Towards a composite ¹⁷¹Yb⁺- ⁸⁸Sr⁺ optical clock

The yearlong excited state lifetime of the electric octupole (E3) transition of ¹⁷¹Yb⁺ promises very long coherent laser interrogation. Motional heating of the trapped ions and laser noise, however, limit the resolvable linewidth and consequently the frequency stability of this clock transition so far. An ancillary ⁸⁸Sr⁺ ion can provide sympathetic cooling of ¹⁷¹Yb⁺ during the interrogation on the E3 transition without introducing major frequency shifts from the sideband cooling lasers. Another important limitation for long coherent interrogation results from collisions of the reference ion with background gas molecules. To estimate the background pressure at the ion position, we determine the reorder rate of two-species ion crystals. In addition to its use as a cooling partner, the co-trapped ⁸⁸Sr⁺ ion can be employed to in situ determine the thermal radiation field causing the largest frequency shift for most optical clocks at room temperature. In preparation for the combined clock operation, we measure the frequency ratio of the ⁸⁸Sr⁺ and the ¹⁷¹Yb⁺ E3 clock transitions with 3*10^-16 uncertainty.