Direct comparison of a Ca+ single ion clock against a Sr optical lattice clock
Abstract
Optical frequency comparison of the 40Ca+ clock transition Ca (2S1/2-2D5/2, 729nm) against the 87Sr optical lattice clock transition Sr(1S0-3P0, 698nm) has resulted in a frequency ratio Ca / Sr = 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of frequency ratio Ca / Sr to reach 1x10-15 in only 1000s and yielded a value consistent with that calculated from separate absolute frequency measurements of Ca 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 report the absolute frequency of 40Ca+ with a systematic uncertainty 14 times smaller than our previous measurement [1].
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