Most quantum hardware coverage tracks qubit counts. Quantum networking has a different bottleneck: a memory that can store the quantum state of light and return it on demand. One family of candidates is rare-earth ions (erbium, ytterbium, europium and others) embedded in crystals, where they behave like nearly isolated atoms controllable with light.
This article goes through the experimental record, ion by ion. Each section links its sources and marks evidence strength: Journal (peer-reviewed) or Preprint (arXiv, not yet formally reviewed).
1. The wavelength constraint
A quantum memory for a network has to talk to optical fiber. Telecom fiber runs at ~1530 nm. Among the ions studied for quantum memories, only erbium has an optical transition in that window. Europium, praseodymium, thulium and ytterbium operate at other wavelengths and require frequency conversion to interface with fiber, which adds loss and complexity.
Sources: Review "Quantum networks using rare-earth ions" (Quantum Sci. Technol. 2025, arXiv version) (Journal, literature review).
2. Erbium: single-ion entanglement at telecom wavelength
In 2025, a Princeton group entangled the spin of a single erbium ion with a photon emitted at 1532.6 nm and estimated an entanglement fidelity of 73% after 15.6 km of fiber.
Sources: PRX 15, 011071 (2025) (Journal) and the 2025 review.
3. Telecom-band storage results
Two recent erbium results define the current storage frontier:
- In an erbium-doped lithium niobate chip, an Illinois group stored single-photon-level light pulses for more than 1 microsecond, across up to 20 temporal modes, with an acceptance bandwidth up to 2.2 GHz.
- A June 2026 preprint transferred telecom optical excitations to the hyperfine spins of 167Er in Y2SiO5 and retrieved them after up to 25 microseconds, with transfer efficiency above 12%, at 0.8 K and 1.1 T. The authors also measure a hyperfine population lifetime of 25 seconds, but that is not the demonstrated storage time. They identify inhomogeneous broadening as the current limit.
Sources: Nano Letters 26, 12056 (2026) (Journal); arXiv 2606.15009 (Preprint).
On duration: the 2025 review reports spin coherence of up to hours in europium (Eu:Y2SiO5), a different ion outside the telecom band. Spin coherence is not the same as an efficient memory for photons. Among the telecom erbium results verifiable here, storage times sit in the microsecond range.
4. Neodymium: real work, older and off-band
Neodymium is the element in the NdFeB magnets of old hard disk drives (used in the voice-coil motor that moves the read head). In a quantum memory it is not a magnet: it is a dilute set of Nd3+ ions in a transparent crystal. A nanophotonic memory built from a neodymium ensemble coupled to a cavity was demonstrated in 2017 (Science). Its transition in Nd:YVO4 sits at 879 nm, outside the telecom band, so fiber interfacing would need frequency conversion. The 2025 review does not list it among the ions of greatest current interest, and this pass found no recent neodymium results. It stays on the watch list.
Sources: Science (2017), PRB 77, 125111 (2008), Stanford Magnets on HDD voice-coil motors, 2025 review.
5. Ytterbium and europium: different strengths
- Geneva (preprint, August 2026): an ytterbium memory in Y2SiO5 stored 8235 modes for 63 microseconds over 5.66 km of urban fiber.
- Another preprint entangled a trapped ytterbium ion with a europium memory 75 m away at 89% fidelity, using frequency conversion.
Sources: arXiv 2608.13177 and arXiv 2603.05836 (Preprints); 2025 review.
6. Side-by-side comparison
| Ion | Wavelength | Telecom? | Results reviewed |
|---|---|---|---|
| Erbium (Er) | ~1530 nm | Yes | Single-ion spin-photon entanglement (73% after 15.6 km); on-chip storage > 1 µs; 167Er spin retrieval at 25 µs, 0.8 K |
| Ytterbium (Yb) | ~980 nm | No | 8235 modes stored for 63 µs over 5.66 km urban fiber (preprint) |
| Europium (Eu) | ~580 nm | No | Spin coherence up to hours in Eu:Y2SiO5 (review); 75 m entanglement with trapped Yb ion, 89% fidelity (preprint) |
| Neodymium (Nd) | ~879 nm | No | Nanophotonic cavity memory (2017); no recent results found |
| Praseodymium, thulium | ~606 / ~794 nm | No | Listed in the 2025 review; not covered in depth |
7. What can and cannot be concluded
Erbium combines a telecom wavelength with on-chip experimental results. The other ions show different strengths, such as very long spin coherence in europium. No ion can be called better in general from these results: they use different setups, temperatures and metrics, and are not comparable number to number.
8. What to watch
- Telecom storage in erbium moving from microseconds to longer times, with measured efficiency and fidelity.
- Entanglement between memories at a distance, with reported rates and operating conditions.
- Less-covered ions (neodymium, praseodymium, thulium) and new materials.
Full sources linked per section above. The complete version of this piece, with per-paragraph source tagging, is on my blog (canonical link in the post metadata).
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