Recently, the Cold Molecular Ion Research Team of the Innovation Academy for Precision Measurement Science and Technology (APM) has made important progress in the precision spectroscopic measurement of beryllium-9 ions (⁹Be⁺). The research team prepared the ionic state through multi-pulse operations in a linear ion trap, realized the measurement of the ground-state hyperfine magnetically insensitive transition of beryllium ion, and successfully improved the measurement accuracy of the ground state hyperfine constant of beryllium ion under weak magnetic fields by one order of magnitude. The relevant research results have been recently published in Physical Review A.

As a few-body three-electron system, the beryllium-9 ion can be calculated with high precision by quantum electrodynamics (QED) theory; meanwhile, it can be directly laser-cooled to form a Coulomb crystal. In this Coulomb crystal, the ions feature high localization and collective radiation, which helps improve the signal-to-noise ratio of spectroscopic measurement. In addition, the beryllium-9 ion has a relatively high charge-to-mass ratio, making it an ideal coolant for low-mass ions and highly ionized ions. It has a nuclear spin of 3/2, and the nuclear structure information can be retrieved through the measurement of the ion's hyperfine structure.

At present, all the highest-precision measurements of the hyperfine constant of the electronic ground state of beryllium ions are completed in strong magnetic fields, and there are inconsistencies between different experimental results. Considering the influence of the diamagnetic effect on the determination of the hyperfine constant, high-precision measurements in weak magnetic fields (especially near zero magnetic field) are particularly important. However, there is a four-order-of-magnitude gap between the current measurement accuracy under weak magnetic fields and that under strong magnetic fields. To bridge this gap, the Cold Molecular Ion Team of APM used the ion trap trapping electrode as a microwave antenna, and realized the ground-state hyperfine magnetically insensitive transition of beryllium-9 ions under weak magnetic fields. By measuring the spectral line of the hyperfine transition frequency varying with the magnetic field, the ground-state hyperfine constant, which does not depend on any physical constants or beryllium ion structural parameters, is obtained through fitting. Its accuracy is improved by one order of magnitude compared with existing results under weak magnetic fields.

This research achievement was published in Physical Review A under the title "Precision measurement of the ground-state hyperfine constant for ⁹Be⁺ in a linear Paul trap via magnetically insensitive hyperfine transitions". AO Zhiyuan and BAI Wenli, PhD candidates from APM, are the co-first authors, while assistant researcher PENG Wencui and researcher TONG Xin are the co-corresponding authors.

This research work was funded by the Ministry of Science and Technology of the People's Republic of China and the National Natural Science Foundation of China.

Link to the article: http://journals.aps.org/pra/abstract/10.1103/4b8w-pxt7