Development And Evaluation Of The Strontium Optical Lattice Clock

The time unit second is the basic unit with the highest measurement accuracy among the seven basic units of the International System of units（SI）.Currently,the cesium atomic fountain clock is used to define the SI second,but its systematic uncertainty is only at the level of E-16,which gradually no longer meets the demand of scientific research.At present,the most accuracy of the optical lattice clock based on neutral atoms has reached the magnitude of E-19,nearly three orders of magnitude higher than the primary cesium fountain clock,so it is expected to become the next generation of time and frequency standards.In order to cope with the changes in the definition of SI seconds in the future,the National Timing Service Center must develop our optical frequency primary clock,so as to ensure that China’s time and frequency reference is always independent,always in line with international standards and always in continuous operation.The strontium atom has two valence electrons in the outermost shell and abundant energy levels and transitions to meet the needs of the optical clock experiment.The forbidden transition（5s²）¹S₀-（5s5p）³P₀,with extremely narrow natural line-width（1 mHz）and ultra-high quality factor（E-17 magnitude）,is an important candidate for optical frequency reference.The working principle of the strontium optical clock is:the cold atoms trapped in the magic wavelength optical lattice as reference frequency of optical clock is detected by the 698 nm clock laser which is locked on an ultra-stable reference cavity.The clock transition spectrum is used to lock the clock laser using feedback control system.The femtosecond optical frequency comb as accurate frequency counter used to read the frequency of the clock laser and translate into microwave signal as the time and frequency standard for practical application.This paper focuses on the development and evaluation of the strontium optical lattice clock of the National Timing Service Center.The main experimental works are:cold atom sample preparation,clock transition spectrum detection and closed-loop locking,stability and uncertainty evaluation and absolute frequency measurement.（1）Cold atom sample preparation:the primary cooling and secondary cooling of strontium atoms were performed by magneto-optical trap technique,and the final atomic number was 2.3×10⁶,and the temperature was about 5μK;Using the standard PDH technology,the 813 nm semiconductor laser was locked into a super-stable reference cavity with a finesse of 30000,and the lifetime of the atoms loaded into optical lattice is 1.6 s;Using the light ofσ^-orσ⁺to pump the atoms populated on all ten Zeeman level of the ground state into the state of m_F=+9/2 or m_F=-9/2,the maximum excitation rate and signal-to-noise ratio of the clock transition spectra are improved by about 7 times and more than 90%of the atoms are pumped into the state of m_F=+9/2 or m_F=-9/2.（2）Clock transition spectrum detection and closed-loop locking:the influence of optical fiber phase noise on the clock laser frequency is reduced to E-17 level,its two orders of magnitude higher than the stability of the clock laser.The line-wide of the sideband resolved spectrum,degenerate spectrum,Zeeman spectrum and spin polarization spectrum detected experimentally are close to the Fourier-limited Rabi line-width.The longitudinal trap temperature of the optical lattice was calculated to be4.2μK based on the sideband resolved spectrum,and the narrowest line-width of the spin polarization spectrum of 3.9 Hz was obtained.The closed loop operation of strontium optical lattice clock was realized by using the polarization spectrum with a line width of 6 Hz,and the linear drift rate of the clock laser was measured to be0.1794 Hz/s.The linear drift of the clock laser is compensated by the acousto-optic modulator,and the in-loop stability is calculated according to the error signal after the closed-loop locking.The Allen deviation variance basically follows the trend of1.6×10^-15/τ^1/2,and reaches 2.8×10^-17 after the averaging times of 2000 s.（3）Strontium optical lattice clock performance evaluation:the stability and systematic uncertainty of strontium optical clock were evaluated by self-comparison method;The calibrated platinum resistance temperature sensor was used to monitor the external surface temperature of the vacuum cavity,and the temperature at the position of the atomic was simulated by the method of finite element analysis.Finally,the total correction of blackbody radiation frequency shift was-2.13（1）Hz with an uncertainty of 2.4×10^-17.The collision frequency shift correction is-0.13 Hz and the uncertainty is 3.1×10^-17.The theoretical calculation shows that the collision frequency shift of p-wave is at least 15 times that of s-wave.By controlling the power of the lattice laser and measuring the frequency shift of the AC Stark of the lattice laser,the obtained magic wavelength of the optical lattice was 368554042 MHz,and the relative uncertainty was 8.0×10^-17.In the actual closed loop experiment,the frequency shift caused by the operation wavelength of the lattice laser was 0.8 Hz,and the total uncertainty was 8.7×10^-17.By measuring the frequency difference between the two polarization peaks after closed loop operation,the second-order Zeeman shift correction of strontium optical clock is 4.1×10^-17,and the uncertainty is 1.8×10^-18.The SI second definition traceability scheme for the absolute transition frequency measurement of strontium optical clock is designed,which can be used for the next step of the absolute transition frequency measurement of the strontium optical clock.