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

We report improved frequency measurement of a single
⁴⁰
Ca
⁺
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
²
S
_1/2
-
²
D
_5/2
clock transition. From frequency comparison between the
⁴⁰
Ca
⁺
clock and an
⁸⁷
Sr 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
⁴⁰
Ca
⁺
clock transition with respect to the SI definition of the second.