European Conference on Trapped Ions (ECTI)

Optical clocks based on mixed-species Coulomb crystals promise reductions of both statistical and systematic uncertainties beyond the state of the art.

We operate an optical clock based on the combination of ${}^{115}$In${}^{+}$ (clock) and ${}^{172}$Yb${}^{+}$ (auxiliary) ions, which we have identified as a candidate for multi-clock-ion operation with ${10}^{-19}$ level systematic uncertainties [1,2].

Our approach uses short linear chains (~10 ions), in which the permutation of species is actively controlled to ensure efficient and reproducible sympathetic cooling conditions [3]. The systematic uncertainty is currently evaluated as $2.5\times {10}^{-18}$ for operation with a single In${}^{+}$ clock ion, which yields an instability of ${\sigma }_y=1.6\times {10}^{-15}/\sqrt{t}$ [4]. The clock has participated in local and international comparisons, and operation with up to four clock ions has been demonstrated.

Besides its use for sympathetic cooling, mixed-species operation also allows the reduction of systematic uncertainties. Fluorescence from the ${}^2$S${}_{1/2}$ to ${}^2$P${}_{1/2}$ cooling transition in Yb${}^{+}$ is used for excess micromotion compensation during clock operation. Uncertainties of the differential polarizability and the ${}^3$P${}_0$ $g$ factor of In${}^{+}$ can be reduced using interleaved interrogation of different transitions in the mixed-species system. These measurements can reduce the frequency uncertainty contributions due to black-body radiation and the 2nd-order Zeeman shift from their respective current values close to $1\times {10}^{-18}$ by more than an order of magnitude each.

[1] N. Herschbach et al., Appl. Phys. B 107, 891 (2012)
[2] J. Keller et al., PRA 99, 013405 (2019)
[3] T. Nordmann et al., in preparation
[4] H. N. Hausser et al., in preparation