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Optical frequency comparison of the 40Ca+ clock transition \nu_{Ca}(2S1/2-2D5/2, 729nm) against the 87Sr optical lattice clock transition\nu_{Sr}(1S0-3P0, 698nm) has resulted in a frequency ratio \nu_{Ca} / \nu_{Sr}= 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowedthe statistical uncertainty of frequency ratio \nu_{Ca} / \nu_{Sr} to reach1x10-15 in only 1000s and yielded a value consistent with that calculated fromseparate absolute frequency measurements of \nu_{Ca} using the InternationalAtomic Time (TAI) link. The total uncertainty of the frequency ratio usingoptical comparison (free from microwave link uncertainties) is smaller thanthat obtained using absolute frequency measurement, demonstrating the advantageof optical frequency evaluation. We report the absolute frequency of ^{40}Ca+with a systematic uncertainty 14 times smaller than our previous measurement[1].

Direct comparison of a Ca+ single ion clock against a Sr optical lattice clock