🔬 optics

Judd-Ofelt analysis and coherent spectroscopy of europium-based molecular complexes for quantum information applications

This paper presents a comprehensive optical characterization and Judd-Ofelt analysis of twelve Eu $^{3+}$ -based molecular complexes, demonstrating that lower point-group symmetry enhances the branching ratio of the coherent transition and identifying a specific complex with an optical coherence time of 2.3 $\mu$ s, thereby establishing key structure-property relations for the rational design of rare-earth molecular systems in quantum information applications.

Original authors: Nicholas Lester Jobbitt, Sascha Weis, Timo Schweizer, Vishnu Unni Chorakkunnath, Evgenij Vasilenko, Tobias Haas, Barbora Brachnakova, Senthil Kumar Kuppusamy, Hartmut Schubert, Mario Ruben, Michael Se

Published 2026-10-12

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Nicholas Lester Jobbitt, Sascha Weis, Timo Schweizer, Vishnu Unni Chorakkunnath, Evgenij Vasilenko, Tobias Haas, Barbora Brachnakova, Senthil Kumar Kuppusamy, Hartmut Schubert, Mario Ruben, Michael Seitz, David Hunger

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

Quantum computers promise to solve problems that are currently impossible for today's machines, but they require a very specific kind of memory to hold information. This memory must be able to store delicate quantum states for a long time without them fading away or becoming scrambled by the environment. Scientists have long looked to rare-earth ions, atoms with unique magnetic properties, as potential candidates for this task. When these ions are placed inside a solid material, they can act as tiny, stable storage units. However, traditional materials made by mixing these ions into a crystal lattice are difficult to control. The ions end up scattered randomly, making it hard to ensure they all behave the same way or to connect them to one another. A newer approach involves building these ions into precise, custom-made molecules. By designing the chemical structure around the ion, researchers can create a uniform environment where every single ion is identical, offering a path toward scalable and reliable quantum memory.

In a recent study, researchers set out to test this idea by examining a collection of twelve different molecular complexes, each containing a europium ion at its center. Europium is particularly useful because it has a specific energy transition that is exceptionally stable and resistant to outside interference, making it a prime candidate for storing quantum information. The team focused on how well these molecules could emit light at a specific wavelength of about 580 nanometers, a color in the visible spectrum. They wanted to see if the chemical structure surrounding the ion could be tuned to make this light emission stronger and more consistent, which is crucial for reading and writing data in a quantum computer. The researchers measured how long the light from these molecules could stay in a coherent state, meaning the waves of light remained synchronized, and they analyzed the chemical environment to understand what made some molecules perform better than others.

The study revealed that the shape and symmetry of the molecule surrounding the europium ion play a decisive role in its performance. The researchers found that when the chemical environment around the ion is less symmetrical, the desired light emission becomes significantly stronger. In several of the molecules they tested, specifically those with lower symmetry, the probability of the ion emitting the useful light was greater than one percent. This is a notable improvement, as the transition is naturally very weak. The team also discovered that the quality of the crystal structure matters immensely. Some of the molecules formed highly ordered crystals with very little internal strain, which allowed the light to remain coherent for longer periods. One specific molecule, labeled 5-Eu, stood out in these tests. At a temperature of 4.2 Kelvin, just above absolute zero, this molecule maintained its optical coherence for 2.3 microseconds. While this duration is fleeting to human perception, it is a substantial amount of time in the world of quantum physics and compares favorably with the best molecular systems reported to date.

However, the researchers also found that a strong light emission does not automatically guarantee a long-lasting memory. They observed that while some molecules emitted bright light, they did not necessarily hold their quantum state for long, and vice versa. The performance of these molecules is also highly sensitive to temperature. The experiments showed that as the temperature rises, even slightly, the coherence time drops rapidly due to the vibrations of the atoms within the molecule. This suggests that for these materials to be truly useful, they must be kept at extremely low temperatures and designed with rigid chemical structures that minimize these vibrations. The study did not find a single "perfect" molecule that excelled in every category, but it provided a clear map of how chemical design influences performance. By showing that lower symmetry leads to stronger signals and that structural order leads to longer coherence, the work offers a guide for chemists to design better materials in the future.

The team concluded that while these molecular crystals are a promising platform, there is still work to be done to optimize them for real-world applications. The current results suggest that the best candidates will likely be those that combine a chemically asymmetric environment to boost the signal with a rigid, ordered structure to protect the quantum state. The researchers also noted that using optical cavities, which are devices that trap light to enhance its interaction with matter, could further boost the weak signals from these molecules. This combination of chemical design and physical engineering could eventually lead to the creation of integrated quantum memories. The study serves as a comprehensive benchmark, proving that the chemical tunability of molecular crystals is a powerful tool for overcoming the limitations of traditional materials, bringing the vision of scalable quantum networks a step closer to reality.

