We measured the ground-state hyperfine splitting of trapped 113 C d + ions to be 15199862855.02799(27) Hz with a fractional uncertainty of 1.8 × 10 − 14 . The ions were trapped and laser-cooled in a linear quadrupole Paul trap. The fractional frequency stability was measured to be 4.2 × 10 − 13 / τ , obtained from Ramsey fringes of high signal-to-noise ratios and taken over a measurement time of nearly 5 h, which is close to the short-term stability limit estimated from the Dick effect. Our result is consistent with previously reported values, but the measurement precision is four times better than the best result obtained to date.

We report on the development of a microwave frequency standard based on a laser-cooled 171 Y b   +   ion trap system. The electronics , lasers, and magnetic shields are integrated into a single physical package. With over 105 ions are stably trapped, the system offers a high signal-to-noise ratio Ramsey line-shape. In comparison with previous work, the frequency instability of a 171 Y b   +   microwave clock was further improved to 8.5 × 10 − 13 / τ for averaging times between 10 and 1000 s. Essential systematic shifts and uncertainties are also estimated.

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