锶原子光晶格钟自旋极化谱线的探测

87Sr原子存在核自旋,在磁场作用下原子能级会分裂成不同塞曼子能级.通过光抽运对原子进行自旋极化,其自旋极化谱线的探测为锶光钟系统的闭环锁定提供精确的频率参考.本文对~(87)Sr原子钟跃迁能级5s~2~1S_0→5s5p~3P_0中的m_F=+9/2和m_F=-9/2的塞曼磁子能级自旋极化谱线进行了探测.经过一级宽带冷却和二级窄线宽冷却与俘获后,锶冷原子温度为3.9μK,原子数目为3.5×10~6.利用邻近"魔术波长"的813.426 nm半导体激光光源实现水平方向的一维光晶格装载.采用归一化探测方法用线宽为Hz量级的698 nm钟激光对~1S_0→~3P_0偶极禁戒跃迁进行探测,在150 ms的探测时间下获得线宽为6.7 Hz的钟跃迁简并谱.在磁光阱竖直方向施加一个300 mGs的偏置磁场获得塞曼分裂谱,并通过689 nm的圆偏振自旋极化光进行光抽运,最终在探测时间为150 ms时,获得左右旋极化谱线线宽分别为6.2 Hz和6.8 Hz.

Interrogation of spin polarized clock transition in strontium optical lattice clock

We demonstrate a spin-polarized clock transition spectrum of the ⁸⁷Sr optical lattice clock. The clock transition 5s^{2 1}S₀→5s5p ³P₀ of isotope ⁸⁷Sr has a hyperfine structure due to non-zero nuclear spin, inducing ten π-polarized transitions from each individual m_F state under the condition of a bias magnetic field along the probing polarization axis. In this experiment, atoms are driven to a certain m_F state by a circular-polarization pump light to maximize the atomic population, which is beneficial to the stability and uncertainty evaluation of the optical lattice clock. After two stages cooling and trapping, about 3.5×10⁶ atoms are trapped in the red magneto-optical trap with a temperature of 3.9 μK. A grating-feedback external cavity diode laser with a tapered amplifier is used to build the optical lattice with a “magic-wavelength” of 813.426 nm. Both waists of the counter-propagating lattice beam along the horizontal direction are overlapped to form a one-dimensional (1D) optical lattice. The lifetime of the atoms trapped in the 1D optical lattice is 1600 ms. The clock laser at 698 nm is a grating-feedback diode laser, which is locked to an ultra-low expansion cavity by the Pound-Drever-Hall technique to stabilize the frequency and phase. As a result, the linewidth of clock laser is narrowed to Hz level. By the normalized shelving method, we obtain a resolved sideband spectrum of ⁸⁷Sr 5s^{2 1}S₀→5s5p ³P₀ transition. According to the spectrum, the lattice temperature along the longitudinal direction is approximately 4.2 μK. After that a linewidth of 6.7 Hz of the degenerate clock transition is obtained at a probing time of 150 ms by utilizing a three-dimensional (3D) bias magnetic field, which is used to eliminate the stray magnetic fields. Then a small bias magnetic field of 300 mGs is applied along the polarization axis of the lattice light to achieve the spectrum of Zeeman magnetic sublevels of the clock transition. Furthermore, the m_F=+9/2 and m_F=-9/2 magnetic sublevels are picked to be respectively pumped by the σ⁺-polarized and σ^--polarized light at 689 nm, a variable liquid crystal wave plate is employed to switch on both polarizations. Finally, the spin polarized clock transition spectrum is obtained at the interrogating pulse of 150 ms, and the linewidths of the m_F=+9/2, m_F=-9/2 magnetic sublevel transitions are 6.8 Hz and 6.2 Hz respectively.