Toward a Transportable Ca+ Optical Clock with a Systematic Uncertainty of 4.8 × 10 ⁻¹⁸

We present the compact long-term nearly continuous operation of a room-temperature ${\mathrm{Ca}}^{+}$-optical-clock setup towards a transportable clock, achieving an overall systematic uncertainty of 4.8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}18}$ and an uptime rate of 97.8% over an 8-day period. The active liquid-cooling scheme is adopted, combined with precise temperature measurement with 13 temperature sensors both inside and outside the vacuum chamber to ensure the accurate evaluation of the thermal environment for the optical clock. The environmental temperature uncertainty is evaluated as 293.31(0.4) K, corresponding to a blackbody-radiation frequency-shift uncertainty of 4.6 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}18}$, which is reduced more than twice compared to our previous work. Through frequency comparison of the room-temperature ${\mathrm{Ca}}^{+}$ optical clock with a cryogenic ${\mathrm{Ca}}^{+}$ optical clock, the overall uncertainty of the clock comparison is 7.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}18}$, including a statistical uncertainty of 4.9 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}18}$ and a systematic uncertainty of 5.7 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}18}$. This work provides a set of feasible implementations for high-precision transportable ion optical clocks.