Improvement in accuracy of a single 40Ca+ optical clock towards the 10−15 level using a magnetic shield

We report a 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 line broadening due to the magnetic field fluctuation, the use of a magnetic shield significantly reduced the spectrum linewidth of the clock transition. By a frequency comparison to an ⁸⁷Sr lattice clock, an Allan deviation of 7 × 10^-16 was observed at an averaging time of 10³ seconds. We preliminarily measured the clock transition frequency to be 411 042 129 776 397 (3) Hz with respect to the SI definition of the second. Further measurements are executed aiming at an accuracy of low parts in the 10^-15 level.