Reducing statistical noise in frequency ratio measurement between Ca⁺ and Sr optical clocks with a frequency-synthesized local oscillator from a Sr optical clock

Optical frequency ratio measurement between optical atomic clocks is essential to precision measurement as well as the redefinition of the second. Currently, the statistical noise in frequency ratio measurement of most ion clocks is limited by the frequency instability of ion clocks. In this work, we reduce the statistical noise in the frequency ratio measurement between a transportable Ca^≁ optical clock and a Sr optical lattice clock down to - 2.2 × 10⁻¹⁵∕pτ. The local oscillator of the Ca^≁ optical clock is frequency-synthesized from the Sr optical lattice clock, enabling a longer probe time for Ca^≁ clock transition. Compared to previous measurement using independent local oscillators, we achieve 10-fold reduction in comparison campaign duration.