Fig. 1. Timing control sequence for (a) Ca+ optical clock and (b) Sr optical lattice clock.

Fig. 2. (a) Schematic of the experiment setup. AOM, acoustic-optic modulator; NCF, noise canceled fiber link; DDS, direct digital synthesizer; OFD, optical frequency divider. The red straight line and the black dashed line stand for the optical path and electric control path, respectively. (b) Noise cancelation scheme for frequency noise of the comb used in OFD. The method to generate the optically referenced time base fCLK is also shown on the right side.

Fig. 3. Measured Rabi spectra of the Ca+ clock transition with 100 times averaging at each frequency point. The probe time is 160 ms (green circles), 320 ms (blue circles), and 640 ms (purple dots), respectively. The dashed lines and shaded regions are the corresponding fits to the measured data using the Rabi model and their 1−σ confidence intervals.

Fig. 4. Overlapping Allan deviation of the frequency ratio measurement. The LO of Ca+ clock is frequency-synthesized from the Sr clock by OFD (red dots and blue squares for Tp=80  ms and 160 ms, respectively). The dashed lines are weighted 1/τ fits to the measured frequency ratio data for an averaging time greater than 100 s. Error bars indicate 1−σ confidence intervals. A previous result with independent LOs for the Ca+ and Sr clocks is shown for reference (purple circles) [36].

Fig. 5. Frequency instability of a single Ca+ clock measured by the time-interleaved self-comparison method with a probe time of 80 ms (red triangles), 160 ms (purple dots), and 320 ms (blue squares). The LO of the Ca+ clock is frequency-synthesized from the cavity-stabilized 698 nm laser with the OFD. The dashed lines are the weighted 1/τ fits to Tp=80  ms, 160 ms, and 320 ms data for an averaging time greater than 100 s. Error bars indicate 1−σ confidence intervals.