Suppression of servo error uncertainty to 10^(-18)level using double integrator algorithm in ion optical clock

A universal locking model for single ion optical clocks was built based on a simple integrator and a double integrator.Different integrator algorithm parameters have been analyzed in both numerical simulations and experiments.The frequency variation measured by the comparison of two optical clocks coincides well with the simulation results for different second integrator parameters.According to the experimental results,the sensitivity of the servo error influenced by laser frequency drift with the addition of a double integrator was suppressed by a factor of 107.In a week-long comparison of optical clocks,the relative uncertainty of the servo error is determined to be 1.9×10^(-18),which is meaningful for the systematic uncertainty of the transportable single^(40)Ca^(+)ion optical clock entering the 10^(-18)level.

Progress on the^40Ca^+ion optical clock

Progress of the ^40Ca^+ion optical clock based on the 4^2S1/2-3d ^2D5/2 electric quadrupole transition is reported.By setting the drive frequency to the“magic”frequencyΩ0,the frequency uncertainty caused by the scalar Stark shift and second-order Doppler shift induced by micromotion is reduced to the 10^-19 level.By precisely measuring the differential static scalar polarizability∆α0,the uncertainty due to the blackbody radiation(BBR)shift(coefficient)is reduced to the 10^-19 level.With the help of a second-order integrating servo algorithm,the uncertainty due to the servo error is reduced to the 10^-18 level.The total fractional uncertainty of the ^40Ca^+ion optical clock is then improved to 2.2×10^-17,whereas this value is mainly restricted by the uncertainty of the BBR shift due to temperature fluctuations.The state preparation is introduced together with improvements in the pulse sequence,and furthermore,a better signal to noise ratio(SNR)and less dead time are achieved.The clock stability of a single clock is improved to 4.8×10^-15√τ(in seconds).