${}^{88}\mathrm{Sr}^{+}$ 445-THz Single-Ion Reference at the ${10}^{-17}$ Level via Control and Cancellation of Systematic Uncertainties and Its Measurement against the SI Second

We describe experiments and measurements on a trapped and laser-cooled single ion of ${}^{88}\mathrm{Sr}^{+}$ which, when probed on its reference $5s{}^{2}S_{1/2}\to 4d{}^{2}D_{5/2}$ transition at 445 THz, provides an optical frequency standard of evaluated accuracy outperforming the current realization of the SI second. Studies are presented showing that micromotion-associated shifts of the standard can be reduced to the ${10}^{-18}$ level and uncertainties in the blackbody-induced shifts for the current system are at the low ${10}^{-17}$ level due to the relatively well-known polarizability of the strontium ion system and careful choice of the trap structure. The current evaluated systematic shifts for the ion transition are at a fractional uncertainty of $2\times {10}^{-17}$. An absolute frequency measurement performed over a two-month period relative to a maser referenced to the SI second via Global Positioning System time transfer has determined the center frequency for the transition at ${\nu }_{SD}=444779044095485.5\pm 0.9\mathrm{Hz}$ ($1\sigma$).

Figure 1

Summary of absolute frequency measurements of $S\mathrm{\text{−}}D$ transition during a two-month period showing weighted mean value with maser uncertainty included (circle) and comparison with measurement performed in Ref 7 (pentagon). The dates of measurement for the individual runs are given on the horizontal axis using the modified Julian day subtracted by 55970.