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Description
Beryllium ions (9Be+) serve as a four-body system with a closed electronic shell and a single valence electron, offering advantages for both theoretical and experimental research. Theoretically, their energy level structure can be accurately calculated using high-precision quantum electrodynamics (QED) methods. Experimentally, laser cooling enables the preparation of ultracold Coulomb crystals, providing an ideal platform for high-precision quantum state manipulation and spectroscopic measurements.
Direct measurements of the ground-state magnetically insensitive hyperfine transition |F=2, mF=0→F=1, mF=0> of 9Be+ ions have been performed using microwave-driven state transfer. The 9Be+ ions are confined and laser-cooled in a linear Paul trap, forming a Coulomb crystal. The transition frequencies have been measured over a magnetic field range of ±0.5 mT centered at zero magnetic field, and the acquired data were fitted accounting for the high-order Zeeman effect. The hyperfine constant A is determined to be −625.008840(35) MHz, achieving a relative precision of 5.6E-8.Combining this with theoretical calculations, the Zemach radius of the beryllium nucleus was derived as 4.03(5) fm.