Development And Application Of A Transportable ⁴⁰Ca⁺ Ion Optical Clock

The second of International system of Units(SI)is defined in the microwave frequency transition of the hyperfine structure of cesium atom,and the SI second is realized by the reproduction of several cesium fountain clocks,which are developed and maintained by laboratories in several countries around the world.The clock transition frequency of optical clock is 4-5 orders of magnitude higher than that of microwave clock.Compared with microwave clock,optical clock can have higher uncertainty and stability.Up to now,some optical clocks in the world are already 2 orders of more accurate in uncertainty and stability than the best cesium fountain clocks,and the future definition of the second may be defined on the optical frequency transition of optical clocks.To this end,we developed a transportable 40Ca+ion optical clock,and cooperated with National Institute of Metrology,China to measure the absolute frequency of 40Ca+ ion optical clock(frequency trace back to cesium fountain clock and SI second respectively)and compare the frequency of 40Ca+ ion optical clock with 87Sr atomic optical lattice clock.The main progresses achieved are as follows:1.A transportable 40Ca+ion optical clock with an uncertainty of 1.3 ×10-17 and a stability of 5×10-15/(?)was designed and established.The clock can operate stably from-20℃ to 40℃ outdoors.2.The frequency of the transportable 40Ca+ ion optical clock was compared with that of the laboratory 40Ca+ ion optical clock,and the gravitational redshift was measured to verify that the optical clock has the height resolution of decimeter level.3.The transportable 40Ca+ion optical clock was transported from Wuhan to Beijing National Institute of Metrology,China(NIM)by trailer.In cooperation with NIM,the frequency of the clock was traced to the international system of units(SI)second directly through UTC(NIM).The absolute frequency Jeasurement value of the 40Ca+ ion optical clock is 411042 129 776 400.41(23)Hz,and the relative uncertainty is 5.6×10-16).The absolute frequency measurement value of the 40Ca+ ion optical clock is measured to the level of E-16 for the first time.The measurement value is adopted by the Consultative Committee for Time and Frequency(CCTF)in March 2021 and this radiation is endorsed by CCTF as secondary representation of the definition of the second(SRS).4.In collaboration with East China Normal University(ECNU)and National Institute of Metrology,China(NIM),the stability of 698nm clock laser of 87Sr optical lattice clock is transferred to the 40Ca+ion optical clock at 729nm by means of the Transfer Oscillator method.The spectral linewidth of clock transition for 40Ca+ ion optical clock is 1.4 Hz and the stability of the transportable 40Ca+ ion optical clock is also improved to 1.4 × 10-15/(?).