pith. sign in

arxiv: 2604.24582 · v1 · submitted 2026-04-27 · 🪐 quant-ph · cond-mat.mtrl-sci

Isotopically enriched epitaxial CaWO₄ thin films for Er³⁺ spin-photon quantum interfaces

Hanlin Tang (1) , Kidae Shin (2) , Ashwin K. Boddeti (3 , 4) , Sebastian P. Horvath (3) , Adam Turflinger (3) , Joseph Alexander (3) , Jeffrey A. Dhas (5)
show 26 more authors
Zihua Zhu (6) Shuhang Pan (2) Jeff D. Thompson (3) Yingge Du (5) Frederick J. Walker (1) Charles H. Ahn (1 2 7) ((1) Department of Applied Physics Yale University New Haven Connecticut 06520 USA. (2) Department of Physics USA. (3) Department of Electrical Computer Engineering Princeton University. (4) Center for Integrated Nanotechnologies Sandia National Laboratories Albuquerque NM 87185 USA. (5) Physical Sciences Division Pacific North National Laboratory Richland Washington 99354 USA. (6) Environmental Molecular Sciences Laboratory USA. (7) Department of Materials Science USA.)
This is my paper

Pith reviewed 2026-05-08 03:57 UTC · model grok-4.3

classification 🪐 quant-ph cond-mat.mtrl-sci
keywords CaWO4 thin filmsisotopic enrichmentEr3+ ionsspin coherencerare-earth quantum interfacesnuclear spin bathnanophotonic devicesphotoluminescence
0
0 comments X

The pith

Isotopically enriched CaWO4 thin films reduce 183W nuclear spins to 1.2% abundance for Er3+ quantum interfaces.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper grows thin films of calcium tungstate doped with erbium ions, first in natural isotopic form and then using a purified tungsten source that removes most of the isotope carrying nuclear spin. They confirm the films remain high quality through narrow optical emission lines of 214 MHz and demonstrate single erbium ion light emission inside nanophotonic structures. The key step is dropping the harmful 183W fraction from 14.3% to 1.2%, which removes the main source of spin decoherence once the system is cooled to millikelvin temperatures where other impurities freeze out. A reader would care because rare-earth spins in oxides are stable and optically addressable, but nuclear noise currently caps their usefulness for connecting distant quantum nodes with light.

Core claim

The authors synthesized isotopically enriched Er3+-doped CaWO4 epitaxial thin films by molecular beam epitaxy from an isotopically purified 186WO3 source. Time-of-flight secondary ion mass spectrometry showed the 183W relative abundance reduced to 1.2%, a factor of ten below natural levels. The non-enriched films exhibited a photoluminescence inhomogeneous linewidth of 214(13) MHz, and single-ion emission was observed after integration with nanophotonic devices. These results establish isotopically engineered CaWO4 thin films as a platform for future studies of nuclear-spin-limited coherence and scalable rare-earth-ion-based quantum nanophotonic devices.

What carries the argument

Epitaxial growth of CaWO4 using isotopically purified 186WO3 source material, which directly suppresses the density of 183W nuclear spins that dephase embedded Er3+ electron spins.

If this is right

  • The reduced nuclear-spin bath should permit direct tests of longer Er3+ spin coherence times at millikelvin temperatures.
  • The thin-film geometry supports straightforward integration with nanophotonic resonators for efficient spin-photon coupling.
  • Single-ion photoluminescence confirms the material can host addressable quantum emitters suitable for quantum networks.
  • The epitaxial process offers a scalable route to isotopically purified oxide hosts without depending on bulk crystal growth.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same purification approach could be applied to other tungstate or oxide hosts to suppress nuclear-spin noise in different rare-earth systems.
  • Improved coherence would enable quantum memory protocols or entanglement distribution using these films as nodes in fiber-linked networks.
  • Strain or heterostructure engineering during epitaxial growth might further tune the optical and spin properties beyond isotopic control alone.

Load-bearing premise

That the observed tenfold drop in 183W abundance will produce a measurable extension of Er3+ spin coherence time once the system reaches millikelvin temperatures where paramagnetic impurities are frozen out.

