Direct comparison of a Ca + single-ion clock against a Sr lattice clock to verify the absolute frequency measurement

Optical frequency comparison of the 40Ca+ clock transition νCa (2S1/2-2D5/2, 729nm) against the 87Sr optical lattice clock transition νSr (1S0-3P0, 698nm) has resulted in a frequency ratio νCa / νSr = 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of frequency ratio νCa / νSr to reach 1 × 10−15 in 1000s and yielded a value consistent with that calculated from separate absolute frequency measurements of νCa using the International Atomic Time (TAI) link. The total uncertainty of the frequency ratio using optical comparison (free from microwave link uncertainties) is smaller than that obtained using absolute frequency measurement, demonstrating the advantage of optical frequency evaluation. We note that the absolute frequency of 40Ca+ we measure deviates from other published values by more than three times our measurement uncertainty.