Abstract
Doping erbium ions into europium-based hosts offers a promising platform for quantum repeater nodes, combining europium’s exceptional coherence properties for long-term quantum storage with erbium’s microwave compatibility and direct telecom-band emission for efficient optical interfacing. In this work, we investigated erbium-doped EuCl3 ⋅ 6H2O stoichiometric crystals as a candidate for such nodes. We demonstrate that erbium doping shifts the optical transition frequencies of nearby Eu3+ ions, producing well resolved satellite lines in the inhomogeneous absorption profile. We experimentally probe the coupling between Er3+ and Eu3+ ions under varying temperature and magnetic field conditions, quantifying the interaction strength, which ranges from tens to hundreds of kilohertz, depending on field orientation, magnitude, and the lattice position of the Eu3+ ions. At 60 mK and a moderate magnetic field of 0.1 T, we observed a strong frozen core effect from Er3+ spins, substantially extending the Eu3+ optical coherence time from 62 μs to 162 μs, approaching the lifetime limit, and enabling hour-long hyperfine state lifetimes. These results underscore the potential of dual-species rare-earth systems for photonic quantum technologies and highlight their promise for precise quantum control.
Data availability
The data for the current study are available via Zenodo (https://zenodo.org/records/18229971). No other publicly available datasets or sequences were used in this study.
Code availability
All data and figures are original and generated by the authors using appropriate licensed software. Scripts and routines used to produce these figures are available from the corresponding author upon reasonable request.
References
Azuma, K. et al. Quantum repeaters: From quantum networks to the quantum internet. Rev. Mod. Phys. 95, 045006 (2023).
Heshami, K. et al. Quantum memories: emerging applications and recent advances. J. Mod. Opt. 63, 2005–2028 (2016).
Sangouard, N., Simon, C., de Riedmatten, H. & Gisin, N. Quantum repeaters based on atomic ensembles and linear optics. Rev. Mod. Phys. 83, 33–80 (2011).
Guo, M. et al. Rare-earth quantum memories: The experimental status quo. Front. Phys. 18, 21303 (2023).
Thiel, C., Böttger, T. & Cone, R. Rare-earth-doped materials for applications in quantum information storage and signal processing. J. Lumin. 131, 353–361 (2011).
Zhong, M. et al. Optically addressable nuclear spins in a solid with a six-hour coherence time. Nature 517, 177–180 (2015).
Ma, Y., Ma, Y.-Z., Zhou, Z.-Q., Li, C.-F. & Guo, G.-C. One-hour coherent optical storage in an atomic frequency comb memory. Nat. Commun. 12, 2381 (2021).
Ortu, A., Holzäpfel, A., Etesse, J. & Afzelius, M. Storage of photonic time-bin qubits for up to 20 ms in a rare-earth doped crystal. npj Quantum Inf. 8, 29 (2022).
Taminiau, T. H. et al. Detection and control of individual nuclear spins using a weakly coupled electron spin. Phys. Rev. Lett. 109, 137602 (2012).
Zhou, Z.-Q. et al. Photonic integrated quantum memory in rare-earth doped solids. Laser Photonics Rev. 17, 2300257 (2023).
Liu, D.-C. et al. On-demand storage of photonic qubits at telecom wavelengths. Phys. Rev. Lett. 129, 210501 (2022).
Saglamyurek, E. et al. A multiplexed light-matter interface for fibre-based quantum networks. Nat. Commun. 7, 11202 (2016).
Saglamyurek, E. et al. Quantum storage of entangled telecom-wavelength photons in an erbium-doped optical fibre. Nat. Photonics 9, 83–87 (2015).
Craiciu, I. et al. Nanophotonic quantum storage at telecommunication wavelength. Phys. Rev. Appl. 12, 024062 (2019).
Ahlefeldt, R., Manson, N. & Sellars, M.Journal of Luminescence133, 152–156 (2013).
Kimiaee Asadi, F. et al. Quantum repeaters with individual rare-earth ions at telecommunication wavelengths. Quantum 2, 93 (2018).
Ahlefeldt, R., Zhong, M., Bartholomew, J. & Sellars, M. Minimizing zeeman sensitivity on optical and hyperfine transitions in EuCl3 ⋅ 6H2O to extend coherence times. J. Lumin. 143, 193–200 (2013).
Ahlefeldt, R. L., Hutchison, W. D., Manson, N. B. & Sellars, M. J. Method for assigning satellite lines to crystallographic sites in rare-earth crystals. Phys. Rev. B 88, 184424 (2013).
Ahlefeldt, R.Evaluation of a stoichiometric rare earth crystal for quantum computing. Ph.D. thesis (2013).
Schulz, M. B. & Jeffries, C. D. Paramagnetic resonance and relaxation of some rare-earth ions in YCl3 6H2O. Phys. Rev. 159, 277–284 (1967).
