Experimental Determination (～mHz) of the Ground-State Hyperfine Separation of Trapped ¹⁹⁹Hg⁺ in a Hyperbolic Paul Trap

The Ramsey fringes on the ground-state hyperfine transition of ¹⁹⁹Hg⁺ (ΔF=1, Δm_F=0) ions trapped in a hyperbolic Paul trap are firstly observed with the method of time-separated oscillatory fields. The full width at half maximum of central Ramsey fringes is about 25 mHz and the corresponding quality factor Q of the line is greater than 10¹² for the trapped ¹⁹⁹Hg⁺ microwave frequency standard. The hyperfine transition frequency shifted by magnetic field is also measured by the high-resolution Ramsey fringes. The final result is Δν_hfs=40507347997.3(0.5) Hz, which is corrected to zero magnetic field.