🔬 materials science

Optical decoherence in Er $^{3+}$ -doped CeO $_2$ spin qubit platforms

This study combines computational modeling and experimental validation to identify Ce $^{3+}$ polarons and their complexes as the primary sources of optical decoherence in Er $^{3+}$ -doped CeO $_2$ spin qubit platforms, revealing that their photoionization at 0.8 eV drives linewidth broadening and photoluminescence quenching.

Original authors: Vrindaa Somjit, Ignas Masiulionis, Gregory D. Grant, Weiguo Jing, Matteo Giantomassi, Supratik Guha, Gian-Marco Rignanese, F. Joseph Heremans, Jiefei Zhang, Giulia Galli

Published 2026-08-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Vrindaa Somjit, Ignas Masiulionis, Gregory D. Grant, Weiguo Jing, Matteo Giantomassi, Supratik Guha, Gian-Marco Rignanese, F. Joseph Heremans, Jiefei Zhang, Giulia Galli

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quest to build a quantum internet, scientists are searching for tiny, stable particles that can hold information like a digital bit, but with the strange, powerful properties of quantum mechanics. One promising candidate is a specific type of atom, erbium, trapped inside a crystal made of cerium and oxygen. This combination is attractive because the surrounding crystal is exceptionally quiet, lacking the magnetic noise that usually scrambles delicate quantum data. Furthermore, erbium has a special ability to talk to light at a frequency used by telecommunications networks, making it a potential bridge between quantum computers and existing fiber-optic cables. However, for this bridge to work, the light emitted by the erbium atoms must be pure and steady. If the light flickers or loses its sharpness, the information carried within it is lost.

For some time, researchers have known that the light from these erbium atoms in cerium oxide is not as sharp as theory predicts it should be. The lines in the light spectrum are too broad, and the atoms stop glowing sooner than expected. The cause of this blurring and fading has remained a mystery, hindering the development of reliable quantum devices. A new study has now pinpointed the culprit: it is not the erbium itself, but rather a specific type of imperfection within the crystal structure that interferes with the light. By combining advanced computer simulations with physical experiments, the researchers identified that tiny clusters of defects, involving missing oxygen atoms and extra electrons, are absorbing the light and disrupting the quantum state. This discovery provides a clear path forward for engineers to clean up the crystal and unlock the full potential of this material for future quantum technologies.

The researchers began by looking closely at the atomic landscape of the cerium oxide crystal. In a perfect crystal, every atom sits in its exact spot, but in reality, crystals often contain missing pieces. In cerium oxide, it is common for an oxygen atom to be missing, leaving behind a vacancy. When this happens, an electron nearby can get trapped in that empty space, forming what is called a polaron. Think of this polaron as a small, localized disturbance in the crystal's electronic fabric. The team used powerful computer models to simulate how these polarons, and the complexes they form with other defects, interact with light. They calculated the energy required to knock an electron out of these defects and into the flow of electricity within the material.

The simulations revealed a critical coincidence. The energy needed to free an electron from these defect clusters is almost exactly the same as the energy of the laser light used to excite the erbium atoms. When the laser shines on the crystal to make the erbium glow, it also accidentally hits these hidden defects. Because the energies match, the laser light can easily strip electrons away from the defects, turning them into charged particles. This process does two damaging things. First, the defects absorb the light that the erbium atoms are trying to emit, effectively stealing the signal and causing the glow to fade. Second, the sudden creation of these new charges introduces electrical noise that jitters the quantum state, blurring the sharp lines of the light spectrum.

To confirm that this theoretical mechanism was actually happening in the real world, the team turned to physical measurements. They grew thin films of cerium oxide doped with erbium and set up a simple experiment to measure the flow of electricity when the material was illuminated with the same laser light used for the quantum experiments. If their theory was correct, shining the laser should generate a measurable electric current, proving that the light was indeed ionizing defects within the crystal. The results were clear: the laser did produce a current, confirming that the defects were being activated by the light.

The researchers then varied the amount of erbium in the samples to see how the interaction changed. They found a surprising pattern: the sample with a lower concentration of erbium produced a stronger electric current relative to the amount of light it received. This happened because, in the sample with less erbium, the light emitted by the excited atoms had a higher chance of hitting a nearby defect before it escaped the crystal. In the sample with more erbium, the light was more likely to be absorbed by another erbium atom instead. This concentration-dependent behavior perfectly matched their computer predictions, validating the idea that these defects are stealing energy from the erbium atoms and causing the optical blurring.

The study concludes that the optical decoherence, or the loss of clarity in the light, is driven by these specific defect clusters. The findings suggest that the very imperfections that make cerium oxide useful for other applications, like energy conversion, are the same ones that hurt its performance as a quantum material. By identifying the exact nature of these defects, the researchers have provided a concrete target for improvement. Future efforts can now focus on engineering the crystal growth process to minimize the formation of these specific oxygen vacancies and polarons. If these defects can be reduced, the erbium atoms in cerium oxide could finally shine with the purity and stability needed to become a cornerstone of the quantum internet.

