Direct frequency comparison between a single Ca+ clock and a Sr lattice clock

Direct comparison between a single ⁴⁰Ca⁺ clock and an Sr lattice clock has resulted in a frequency ratio of 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of the frequency ratio Ca⁺/Sr to be 1 × 10-¹⁵ in 1000 s and the value is consistent with that calculated from separate frequency measurements of Ca⁺ using the International Atomic Time (TAI) link. The total uncertainty of the measured ratio is smaller than that obtained at the absolute frequency measurement. The absolute frequency of Ca⁺ we measured deviates from other published values by more than three times our measurement uncertainty.