Precision measurement of D₂ line transition in a single trapped ²⁵Mg⁺ ion

Precision spectroscopy measurement plays a prominent part in the study of fundamental physical constants and atomic structure. In quasar absorption spectra, Mg⁺ ion doublet lines act as "anchor line" in the many multiplet method to find the time variation of fine structure constant α [1]. With the precision frequency data of Mg⁺ ions, we can get the isotope shifts of the three isotopes (mass number: 24, 25 and 26) and the hyperfine structure constants of ²⁵Mg⁺.

We employ the decoherence-assisted method [2] to obtain the D₂ hyperfine transitions of a single ²⁵Mg⁺ ion. The hyperfine transitions (|²S_1/2, m_F=0 > to| ²P_3/2, m_F=0>) we measured are insensitive to magnetic field and the branching ratios of the spontaneous decay. And the ion is trapped in a linear Paul trap and cooled to the Doppler cooling limit to reduce the inhomogeneous broadening. To minimize the influence of the laser linewidth, the spectroscopy laser is locked to an optical frequency comb that is referenced to a Hydrogen maser, and the frequency of the locked laser is measured by another frequency comb. In addition, we compensate the polarization of the spectroscopy laser to avoid undesired transitions in the D₂ line. Finally, we get symmetric Voigt line profile with high signal-to-noise ratios by preparing the ²⁵Mg⁺ ion to different initial state. With these hyperfine transitions, we obtain a preliminary D₂ transition frequency of 1072084555.5 MHz with an uncertainty of 0.6 MHz, which is 8 times better than the result of G. Clos et al. [2].