Strontium Repumping Scheme, Combining 18 Zeeman Sublevels, Maximizes Photon Scattering Rate for Trapped Ion Technologies

Laser-cooled trapped ions underpin many emerging technologies, yet accurately predicting their behaviour requires moving beyond simplified theoretical models. Valentin Martimort, Sacha Guesne, and Derwell Drapier, alongside colleagues at their respective institutions, investigate the complex cooling dynamics of strontium ions, focusing on a process called repumping.

The team demonstrates that standard descriptions of atomic behaviour, which assume only two energy levels are important, fail to capture the full picture, as observed fluorescence spectra deviate significantly from predictions. By combining precise spectroscopic measurements with detailed numerical modelling, they identify optimal conditions for maximising the rate at which ions scatter light, paving the way for improved control and performance in future quantum technologies and precision sensors. Doppler cooling of strontium ions involves multiple electronic levels and repumping channels that significantly influence fluorescence. This work investigates a repumping scheme for strontium ions, combining precise single-ion spectroscopy with comprehensive numerical modelling based on the Optical Bloch equations, including Zeeman sublevels. The results demonstrate that, although observed fluorescence spectra resemble those of a simple three-level system, detailed analysis reveals a complex interplay between repumping rates and dark state populations. This understanding is crucial for optimising cooling efficiency and achieving high fidelity control of individual ions, ultimately advancing the development of scalable quantum computing architectures.

Trapped Ion Qubit Control and Spectroscopy This research focuses on the control and manipulation of trapped ions for quantum information processing and precision spectroscopy. Trapped ion technology, utilising ions like strontium and calcium, serves as a platform for building qubits and conducting high-precision measurements. A key aspect of this work is the precise control of the internal quantum states of these ions, achieved through techniques like Doppler and sideband cooling to minimise decoherence, and laser manipulation to drive transitions between energy levels. Raman transitions are also employed for state control and manipulation, with accurate quantum state measurement essential for these processes. A significant focus lies on using trapped ions for extremely precise measurements of atomic properties, relevant for fundamental physics tests and atomic clocks. Various spectroscopic techniques, including laser-induced fluorescence and saturated absorption spectroscopy, are used to probe the ions and determine properties like branching fractions and lifetimes of excited states. The research employs the Optical Bloch Equations as a theoretical framework for modelling the interaction of light with the ions, requiring accurate calculations of atomic properties, and addresses the micromotion of ions within the trap for high-precision measurements. Single-ion thermometry, measuring the temperature of a single ion, is a challenging but important task for characterising the system and optimising performance. Understanding and overcoming the limitations of Doppler cooling is crucial for achieving the lowest possible temperatures. The research also considers the Lamb shift, a small energy shift due to quantum electrodynamic effects, for accurate spectroscopy and utilises microwave radiation to manipulate the quantum states of the ions.

Strontium Ion Cooling Reveals Repumping Limitations Scientists have achieved a detailed understanding of laser cooling dynamics in strontium ions, revealing limitations in the commonly used two-level atom model. The research focused on repumping schemes essential for maintaining efficient cooling, specifically examining both coherent and incoherent approaches to return ions to the cooling cycle after they enter a metastable state. Experiments involved precise spectroscopic measurements of single laser-cooled strontium ions, utilising fast acquisition techniques to minimise experimental artefacts and accurately determine photon collection efficiency. Analysis of the spectral lineshape revealed a Lorentzian profile with a full width at half-maximum of 30MHz. However, comparisons with predictions based on a simple two-level atom model revealed discrepancies, as calculations indicated a saturation parameter of 0. 14, corresponding to a predicted linewidth of 22MHz, significantly lower than the measured 30MHz. Even accounting for a small magnetic field of 0. 045 mT could not reconcile the observed linewidth with the two-level model. To address these discrepancies, the researchers developed a comprehensive model based on the Optical Bloch Equations, incorporating eighteen Zeeman sublevels to accurately describe the complex atom-laser interactions within the incoherent repumping scheme. This detailed model promises to improve understanding of techniques like Doppler recooling and provide a more accurate framework for characterising ion trap heating rates.

Zeeman Sublevel Effects on Laser Cooling This research demonstrates that established models for laser cooling of trapped ions, often relying on simplified two-level atom descriptions, are insufficient to fully explain experimental observations. By combining precise spectroscopic measurements of strontium ions with a comprehensive numerical model incorporating eighteen Zeeman sublevels, scientists have revealed the complexities of the repumping process crucial for efficient cooling. While fluorescence spectra appear Lorentzian, consistent with a two-level system, the observed signal width and amplitude deviate significantly from predictions based on this simplification. Notably, the study identifies optimal conditions for maximising photon scattering rates, demonstrating that increasing laser power does not always lead to improved cooling efficiency, a counterintuitive finding. The research establishes a quantitative understanding of Doppler cooling in ions possessing metastable states and offers practical guidance for enhancing precision spectroscopy techniques, with implications for metrology and quantum information experiments utilising various atomic species. The work underscores the importance of considering multi-level atomic structure, even when spectral lines resemble those of simpler, two-level systems. The authors acknowledge that their model, while significantly more accurate than previous approaches, still relies on certain approximations, and future work could explore the impact of additional factors, such as the spatial distribution of the ion cloud and the effects of collisions, on the cooling process.

This research provides a foundation for further refinement of cooling techniques and optimisation of experimental parameters in advanced quantum technologies. 👉 More information 🗞 Incoherent repumping scheme in the Sr five-level manifold 🧠 ArXiv: https://arxiv.org/abs/2512.08710 Tags: