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Here we present a liquid nitrogen-cooled Ca^+ optical clock with an overallsystematic uncertainty of 3*10^-18. In contrast with the room-temperature Ca^+optical clock that we have reported previously, the temperature of theblackbody radiation (BBR) shield in vacuum has been reduced to 82(5) K usingliquid nitrogen. An ion trap with a lower heating rate and improved coolinglasers were also introduced. This allows cooling the ion temperature to theDoppler cooling limit during the clock operation, and the systematicuncertainty due to the ion's secular (thermal) motion is reduced to < 1*10^-18.The uncertainty due to the probe laser light shift and the servo error are alsoreduced to < 1*10^-19 and 4*10^-19 with the hyper-Ramsey method and thehigher-order servo algorithm, respectively. By comparing the output frequencyof the cryogenic clock to that of a room-temperature clock, the differentialBBR shift between the two was measured with a fractional statisticaluncertainty of 7*10^-18. The differential BBR shift was used to calculate thestatic differential polarizability, and it was found in excellent agreementwith our previous measurement with a different method. This work suggests thatthe BBR shift of optical clocks can be well suppressed in a liquid nitrogenenvironment. This is advantageous because conventional liquid-helium cryogenicsystems for optical clocks are more expensive and complicated. Moreover, theproposed system can be used to suppress the BBR shift significantly in othertypes of optical clocks such as Yb^+, Sr^+, Yb, Sr, etc.

A liquid nitrogen-cooled Ca^+ optical clock with systematic uncertainty of 3*10^-18