Progress towards a 40Ca+ optical clock with a fractional uncertainty at the 10−18 level

We recently reported a 5×10
^-17
level frequency comparison of two
⁴⁰
Ca
⁺
optical clocks, one of which has fractional frequency uncertainty of 3.4×10
^-17
, limited by the BBR effect. Here we present progress towards a
⁴⁰
Ca
⁺
optical clock with uncertainty at the 10
^-18
level. The differential static scalar polarizability Δα0 of the
⁴⁰
Ca
⁺
ion clock transition is obtained by precisely measuring the “magic” rf drive frequency. Δα0 is measured as -7.2653(44)x10
^-40
J m
²
/V
²
, the blackbody radiation shift is then calculated to be 0.3790(2) Hz at 300 K. The contribution of the blackbody shift coefficient to the uncertainty of the optical clock at room temperature is reduced to 5.6×10
^-19
, the excess micromotion induced clock uncertainty is also reduced to the 10
^-19
level by choosing the appropriate trap drive frequency. With a suppression of the BBR field temperature uncertainty to the sub-Kelvin level and improved clock stability, a robust, relatively low-cost
⁴⁰
Ca
⁺
ion clock with uncertainty at 10
^-18
level can be made.