Improved measurement of the 40Ca+ clock transition frequency towards an uncertainty of 10−15 level

We report improved frequency measurement of a single ⁴⁰Ca⁺ optical clock towards the 10^-15 uncertainty level. In contrast with our previous measurement, in which the accuracy was limited by the magnetic field fluctuation, the use of a magnetic shield significantly reduced the observed linewidth of its ²S_1/2-²D_5/2 clock transition. From frequency comparison between the ⁴⁰Ca⁺ clock and an ⁸⁷Sr lattice clock, an Allan deviation of 7 × 10^-16 at an averaging time of one thousand seconds was measured. We now measure the absolute frequency of the ⁴⁰Ca⁺ clock transition with respect to the SI definition of the second.