Quantum Coherence in Rare-Earth Ions Enables Heterogeneous Photonic Interfaces with Reduced Inhomogeneous Linewidths

Controlling the quantum properties of materials represents a significant challenge in developing advanced technologies, and recent research focuses on integrating rare-earth ions into hybrid material systems. Henry C. Hammer, Hassan A. Bukhari, Yogendra Limdu, and colleagues at the University of Iowa and Oak Ridge National Laboratory investigate the factors limiting the optical coherence of rare-earth ions within these complex interfaces. Their work examines erbium-doped titanium oxide films grown on semiconductor substrates, revealing how strain and material defects impact the ions’ quantum behaviour. By combining theoretical modelling with experimental spectroscopy, the team demonstrates a direct link between the distance of erbium ions from the interface and their spectral properties, ultimately providing crucial insight into overcoming decoherence and paving the way for more robust and scalable quantum technologies.

Harnessing rare-earth ions in oxides for quantum networks requires integrating them with bright emitters in III-V semiconductors, but local disorder and interfacial noise limit their optical coherence. This work investigates the microscopic origins of the spectrum in Er3+:TiO2 epitaxial thin films on GaAs and GaSb substrates. Calculations, combined with noise-Hamiltonian modelling and simulations, quantify the effects of interfacial and bulk spin noise and local strain on erbium crystal-field energies and inhomogeneous linewidths. Photoluminescence spectroscopy reveals how these factors contribute to spectral broadening and provides insights into optimising material growth for enhanced quantum coherence.

Erbium-doped TiO2 Growth on III-V Semiconductors

This research details the growth and characterization of Erbium-doped TiO2 (Er:TiO2) thin films on III-V semiconductors, aiming to create hybrid quantum photonic interfaces. The team successfully grew high-quality Er:TiO2 films on these substrates using pulsed laser deposition, achieving narrow optical linewidths in the Er3+ emission, indicating reduced spectral diffusion and improved coherence, crucial for creating well-defined single photons. The study identified that decoherence mechanisms depend strongly on the depth of the Er3+ ions within the TiO2 film, with surface and interface defects, as well as dopants like Gallium (Ga), contributing significantly to decoherence near the surface. Ga doping in TiO2 creates paramagnetic defects that contribute to spin decoherence, acting as noise sources that broaden the optical linewidth and reduce coherence.

Researchers employed sophisticated calculations, including Density Functional Theory and a refined Hamiltonian for Lanthanide ions, to understand the crystal field splitting, electronic structure, and magnetic interactions of Er3+ ions in TiO2. This modelling helped interpret experimental results and identify decoherence mechanisms, highlighting the importance of minimizing surface and interface defects to improve coherence, demonstrating that the decoherence rate is higher near the surface due to the presence of these defects and dopants. The research team meticulously investigated erbium-doped titanium dioxide films grown on both gallium arsenide and gallium antimonide, quantifying the impact of interfacial strain and defects on the optical properties of the embedded ions. Measurements reveal a systematic blue shift of the erbium transition as ions are positioned further from the oxide/semiconductor interface, consistent with theoretical predictions of strain relaxation. Experiments demonstrate that thermal annealing induces a compensating red shift and narrowing of the spectral linewidth, effectively isolating the contributions of oxygen-vacancy and gallium-diffusion noise to decoherence.

Specifically, the team measured a characteristic dependence of the energy level splitting on local defects and strain relaxation using crystal-field calculations, employing large supercells to model the material structure. These calculations, combined with photoluminescence excitation spectroscopy at 5 Kelvin, provide detailed insight into the microscopic origins of decoherence mechanisms. The team’s measurements show that the choice of substrate, gallium arsenide or gallium antimonide, influences the strain state of the titanium dioxide film, with rutile films exhibiting mixed orientations and anatase films displaying biaxial compressive strain. These distinct strain states, ranging from 0.

12% to 0. 48%, provide a controllable means of tuning interfacial stress and defect formation, ultimately impacting the optical coherence of the embedded erbium ions. The research establishes a quantitative link between interfacial chemistry, strain engineering, and the optical properties of rare-earth ions, paving the way for the design of coherent oxide/semiconductor heterostructures for advanced quantum applications.

Interface Strain Controls Erbium Ion Energy Levels

This research details a microscopic investigation into the origins of optical decoherence in erbium-doped titanium dioxide films grown on semiconductor substrates. Scientists successfully quantified how imperfections and strain within the material affect the optical properties of the erbium ions, revealing that the position of these ions relative to the interface between the oxide and the semiconductor significantly impacts their energy levels. Specifically, the team demonstrated that erbium ions further from the interface exhibit a systematic shift in their optical transitions, consistent with theoretical predictions of strain relaxation. Through a combination of advanced calculations and spectroscopic analysis, the researchers isolated the contributions of various noise sources, including oxygen vacancies and gallium diffusion, to the broadening of the optical signals.

Thermal processing was shown to narrow these signals, demonstrating a pathway to improve the coherence of the system. These findings provide fundamental insight into the mechanisms governing decoherence in these hybrid materials, offering crucial information for the development of scalable quantum technologies. Future work will likely focus on optimizing the material growth process to minimize defects and enhance the optical coherence of the erbium ions, ultimately paving the way for brighter and more stable quantum emitters.