What would settle it

Measure the electron spin coherence time of Er3+ ions in the isotopically enriched films versus natural-abundance films at temperatures below 1 K using spin-echo sequences; no significant improvement would falsify the claim that nuclear spins are now the dominant limit.

read the original abstract

Rare earth ion (REI)-doped oxide thin films are attractive for the application of quantum interconnects due to their stable optical levels and scalability$^{1-3}$. Among them, Er$^{3+}$ doped CaWO$_{4}$ is promising because it possesses narrow optical linewidth transitions and a long spin coherence time$^{4-6}$. The electron spin coherence is limited at high temperatures by paramagnetic impurities and by the presence of the 14.3% $^{183}$W nuclear spin. To further increase the spin coherence time at millikelvin temperatures, where the paramagnetic impurities are frozen out, our approach is to synthesize chemically and isotopically purified thin films as a host material. We first grow non-isotopically enriched Er$^{3+}$ doped CaWO$_{4}$ thin films, which exhibit a 214(13) MHz photoluminescence (PL) inhomogeneous linewidth, indicating the thin film has high crystalline quality. We then grow isotopically enriched CaWO$_{4}$ thin films using an isotopically purified $^{186}$WO$_{3}$ source. Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to measure the relative concentration of W isotopes. $^{183}$W, the only W isotope that has a net nuclear spin and is the major cause of spin decoherence, was at a relative abundance of 1.2%, a factor of 10 lower than natural abundance. We also observed PL emission from single ions after integrating nano-photonic devices with the thin film. These results establish isotopically engineered CaWO$_{4}$ thin films as a promising platform for future studies of nuclear-spin-limited coherence and for scalable rare-earth-ion-based quantum nanophotonic devices.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript reports the epitaxial growth of Er³⁺-doped CaWO₄ thin films, both with natural isotopic composition and isotopically enriched via a purified ¹⁸⁶WO₃ source. Non-enriched films exhibit a photoluminescence (PL) inhomogeneous linewidth of 214(13) MHz. ToF-SIMS measurements show the ¹⁸³W nuclear-spin-bearing isotope reduced to 1.2% relative abundance (a factor of ~10 below natural). Single Er³⁺ ion PL is observed in nanophotonic devices fabricated on the films. The central claim is that these results establish isotopically engineered CaWO₄ thin films as a promising platform for future studies of nuclear-spin-limited coherence and scalable rare-earth quantum nanophotonic devices.

Significance. If the reported growth, isotope purification, and optical characterization hold, the work supplies a concrete thin-film route to suppress the dominant ¹⁸³W nuclear-spin bath in a host already known for narrow Er³⁺ optical transitions. The direct ToF-SIMS quantification and the 214 MHz linewidth plus single-ion emission constitute measurable progress toward millikelvin spin-coherence experiments. The manuscript appropriately frames the coherence-time improvement as future work rather than a completed result.

major comments (1)
  1. The ToF-SIMS isotope-abundance data (1.2% for ¹⁸³W) are presented without reported uncertainties, calibration standards, or raw spectral integration details. Because the factor-of-10 reduction is the central experimental result supporting the 'promising platform' claim, this omission is load-bearing for reproducibility and for any subsequent coherence modeling.
minor comments (2)
  1. Growth parameters (substrate temperature, oxygen partial pressure, deposition rate, and post-growth annealing) are referenced only qualitatively; explicit values and run-to-run reproducibility metrics should be added to the methods section.
  2. The PL linewidth error bar of (13) MHz is stated without describing the fitting procedure or number of sampled ions/areas; a brief methods paragraph would clarify whether this reflects inhomogeneous broadening across the film or statistical uncertainty.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their positive assessment of our work and the recommendation for minor revision. We address the single major comment below.

read point-by-point responses

  1. Referee: The ToF-SIMS isotope-abundance data (1.2% for ¹⁸³W) are presented without reported uncertainties, calibration standards, or raw spectral integration details. Because the factor-of-10 reduction is the central experimental result supporting the 'promising platform' claim, this omission is load-bearing for reproducibility and for any subsequent coherence modeling.

    Authors: We agree that additional methodological details are required to substantiate the central isotopic purification result. In the revised manuscript we will report the uncertainties on the 1.2% ¹⁸³W abundance (derived from repeated depth profiles), describe the calibration against natural-abundance reference standards, and provide the spectral integration procedure together with representative raw spectra. These additions will be incorporated into the Methods section (and, if space permits, the supplementary information) to enable reproducibility and subsequent nuclear-spin-bath modeling. revision: yes

Circularity Check

0 steps flagged

No significant circularity; purely experimental report

full rationale

The manuscript is an experimental materials-science report. It describes thin-film growth of CaWO4, direct ToF-SIMS measurement of 183W abundance (reduced to 1.2 %), PL linewidth quantification (214 MHz), and observation of single-ion emission in nanophotonic devices. No equations, parameter fits, predictions, or derivations appear in the provided text. The concluding claim that the films constitute a “promising platform for future studies” is a forward-looking statement that does not rest on any internal derivation or self-referential step. External citations are used only for background context and do not form a load-bearing chain within the paper itself.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

No free parameters or invented entities are introduced; the work relies on standard thin-film growth and mass spectrometry techniques whose validity is assumed from prior literature.

axioms (1)
  • domain assumption ToF-SIMS provides accurate relative isotope abundances when calibrated against standards
    Invoked when reporting the 1.2% 183W value from the enriched films.

pith-pipeline@v0.9.0 · 5829 in / 1165 out tokens · 79294 ms · 2026-05-08T03:57:44.601472+00:00 · methodology

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.

