Researchers have recently developed a method to prepare the ionic state using multi-pulse operations in a linear ion trap, enabling the measurement of the ground-state hyperfine magnetically insensitive transition of beryllium ions.

The method has successfully improved the measurement accuracy of the ground-state hyperfine constant of beryllium ions under weak magnetic fields by one order of magnitude.

Led by the Cold Molecular Ion Research Team of the Innovation Academy for Precision Measurement Science and Technology (APM) of the Chinese Academy of Sciences (CAS), the research has been 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 Research Team of APM used the ion trap electrode as a microwave antenna to realize 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 as a function of the magnetic field, the ground-state hyperfine constant, which does not depend on any physical constants or beryllium ion structural parameters, was obtained through fitting. The measurement accuracy was improved by one order of magnitude compared with previous results obtained under weak magnetic fields.

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.