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Ultra-narrow optical linewidths in rare-earth molecular crystals


Rare-earth ions (REIs) are promising solid-state systems for building light–matter interfaces at the quantum level1,2. This relies on their potential to show narrow optical and spin homogeneous linewidths, or, equivalently, long-lived quantum states. This enables the use of REIs for photonic quantum technologies such as memories for light, optical–microwave transduction and computing3,4,5. However, so far, few crystalline materials have shown an environment quiet enough to fully exploit REI properties. This hinders further progress, in particular towards REI-containing integrated nanophotonics devices6,7. Molecular systems can provide such capability but generally lack spin states. If, however, molecular systems do have spin states, they show broad optical lines that severely limit optical-to-spin coherent interfacing8,9,10. Here we report on europium molecular crystals that exhibit linewidths in the tens of kilohertz range, orders of magnitude narrower than those of other molecular systems. We harness this property to demonstrate efficient optical spin initialization, coherent storage of light using an atomic frequency comb, and optical control of ion–ion interactions towards implementation of quantum gates. These results illustrate the utility of rare-earth molecular crystals as a new platform for photonic quantum technologies that combines highly coherent emitters with the unmatched versatility in composition, structure and integration capability of molecular materials.

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Fig. 1: Material and low-temperature optical spectroscopy.
Fig. 2: Ultra-narrow optical homogeneous linewidths.
Fig. 3: Optically addressable nuclear spins.
Fig. 4: Coherent light storage and optically controlled ion–ion interactions.

Data availability

Datasets generated and/or analysed during the current study are available in the Zenodo repository ( Source data are provided with this paper.


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We thank M. Afzelius for useful discussions, and N. Harada and P. Vermaut for assistance during scanning electron microscopy measurements. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 820391 (SQUARE) and EUCOR Marie Skłodowska-Curie COFUND project number 847471 (QUSTEC), the French Agence Nationale de la Recherche under grant ANR-20-CE09-0022 (UltraNanOSpec), the Frontiers Research in Chemistry Foundation CIRFC number 93 "Optically controlled qudits" and KIT Future Fields Project "Optically addressable qubits".

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Authors and Affiliations



P.G., M.R. and D.H. conceived and supervised the project. D.S. and S.K.K. were involved in the conceptual development of the project. S.K.K. and M.R. were responsible for the synthesis and characterization of the isotopologue complexes. B.H. performed powder X-ray diffraction studies and indexed the patterns. O.F. solved the X-ray structure of the complex. D.S. and P.G. performed the optical experiments and analysed the results. D.S. and P.G. wrote the manuscript with input from all authors.

Corresponding authors

Correspondence to Diana Serrano, Senthil Kumar Kuppusamy, Mario Ruben or Philippe Goldner.

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The authors declare no competing interests.

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Nature thanks David Mills and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Eu3+ complex preparation and X-ray crystal structure.

Upper: Schematic representation of the preparation of the europium complex discussed in this study. Lower: X-ray crystal structure of the complex. As shown, the piperidin-1-ium cation is involved in hydrogen bonding interactions with two oxygen atoms of benzoylacetonate ligands.

Extended Data Fig. 2 Photostability of the Eu3+ complex.

5D0 photoluminescence (PL) intensity under continuous wave excitation measured at 8 K for the 151Eu3+ isotopically enriched complex. The constant PL signal confirms absence of photobleaching. See SI section 3 for more details.

Source data

Extended Data Fig. 3 2-pulse photon echo decay from the 5% Eu3+-95%Y3+ diluted complex.

The experimental decay (black circles) was fitted with a double exponential model (red curve) with T2,fast = 18 μs and T2,slow = 68 μs decay time constants.

Source data

Extended Data Fig. 4 Spectral tailoring prior to AFC storage.

a. Spectral pit of 9 MHz dug in the absorption profile at 30 MHz to create a high absorption region at 0 MHz. b. Atomic frequency comb (AFC) with teeth of 0.9 MHz separated by 1.75 MHz (finesse F = 1.9). The FFT of the storage pulse is presented over the AFC (blue line) showing good spectral overlap. The input pulse intensity was estimated by sending it through the spectral pit in a, taking advantage of the pit’s almost full transparency. A correction was made to account for residual absorption.

Source data

Supplementary information

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This file contains the table of contents; supplementary text, figures, tables and references.

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Serrano, D., Kuppusamy, S.K., Heinrich, B. et al. Ultra-narrow optical linewidths in rare-earth molecular crystals. Nature 603, 241–246 (2022).

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