Reference graph

Works this paper leans on

44 extracted references · 44 canonical work pages

  1. [1]

    Department of Applied Physics, Yale University, New Haven, CT 06520, USA

    work page

  2. [2]

    Department of Physics, Yale University, New Haven, CT 06520, USA

    work page

  3. [3]

    Department of Electrical and Computer Engineering, Princeton University, Princeton, NJ 08544

    work page

  4. [4]

    Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, NM, 87185, USA

    work page

  5. [5]

    Physical Sciences Division, Pacific North National Laboratory, Richland, Washington 99354, USA

    work page

  6. [6]

    Environmental Molecular Sciences Laboratory, Pacific North National Laboratory, Richland, Washington 99354, USA

    work page

  7. [7]

    Rare earth ion (REI) -doped oxide thin films are attractive for the application of quantum interconnects due to their stable optical levels and scalability1–3

    Department of Materials Science, Yale University, New Haven, CT 06520, USA. Rare earth ion (REI) -doped oxide thin films are attractive for the application of quantum interconnects due to their stable optical levels and scalability1–3. Among them, Er3+ doped CaWO4 is promising because it possesses narrow optical linewidth transitions and a long spin coher...

    work page

  8. [8]

    Small Laue oscillations around the CaWO4 substrate (004) peak may originate from local crystal distortions due to deviations of the thin film lattice from the bulk

    Among them, the thin film grown using 12% excess CaO has a smooth X-ray reflectivity curve, suggesting the average density and stoichiometry of the thin film match those of the substrate. Small Laue oscillations around the CaWO4 substrate (004) peak may originate from local crystal distortions due to deviations of the thin film lattice from the bulk. As s...

    work page doi:10.46936/cpcy.proj.2023.60681/60008771 2023

  9. [9]

    Simon, C. et al. Quantum memories. Eur. Phys. J. D 58, 1–22 (2010)

    work page 2010

  10. [10]

    Kindem, J. M. et al. Control and single-shot readout of an ion embedded in a nanophotonic cavity. Nature 580, 201–204 (2020)

    work page 2020

  11. [11]

    D., Hanson, R., Wrachtrup, J

    Awschalom, D. D., Hanson, R., Wrachtrup, J. & Zhou, B. B. Quantum technologies with optically interfaced solid-state spins. Nat. Photonics 12, 516–527 (2018)

    work page 2018

  12. [12]

    Ourari, S. et al. Indistinguishable telecom band photons from a single Er ion in the solid state. Nature 620, 977–981 (2023)

    work page 2023

  13. [13]

    Le Dantec, M. et al. Twenty-three–millisecond electron spin coherence of erbium ions in a natural-abundance crystal. Sci. Adv. 7, eabj9786 (2021)

    work page 2021

  14. [14]

    Raha, M. et al. Optical quantum nondemolition measurement of a single rare earth ion qubit. Nat. Commun. 11, 1605 (2020)

    work page 2020

  15. [15]

    Wolfowicz, G. et al. Quantum guidelines for solid-state spin defects. Nat. Rev. Mater. 6, 906–925 (2021)

    work page 2021

  16. [16]

    Weber, J. R. et al. Quantum computing with defects. Proc. Natl. Acad. Sci. U. S. A. 107, 8513–8518 (2010)

    work page 2010

  17. [17]

    C., Exarhos, A

    Alkauskas, A., Bassett, L. C., Exarhos, A. L. & Fu, K.-M. C. Defects by design: Quantum nanophotonics in emerging materials. Nanophotonics 8, 1863–1865 (2019)

    work page 2019

  18. [18]

    Stevenson, P. et al. Erbium-implanted materials for quantum communication applications. Phys. Rev. B 105, 224106 (2022)

    work page 2022

  19. [19]

    Awschalom, D. et al. Development of Quantum Interconnects (QuICs) for Next- Generation Information Technologies. PRX Quantum 2, 017002 (2021)

    work page 2021

  20. [20]

    & Gisin, N

    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)

    work page 2011

  21. [21]

    Sipahigil, A. et al. An integrated diamond nanophotonics platform for quantum- optical networks. Science 354, 847–850 (2016)

    work page 2016

  22. [22]

    Uysal, M. T. et al. Spin-Photon Entanglement of a Single ${\mathrm{Er}}^{3+}$ Ion in the Telecom Band. Phys. Rev. X 15, 011071 (2025)

    work page 2025

  23. [23]

    Shin, K. et al. Er-doped anatase TiO 2 thin films on LaAlO 3 (001) for quantum interconnects (QuICs). Appl. Phys. Lett. 121, 081902 (2022)

    work page 2022

  24. [24]