Altner, S. B., Wild, U. P. & Mitsunaga, M. Photon-echo demolition spectroscopy in Eu3+: Pr3+: Nd3+: Y2SiO5. Chem. Phys. Lett. 237, 406–410 (1995).
Ahlefeldt, R. L., McAuslan, D. L., Longdell, J. J., Manson, N. B. & Sellars, M. J. Precision measurement of electronic ion-ion interactions between neighboring Eu3+ optical centers. Phys. Rev. Lett. 111, 240501 (2013).
Longdell, J. J., Sellars, M. J. & Manson, N. B. Demonstration of conditional quantum phase shift between ions in a solid. Phys. Rev. Lett. 93, 130503 (2004).
Könz, F. et al. Temperature and concentration dependence of optical dephasing, spectral-hole lifetime, and anisotropic absorption in Eu3+: Y2SiO5. Phys. Rev. B 68, 085109 (2003).
Kunkel, N. et al. Dephasing mechanisms of optical transitions in rare-earth-doped transparent ceramics. Phys. Rev. B 94, 184301 (2016).
Abragam, A. & Bleaney, B.Electron paramagnetic resonance of transition ions (OUP Oxford, 2012).
Mitsunaga, M., Yano, R. & Uesugi, N. Stimulated-photon-echo spectroscopy. ii. echo modulation in Pr3+:YAlO3. Phys. Rev. B 45, 12760–12768 (1992).
Probst, S. et al. Hyperfine spectroscopy in a quantum-limited spectrometer. Magn. Reson. 1, 315–330 (2020).
Wang, F. et al. Nuclear spins in a solid exceeding 10-hour coherence times for ultra-long-term quantum storage. PRX Quantum 6, 010302 (2025).
Mims, W. B. Phase memory in electron spin echoes, lattice relaxation effects in CaWO4: Er, Ce, Mn. Phys. Rev. 168, 370–389 (1968).
Couture, L. & Rajnak, K. Parametric analysis of the energy levels of Er3+ ions in hydrated erbium chloride ErCl3 ⋅ 6H2O and in frozen dilute aqueous solutions of erbium chloride. Chem. Phys. 85, 315–332 (1984).
Rančić, M., Hedges, M. P., Ahlefeldt, R. L. & Sellars, M. J. Coherence time of over a second in a telecom-compatible quantum memory storage material. Nat. Phys. 14, 50–54 (2018).
Ruskuc, A., Wu, C. J., Rochman, J., Choi, J. & Faraon, A. Nuclear spin-wave quantum register for a solid-state qubit. Nature 602, 408 – 413 (2022).
Schwartz, I. et al. Robust optical polarization of nuclear spin baths using hamiltonian engineering of nitrogen-vacancy center quantum dynamics. Sci. Adv. 4, eaat8978 (2018).
Choi, J. et al. Robust dynamic hamiltonian engineering of many-body spin systems. Phys. Rev. X 10, 031002 (2020).
Freeman, J., Crosby, G. & Lawson, K. The effect of deuterium on the luminescence decay times of solvated rare earth chlorides. J. Mol. Spectrosc. 13, 399–406 (1964).
Ahlefeldt, R., Hutchison, W. & Sellars, M. Eu3+ superhyperfine structure due to magnetic dipole-dipole interactions with Nd3+ in Nd3+:EuCl3 ⋅ 6H2O. J. Lumin. 130, 1594–1597 (2010).
Acknowledgements
This work was supported by the Quantum Science and Technology -National Science and Technology Major Project (No. 2021ZD0301204), the National Natural Science Foundation of China (Grant No. 11904159, 12004168 and 12304454), Guandong Innovative and Entrepreneurial Research Team Program (Grant No. 2019ZT08X324), Guangdong Basic and Applied Basic Research Foundation (Grant No. 2021A1515110191), and the Key- Area Research and Development Program of Guangdong Province (Grant No. 2018B030326001), the Science, Technology and Innovation Commission of Shenzhen Mu- nicipality (KQTD20210811090049034).
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F. Wang, R. Ahlefeldt, M. Sellars and M. Zhong conceived the project. W. Xiao, with assistance from Z. Li and S. Liu, prepared the crystal samples. M. Guo and W. Xiao performed the experiments, with help from F. Wang. M. Guo completed the theoretical modeling and analysis with assistance of W. Sun, F. Wang, P. Wang and M. Zhong. M. Guo wrote the manuscript with help from S. Liu, F. Wang, and M. Zhong. All authors discussed the results and contributed to the final manuscript.
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Guo, M., Xiao, W., Li, Z. et al. Towards telecom-compatible quantum nodes using erbium-doped stoichiometric EuCl3 ⋅ 6H2O crystals. npj Quantum Inf (2026). https://doi.org/10.1038/s41534-026-01203-4
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DOI: https://doi.org/10.1038/s41534-026-01203-4
