Interrogation Of ¹S₀-³P₀ Forbidden Transition In Strontium Optical Lattice Clock

⁸⁷Sr is fermion,it has a hyperfine structure due to it’s nuclear spin is 9/2.The ⁸⁷Sr transition 5s^{2 1}S₀（F=9/2）→5s5p ³P₀（F=9/2）is the clock transition of optical lattice clock.Because of its ultra-narrow natural line width and super high quality factor(10¹⁷),this transition can be used as the clock transition energy level of the optical clock,providing a high precision frequency reference for the strontium optical lattice clock.The splitting of a spectral line into several components in the presence of a external magnetic field,and the corresponding ground state and excited state energy level can be divided into different magneton energy levels.In the experiment,we polarize all atoms by the method of optical pump.Atoms are drived to the corresponding-9/2 or+9/2 m_F states of the ground state with polarized pumping pulses.At this time,the normalized detection of the atom can obtain a the spin-polarized spectra of clock transitions.As the quantum reference system of the optical clock system,the spin-polarized spectra is the necessary condition for the closed-loop locking of the optical clock frequency.In this paper,the concrete process and corresponding principle of ⁸⁷Sr optical lattice clock are introduced,and the detailed description is written according to the detection process of the clock transition spectra.The specific work include:Firstly,we obtion the cold atoms of strontium by the laser cooling and trapping.After the two-dimensional collimation、zeeman deceleration、cooling and pump,the atomic number in the blue MOT is 2.3 x 10⁷,the temperature of cold atoms is 5 mK.After that,the two stage cooling atoms are obtained by 689 nm laser.The atomic temperature is 3.9μK and the atomic number is 3.5x10⁶.About 10⁴ atoms are loaded into an 1D optical lattice formed by lattice laser with the wavelength is 813.426 nm.The optical lattice life t>1600 ms was obtained by using the attenuation function for the data fitting of atomic fluorescence with different time delays.With the normalized shelving method,the clock transition spectra of ⁸⁷Sr 5s²¹S⁰→5s5p ³P₀ transition is detected and the resolved sideband spectra is obtained.Then eliminate the Zeeman effect of the detection of spectral line by the magnetic field compensation device.When probing time is 150 ms,we obtion the narrow spectrum of the clock transtion by reducing the power of the clock laser and scanning step,and the linewidth is 6.7 Hz.Then a compensation magnetic field of 300 mGs is added to MOT and the spectrum of Zeeman spectrum of the clock transition is achieved.Due to the atomic transition probability of zeeman spectrum of clock transtion is small,the atom utilization rate is low.By the method of optical pump,we polarize all atoms to the quantum state required for the experiment.Finally the spin-polarized spectrum of the clock transition is detected,the linewidths of spectral line corresponding to the left and right circular polarization are 6.8 Hz and 6.2 Hz respectively.When the probing time is 300 ms,the linewidth of spectral line is 3.9 Hz by the Lorentzian fit.Adopt the method of bilateral lock,we complete closed-loop locking of strontium optical clock frequency.Finally,when the averaging time is 3000 s,the frequency instability of ⁸⁷Sr optical lattice clock is 5.7×10^-17.