    Phenicie, C. M. et al. Narrow Optical Line Widths in Erbium Implanted in TiO2. Nano Lett. 19, 8928–8933 (2019)

    work page 2019

  25. [25]

    Ji, C. et al. Nanocavity-Mediated Purcell Enhancement of Er in TiO2 Thin Films Grown via Atomic Layer Deposition. ACS Nano 18, 9929–9941 (2024)

    work page 2024

  26. [26]

    Xie, T. et al. Characterization of Er 3 + : YV O 4 for microwave to optical transduction. Phys. Rev. B 104, 054111 (2021)

    work page 2021

  27. [27]

    Rančić, M. et al. Electron-spin spectral diffusion in an erbium doped crystal at millikelvin temperatures. Phys. Rev. B 106, 144412 (2022)

    work page 2022

  28. [28]

    Xu, H. et al. Coherent control of interacting solid-state spins below the diffraction limit. Preprint at https://doi.org/10.48550/arXiv.2508.09122 (2025)

    work page doi:10.48550/arxiv.2508.09122 2025

  29. [29]

    Masiulionis, I. et al. Microstructural and preliminary optical and microwave characterization of erbium doped CaMoO$_4$ thin films. Preprint at https://doi.org/10.48550/arXiv.2508.15122 (2025)

    work page doi:10.48550/arxiv.2508.15122 2025

  30. [30]

    Tiranov, A. et al. Sub-second spin and lifetime-limited optical coherences in $^{171}$Yb$^{3+}$:CaWO$_4$. Preprint at https://doi.org/10.48550/arXiv.2504.01592 (2025)

    work page doi:10.48550/arxiv.2504.01592 2025

  31. [31]

    Tyryshkin, A. M. et al. Electron spin coherence exceeding seconds in high-purity silicon. Nat. Mater. 11, 143–147 (2012)

    work page 2012

  32. [32]

    O’Sullivan, J. et al. Individual solid-state nuclear spin qubits with coherence exceeding seconds. Nat. Phys. 21, 1794–1800 (2025)

    work page 2025

  33. [33]

    Marcks, J. C. et al. Nuclear spin engineering for quantum information science. J. Mater. Res. 40, 1433–1448 (2025)

    work page 2025

  34. [34]

    M., Carroll, M

    Witzel, W. M., Carroll, M. S., Morello, A., Cywiński, Ł. & Das Sarma, S. Electron Spin Decoherence in Isotope-Enriched Silicon. Phys. Rev. Lett. 105, 187602 (2010)

    work page 2010

  35. [35]

    Zhang, J. et al. Optical and spin coherence of Er spin qubits in epitaxial cerium dioxide on silicon. Npj Quantum Inf. 10, 1–9 (2024)

    work page 2024

  36. [36]

    Grant, G. D. et al. Optical and microstructural characterization of Er$^{3+}$ doped epitaxial cerium oxide on silicon. Preprint at https://doi.org/10.48550/arXiv.2309.16644 (2023)

    work page doi:10.48550/arxiv.2309.16644 2023

  37. [37]

    Seth, S. K. et al. Spin Decoherence Dynamics of ${\mathrm{Er}}^{3+}$ in ${\mathrm{CeO}}_{2}$ Films. Phys. Rev. Lett. 135, 266901 (2025)

    work page 2025

  38. [38]

    Kanai, S. et al. Generalized scaling of spin qubit coherence in over 12,000 host materials. Proc. Natl. Acad. Sci. U. S. A. 119, e2121808119 (2022)

    work page 2022

  39. [39]

    Tang, H. et al. Homoepitaxial growth of CaWO4. J. Vac. Sci. Technol. A 42, 022701 (2024)

    work page 2024

  40. [40]

    Spectroscopic Ellipsometry: Principles and Applications

    Fujiwara, H. Spectroscopic Ellipsometry: Principles and Applications. (John Wiley & Sons, 2007)

    work page 2007

  41. [41]

    M., Raha, M., Phenicie, C

    Dibos, A. M., Raha, M., Phenicie, C. M. & Thompson, J. D. Atomic Source of Single Photons in the Telecom Band. Phys. Rev. Lett. 120, 243601 (2018)

    work page 2018

  42. [42]

    & Jayich, A

    Bluvstein, D., Zhang, Z. & Jayich, A. C. B. Identifying and Mitigating Charge Instabilities in Shallow Diamond Nitrogen-Vacancy Centers. Phys. Rev. Lett. 122, 076101 (2019)

    work page 2019

  43. [43]

    Billaud, E. et al. Electron paramagnetic resonance spectroscopy of a scheelite crystal using microwave-photon counting. Phys. Rev. Res. 7, 013011 (2025)

    work page 2025

  44. [44]

    Becker, F. et al. Spectroscopic investigations of multiple environments in Er: CaWO 4 through charge imbalance. Phys. Rev. Mater. 9, 076203 (2025)

    work page 2025