In+115−Yb+172 Coulomb Crystal Clock with 2.5×10−18 Systematic Uncertainty

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We present a scalable mixed-species Coulomb crystal clock based on the S01↔P30 transition in In+115. Yb+172 ions are cotrapped and used for sympathetic cooling. Reproducible interrogation conditions for mixed-species Coulomb crystals are ensured by a conditional preparation sequence with permutation control. We demonstrate clock operation with a 1In+−3Yb+ crystal, achieving a relative systematic uncertainty of 2.5×10−18 and a relative frequency instability of 1.6×10−15/τ/1 s. We report on absolute frequency measurements with an uncertainty of 1.3×10−16 and optical frequency comparisons with clocks based on Yb+171 (E3) and Sr87. With a fractional uncertainty of 4.4×10−18, the former is—to our knowledge—the most accurate frequency ratio value reported to date. For the In+115/Sr87 ratio, we improve upon the best previous measurement by more than an order of magnitude. We also demonstrate operation with four In+115 clock ions, which reduces the instability to 9.2×10−16/τ/1 s. Published by the American Physical Society 2025