Fig. 3

Typical spectrum of the |i〉 − |f〉 transition. Average of the ion signals and its standard deviations of mean (shown in black) are plotted against the microwave frequency. The total number of original data points used to make this plot is ~95,000, which corresponds to 6 h of data collection. To find the central frequency, the spectrum is fitted to a Gaussian function (magenta curve). Subtracting the Gaussian function from the ion counts gives the clear view of the residuals. The full-width-of-half-maximum (FWHM) of the spectrum is about 50 Hz, which is limited by the microwave pulse duration of 16 ms