115In+-172Yb+ Coulomb crystal clock with 2.5×10-18 systematic uncertainty
Abstract
We present a scalable mixed-species Coulomb crystal clock based on the 1S0 3P0 transition in 115In+. 172Yb+ ions are co-trapped 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 171Yb+ (E3) and 87Sr. 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 115In+/87Sr ratio, we improve upon the best previous measurement by more than an order of magnitude. We also demonstrate operation with four 115In+ clock ions, which reduces the instability to 9.2×10-16/τ/1\;s.