Speaker
Description
Mixed-species Coulomb crystal operation can reduce both statistical and systematic uncertainties in optical clocks. Increased clock ion numbers provide a lower quantum projection noise limited instability and are the prerequisite for various advanced interrogation schemes, while auxiliary ions can be used for sympathetic cooling and systematic shift characterization.
We operate a clock based on a linear chain of $^{115}$In$^+$ (clock) and $^{172}$Yb$^+$ (auxiliary) ions, which we have identified as a favorable system for high-accuracy multi-clock-ion operation [1], capable of reaching a fractional systematic uncertainty below $1\times10^{−19}$ [2].
With an evaluated uncertainty of $2.5\times10^{−18}$, the clock has participated in the first inter-species frequency ratio measurement [3] to fulfill the $< 5 \times10^{−18}$ requirement stated by the CCTF roadmap toward the redefinition of the second [4].
The instability in multi-clock-ion operation was initially limited by unfavorable scaling of dead time due to a state preparation scheme based on spontaneous decay [5]. We have therefore implemented a repumping step using the $^3\mathrm{P}_0$ to $^1\mathrm{P}_1$ M1 transition at 482 nm, which reduces the effective $^3\mathrm{P}_0$ lifetime to 3.5 ms. The instability now follows the expected $1/\sqrt{N}$ scaling with ion number $N$ and reaches $\sigma_y = 6(2) \times 10^{−16}/\sqrt{t}$ in operation with 4 In$^+$ ions.
[1] N. Herschbach et al., Appl. Phys. B 107, 891 (2012)
[2] J. Keller et al., Phys. Rev. A 99, 013405 (2019)
[3] H. N. Hausser et al., Phys. Rev. Lett. 134, 023201 (2025)
[4] N. Dimarcq et al., Metrologia 61, 012001 (2024)
[5] J. Keller et al., J. Phys.: Conf. Ser., 2889, 01205 (2024)