Technical Summary: Judd-Ofelt Analysis and Coherent Spectroscopy of Europium-Based Molecular Complexes for Quantum Information Applications

Problem Statement
Rare-earth-ion (REI) based solid-state materials are promising candidates for quantum computing and networking due to their exceptional optical and spin coherence times. However, traditional REI-doped inorganic crystals face limitations regarding low oscillator strengths, poor branching ratios for coherent transitions, and challenges in scaling due to the random distribution of ions. While REI-based molecular crystals offer a potential alternative through self-assembly and chemical tunability, a comprehensive understanding of the structure-property relationships governing their optical coherence and transition strengths is lacking. Specifically, there is a need to systematically characterize how local point-group symmetry, ligand fields, and crystalline environments influence the coherent $^5D_0 \rightarrow ^7F_0$ transition of Eu $^{3+}$ ions, which is critical for qubit applications.

Methodology
The authors present a comprehensive optical characterization of twelve Eu $^{3+}$ -based molecular complexes (designated 1-Eu through 12-Eu), including both mononuclear and binuclear species. The study employed the following experimental and analytical approaches:

Sample Preparation: Synthesis of microcrystalline powders via coordination of Eu $^{3+}$ salts with organic chelating ligands, followed by isolation and purification.
Luminescence Spectroscopy: Room-temperature fluorescence spectra were recorded using 532 nm excitation to map the $^5D_0 \rightarrow ^7F_J$ ( $J=0-6$ ) transitions.
Judd-Ofelt Analysis: Spectral intensities were analyzed to calculate Judd-Ofelt parameters ( $\Omega_2, \Omega_4, \Omega_6$ ), radiative lifetimes, and branching ratios. This provided insights into local symmetry, bonding types, and ligand-field environments.
Linewidth and Coherence Measurements:
- Inhomogeneous linewidths ( $\Gamma_{inh}$ ) were measured at both room temperature and 4.2 K by fitting fluorescence spectra with Lorentzian functions.
- Optical coherence times ( $T_2$ ) were determined via spectral hole burning (SHB), free-induction decay (FID), and two-pulse photon-echo measurements at 4.2 K for the most promising candidate.
Lifetime Measurements: Optical lifetimes ( $T_1$ ) were measured at room temperature to calculate intrinsic quantum efficiencies.

Key Contributions and Results

Systematic Characterization: The study provides the first comprehensive dataset for twelve Eu $^{3+}$ molecular complexes, establishing a baseline for comparing structural variations (e.g., ligand types, symmetry) against optical properties.
Symmetry and Branching Ratios: A clear correlation was found between local point-group symmetry and the branching ratio ( $\beta_0$ ) of the nominally forbidden $^5D_0 \rightarrow ^7F_0$ transition. Complexes with lower effective symmetries (specifically $C_1$ and $C_2$ ) exhibited higher branching ratios, exceeding 1% in several cases (e.g., 1-Eu, 2-Eu, 5-Eu, 8-Eu, 11-Eu). This is attributed to increased mixing between different $J$ -manifolds in lower-symmetry ligand fields.
Optical Coherence: Among the newly investigated complexes (1-Eu to 6-Eu), only 5-Eu exhibited a measurable optical coherence signal at 4.2 K.
- 5-Eu Performance: It demonstrated a narrow inhomogeneous linewidth of 6.4(0.2) GHz at 4.2 K, indicative of a highly ordered crystalline environment.
- Coherence Time: Two-pulse photon-echo measurements yielded an optical coherence time of $T_2 = 2.3(0.3)$ µs at 4.2 K. This value is comparable to the best-performing molecular systems reported to date (e.g., 9-Eu).
Judd-Ofelt Parameters: The analysis revealed that $\Omega_2$ values correlate with the intensity of the hypersensitive $^5D_0 \rightarrow ^7F_2$ transition, reflecting local asymmetry and covalency. Quantum efficiencies were generally found between 50% and 75%, with the exception of 12-Eu (23.8%), suggesting significant non-radiative losses via high-energy C-H vibrational modes in some complexes.
Trade-offs: The study highlights that a high branching ratio does not automatically guarantee long coherence times. While lower symmetry enhances the transition strength, it may also introduce greater sensitivity to local field fluctuations.

Significance and Claims
The paper claims that systematic characterization and quantitative comparison are essential for guiding the rational design of optimized molecular complexes for quantum information applications. The authors assert that:

Eu $^{3+}$ molecular crystals are a viable platform for quantum memories and spin-photon interfaces, combining long coherence times with chemical tunability.
Structure-Property Relations: Lower point-group symmetry is a key design parameter for enhancing the oscillator strength of the coherent transition, but this must be balanced against the need for a rigid, homogeneous crystalline environment to minimize dephasing.
Optimization Pathways: Future improvements should focus on minimizing high-energy vibrational modes to reduce non-radiative relaxation and controlling crystal packing to limit spectral diffusion.
Complementary Approaches: While chemical modification is limited by the doubly forbidden nature of the transition, optical resonators (Purcell effect) offer a complementary route to enhance emission rates and branching ratios without altering the molecular structure.

The authors conclude that while 5-Eu shows promise with its combination of a >1% branching ratio and 2.3 µs coherence time, further work involving temperature-dependent studies, single-crystal structural characterization, and crystal-field calculations is necessary to establish predictive design rules for next-generation quantum materials.

More like this