$^{115}$In$^+$-$^{172}$Yb$^+$ Coulomb crystal clock with $2.5\times10^{-18}$ systematic uncertainty

TL;DR

This paper reports a highly precise mixed-species Coulomb crystal optical clock using indium-115 and ytterbium-172 ions, achieving record low systematic uncertainty and improved frequency ratio measurements compared to previous work.

Contribution

The authors develop a scalable mixed-species Coulomb crystal clock with systematic uncertainty of 2.5×10⁻¹⁸, demonstrating advanced interrogation techniques and improved frequency ratio accuracy.

Findings

Achieved a systematic uncertainty of 2.5×10⁻¹⁸.

Reported the most accurate frequency ratio between $^{115}$In$^+$ and $^{87}$Sr.

Demonstrated clock operation with four $^{115}$In$^+$ ions, reducing instability.

Abstract

We present a scalable mixed-species Coulomb crystal clock based on the $^1S_0$ $\leftrightarrow$ $^3P_0$ transition in $^{115}$In$^+$. $^{172}$Yb$^+$ 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 $2.5\times10^{-18}$ and a relative frequency instability of $1.6\times10^{-15}/\sqrt{\tau/1\;\mathrm{s}}$. We report on absolute frequency measurements with an uncertainty of $1.3\times10^{-16}$ and optical frequency comparisons with clocks based on $^{171}$Yb$^+$ (E3) and $^{87}$Sr. With a fractional uncertainty of $4.4\times10^{-18}$, the former is - to our knowledge - the most accurate frequency ratio value reported to date. For the $^{115}$In$^+$/$^{87}$Sr ratio, we improve upon the best previous measurement by more than an order of magnitude. We also demonstrate operation with four $^{115}$In$^+$ clock ions, which reduces the instability to $9.2\times10^{-16}/\sqrt{\tau/1\;\mathrm{s}}$.