Europium Crystal Isolates Single Frequencies For Improved Optical Memory Storage

The persistent demand for robust and long-lasting data storage motivates continued investigation into novel memory platforms, with europium-doped crystals emerging as a promising candidate due to their exceptional coherence properties. Jingjing Chen, Mikael Afzelius, and colleagues from the Department of Applied Physics at the University of Geneva detail a comprehensive analysis of these crystals under magnetic fields, presenting both computational modelling and experimental validation of optical transitions. Their work, entitled ‘Optical pumping simulations and optical Rabi frequency measurements in under magnetic field’, focuses on manipulating the complex interplay of optical and hyperfine levels within the crystal, demonstrating precise control over spectral features and ultimately, the potential for isolating specific frequencies crucial for data storage applications. The team’s measurements of the optical Rabi frequency across 21 transitions, alongside a derived optical dipole moment, provide a rigorous test of existing spin Hamiltonian models and refine understanding of energy level structures within these materials.

Quantum memories constitute a vital component in the development of quantum networks, facilitating the storage and subsequent retrieval of quantum information for long-distance communication via quantum repeaters. Rare-earth doped crystals, notably those incorporating ions like Praseodymium (Pr³⁺) and Europium (Eu³⁺), are receiving considerable attention as potential materials for ensemble-based quantum memories due to their prolonged optical and spin coherence times, coupled with a significant capacity for multimode storage. Researchers exploit hyperfine levels, which arise from the interaction between the electron and nuclear spin, within the electronic structure of ions like Pr³⁺ and Eu³⁺ to store quantum information as spin waves. The nuclear spin creates multiple hyperfine levels that can be manipulated and used to encode quantum bits, or qubits, and other quantum states.

Application of a magnetic field to these crystals introduces complexity, splitting the hyperfine levels and creating numerous possible optical-hyperfine transitions. However, even weak fields can enhance spin coherence times, although this comes at the cost of oscillations in storage efficiency. Conversely, strong magnetic fields fully resolve the hyperfine transitions, potentially enabling longer coherence times through Zero First-Order Zeeman transitions – transitions where the energy shift due to the magnetic field is zero – but demand precise control over the field’s magnitude and orientation. The anisotropy of the doping site within the host crystal significantly influences the frequencies and strengths of these optical-hyperfine transitions, necessitating accurate characterisation of the magnetic field vector relative to the crystal’s symmetry axis to optimise storage and retrieval processes. Techniques such as Raman heterodyne scattering and spectral hole burning are employed to precisely determine these field parameters and validate the underlying spin Hamiltonian models, which describe the energy levels and interactions within the material.

Yttrium orthosilicate (Y₂SiO₅) crystals doped with europium represent a promising platform for developing long-duration optical quantum memories. Investigations centre on understanding and manipulating the complex interplay between electronic and nuclear spins within the europium ions, crucial for reliable quantum information storage. The material exhibits 36 distinct optical-hyperfine transitions, creating a complex system requiring detailed characterisation. Researchers achieve isolation of specific frequency classes of ions within this complex system through carefully designed optical pumping schemes, applicable to any multi-level atom experiencing inhomogeneous broadening – a phenomenon where the transition frequency varies slightly across the sample due to local environment differences.

The work details methodologies for accurately determining the magnetic field vector, enabling high-precision prediction of spectral features based on established spin Hamiltonians. Experimental measurements focus on quantifying the optical Rabi frequency – a measure of the rate of oscillation between quantum states – across 21 of the 36 possible optical-hyperfine transitions. These measurements allow construction of a 6×6 branching ratio matrix, detailing the relative strengths of each transition, and from these Rabi frequency values, researchers derive the optical dipole moment for the transitions, obtaining a key parameter for understanding the interaction between light and the material.

The results provide a rigorous validation of the spin Hamiltonian models, demonstrating a high degree of accuracy in predicting energy levels and relative transition strengths. This precision is essential for applications requiring operation under magnetic fields, as it allows for precise control and manipulation of the quantum states. Coherence exceeding one second, and even several hours when utilising nuclear spins as the storage medium, is achieved, representing a substantial improvement in the viability of solid-state quantum memories.

Future work will likely focus on scaling these systems to accommodate a greater number of qubits, investigating alternative rare-earth ions and optimising crystal growth techniques to minimise defects. Furthermore, exploring integration with other quantum technologies, such as superconducting circuits, will be crucial for building practical quantum information processing systems.