fiber cavity. These are single-ion laboratory experiments.
Why single rare-earth ions
The Nature Communications abstract says optically interfaced spins in the solid state are a promising platform, and that erbium ions are attractive because of a telecom-wavelength optical transition and compatibility with silicon nanophotonic circuits. A difficulty is that erbium and other rare-earth ions generally lack strong cycling optical transitions, so the usual fluorescence readout does not work well.
2018: neodymium in a photonic crystal resonator
The Physical Review Letters abstract (Zhong and coauthors) reports optical probing of spectrally resolved single neodymium ions in yttrium orthovanadate. The ions couple to a photonic crystal resonator and show strong enhancement of emission through the Purcell effect, giving near radiatively limited single-photon emission. The measured high coupling cooperativity between a single photon and the ion allows observation of coherent optical Rabi oscillations. The authors say this could enable optically controlled spin qubits, gates and spin-photon interfaces, which is an outlook.
2020: reading out an erbium spin
The authors demonstrate single-shot spin readout of a single erbium ion. They show that modifying the electromagnetic environment around the ion can increase the strength and cyclicity of its optical transition by several orders of magnitude, enabling single-shot quantum nondemolition readout of the ion spin with 94.6% fidelity. They then use the readout to probe coherent dynamics and relaxation of the spin. The abstract does not name the host material.
Erbium-doped nanoparticles in a fiber cavity
An exa.ai library page reproduces the abstract of a publication on Purcell-enhanced detection of single solid-state ions in erbium-doped nanoparticles placed in a fiber cavity, emitting at 1536 nm. The page does not show the journal or year in the text read here, so the venue is unconfirmed and the claims should be treated with that caveat. The authors say the cavity design is fully tunable in space and frequency, letting them select individual particles and ions, and that the ions are confined in a volume two orders of magnitude smaller than in previous realizations, which raises the chance of finding ions a few nanometers apart that could interact. They report individual spectral features with saturation of emission count rate and linewidth, as expected for two-level systems, and an uncorrected second-order correlation of 0.24 (uncertainty 0.05), which they say confirms a single emitter.
Limits
The three experiments use different ions and hosts. Only the 2020 paper reports a spin measurement. Hosts, years and venues differ, and the nanoparticle result could not be cross-checked against its original journal page.
FAQ
Why are rare-earth ions hard to detect?
They lack strong cycling transitions, so emission is weak, per the 2020 abstract.
How was erbium read out?
Single-shot, with 94.6% fidelity, by modifying the electromagnetic environment.
What is the Purcell effect here?
An enhancement of the emission rate when an ion couples to a resonator.
What proves a single emitter?
The nanoparticle authors cite a second-order correlation of 0.24, uncorrected.
Is this a quantum network?
No. These are single-ion detection and readout experiments.
All field notes