Technical Summary: Optical Decoherence in Er³⁺-Doped CeO₂ Spin Qubit Platforms

Problem Statement
Erbium ions (Er³⁺) embedded in cerium dioxide (CeO₂) are promising candidates for quantum spin-photon interfaces due to their telecom-C band transitions and long predicted spin coherence times. However, the mechanisms limiting the optical coherence of these systems remain poorly understood. Experimental observations indicate that the optical homogeneous linewidth of Er³⁺ in CeO₂ is approximately $10^3$ times broader than its lifetime-limited linewidth, and the origin of this broadening, as well as the source of specific electron paramagnetic resonance (EPR) peaks (e.g., $g=2.04$ ), has not been fully clarified. While spin decoherence is partially understood, the specific defect-mediated pathways causing optical decoherence—potentially involving charged defects, electric field gradients, and strain—require identification to enable the realization of high-performance quantum memories.

Methodology
The study employs a combined computational and experimental approach:

Computational Framework:
- Theory: Periodic hybrid density functional theory (DFT) calculations were performed using the Quantum Espresso code with the dielectric-dependent hybrid (DDH) exchange-correlation functional. The Hartree-Fock mixing parameter was set to $\alpha = \varepsilon_{\infty}^{-1} = 0.188$ , based on the experimental high-frequency dielectric constant of CeO₂.
- Defect Modeling: The study investigated isolated electron polarons (Ce³⁺), oxygen vacancies ( $V_O$ ) in various charge states, and complexes formed between Er³⁺ dopants and oxygen vacancies.
- Key Metrics: The authors calculated thermodynamic charge transition levels (CTLs) and optical charge transition levels (VTLs). Crucially, they applied finite-size corrections to account for artificial electrostatic interactions in supercells, ensuring accurate defect energy levels.
- Photoionization: Photoionization cross-sections were computed using the WEST code, incorporating electron-phonon coupling via one-dimensional configurational coordinate diagrams (1D-CCD) to model vibrational broadening.
Experimental Validation:
- Samples: Single-crystal CeO₂ films (~750 nm thick) doped with 2 ppm and 10 ppm Er³⁺ were grown via molecular beam epitaxy.
- Measurements: Photocurrent measurements were conducted under 0.8 eV (1530 nm) laser illumination, corresponding to the Er³⁺ excitation wavelength. Ti/Au gates were fabricated to apply a bias across the films, and current-voltage characteristics were recorded at 300 K to detect photoexcited carriers.

Key Results

Defect Energy Levels: The calculations reveal that the thermodynamic charge transition levels (CTLs) for isolated Ce³⁺ polarons and their complexes with oxygen vacancies and Er³⁺ are located below 0.8 eV relative to the conduction band minimum (CBM). Specifically, the isolated polaron CTL is at 0.62 eV, and the complexes are at $\lesssim 0.84$ eV.
Photoionization Cross-Sections: The computed photoionization cross-sections show a finite value at 0.8 eV for the isolated polaron and likely for the defect complexes (considering temperature effects and computational uncertainties). This indicates that the 0.8 eV laser used to excite Er³⁺ can photoionize these defects.
Mechanism Identification:
- Photoluminescence Quenching: The defects can absorb the 0.8 eV emission from Er³⁺, leading to non-radiative quenching.
- Charge Noise: Photoionization of these defects generates free carriers and creates new polaron-defect complexes, introducing charge noise.
- Spectral Broadening: These processes collectively lead to the broadening of optical linewidths and the reduction of excited-state lifetimes.
- EPR Origin: The negative VTL for the isolated polaron ( $\mu(0/-1) = -0.05$ eV) suggests rapid vibrational relaxation to form a localized polaron, supporting the hypothesis that the observed $g=2.04$ EPR peak arises from Ce³⁺ polarons rather than delocalized electrons.
Experimental Confirmation: Photocurrent measurements confirmed the presence of photoionizable charged defects in Er³⁺-doped CeO₂ under 0.8 eV illumination.
- Concentration Dependence: The 2 ppm sample exhibited a higher photocurrent and saturation behavior at high laser powers compared to the 10 ppm sample.
- Interpretation: In the 2 ppm sample, the relative concentration of Ce³⁺ (from interfacial CeOₓ-SiOᵧ layers) to Er³⁺ is higher, facilitating efficient energy transfer from excited Er³⁺ to Ce³⁺, leading to ionization and saturation. In the 10 ppm sample, energy transfer is more likely to occur between Er³⁺ ions, leaving more Ce³⁺ available for direct laser ionization and preventing saturation.

Significance and Claims
The paper identifies a concrete defect-engineering target for improving the Er³⁺-doped CeO₂ quantum platform. The authors claim that the optical decoherence observed in these systems is likely driven by the photoionization of Ce³⁺ polarons and their complexes with oxygen vacancies under standard operating conditions (0.8 eV illumination).

The study posits that defects beneficial for energy conversion applications (oxygen vacancies and polarons) are detrimental to quantum applications in this specific host. By validating this decoherence pathway through both first-principles calculations and concentration-dependent photocurrent measurements, the work provides a mechanistic explanation for the broad optical linewidths and short excited-state lifetimes previously observed. The authors suggest that this mechanism is likely relevant to other Er³⁺-doped multivalent-oxide quantum platforms, such as TiO₂ and CaWO₄, where mixed-valent cations and oxygen non-stoichiometry are present. Future work is proposed to quantify energy transfer rates and develop charge noise models to guide mitigation strategies.

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