Absolute Frequency Measurement of theCa+404sS1/22−3dD5/22Clock Transition

We report on the first absolute transition frequency measurement at the ${10}^{\\ensuremath{-}15}$ level with a single, laser-cooled $^{40}\\mathrm{Ca}^{+}$ ion in a linear Paul trap. For this measurement, a frequency comb is referenced to the transportable Cs atomic fountain clock of LNE-SYRTE and is used to measure the $^{40}\\mathrm{Ca}^{+}$ $4s\\text{ }^{2}S_{1/2}\\ensuremath{-}3d\\text{ }^{2}D_{5/2}$ electric-quadrupole transition frequency. After the correction of systematic shifts, the clock transition frequency ${\\ensuremath{\\nu}}_{{\\mathrm{Ca}}^{+}}=411\\text{ }042\\text{ }129\\text{ }776\\text{ }393.2(1.0)\\text{ }\\text{ }\\mathrm{Hz}$ is obtained, which corresponds to a fractional uncertainty within a factor of 3 of the Cs standard. In addition, we determine the Land\\'e $g$ factor of the $3d^{2}D_{5/2}$ level to be ${g}_{5/2}=1.200\\text{ }334\\text{ }0(3)$.