利用双色Mach-Zehnder干涉仪实现镱光钟跃迁几率的非破坏测量

Optical lattice clocks based on neutral atoms currently have the best stability and accuracy. The optical lattice clocks can interrogate a large number of atoms simultaneously, and thus have very low quantum projection noise limits. However, due to the Dick effect, the frequency stability of most optical clocks is far above this limit. The Dick effect can be suppressed by reducing the laser noise, which is technically very challenging, or by increasing the clock duty cycle, defined as the ratio of the interrogation time to the total cycle time. A nondestructive detection of transition probability can significantly increase the duty cycle by avoiding the loss of atoms and reusing the atoms for multiple clock cycles. .This project aims at the nondestructive detection of transition probability for an Ytterbium (Yb) clock and provides a new nondestructive detection scheme. The main idea relies on the differentially extraction of the phase shifts of two near-resonance lights induced by the atoms in the ground state, by using a two-color Mach-Zehnder interferometer. The innovation of our scheme lies in the cancellation of the locking process of the pass-length differences between the two arms of the interferometer. Besides, the phase shift signal is completely immune to external phase noises caused by vibration, temperature changes, and the like. These features make our method very robust and reliable. Compared to the scheme based on single probe beam, our strategy also has the following advantages: lower noise level, shorter detection duration, weaker heating effect, and higher signal-to-noise ratio. The application of this nondestructive measurement scheme in our Yb clock will significantly increase the operational duty cycle and sufficiently suppress the Dick effect, thus making the frequency stability of our clock approach the quantum projection noise limit.

中性原子晶格光钟目前具有最好的频率稳定度和不确定度。晶格光钟可以同时探询大量原子，因此具有极低的量子投影噪声极限。然而由于Dick效应的限制，目前大多数原子光钟的频率稳定度还远高于这一极限。利用非破坏性的探测方式进行钟跃迁几率的测量可以实现冷原子的重复利用，从而提高探询占空比，有效抑制Dick效应。本项目针对镱光钟跃迁几率的测量，提出一种新的非破坏探测方案：利用双色Mach-Zehnder干涉仪差分提取原子对两束近共振探测光产生的相移。项目的最大特色是在探测过程中无需锁定干涉仪两臂的相位差，且相移信号对外部相位扰动完全免疫。因此我们的方案具有很高的鲁棒性和可靠性。相比使用单束探测光的方案，我们的方案还具有以下优势：更低的噪声水平、更短的探测时间、更弱的加热效应及更高的信噪比。本项目的成功实施将大幅提高镱光钟的探询占空比，充分抑制Dick效应，使镱光钟的频率稳定度接近量子投影噪声极限。

原子晶格光钟可以同时探询大量原子，具有极低的量子投影噪声极限。然而由于Dick效应的限制，目前大多数原子光钟的频率稳定度还远高于这一极限。提高光钟的占空比是抑制Dick效应的有效方法。当前光钟跃迁几率的测量是通过对基态原子进行荧光探测来实现的，这种破坏性的探测方式使得光钟在每个周期都需要花费很多时间用以制备原子，从而限制了占空比。因此，发展非破坏性的探测方式进行基态原子数的测量，可以实现冷原子的重复利用，从而大幅提高探询占空比和有效抑制Dick效应。.本项目旨在通过使用双色Mach-Zehnder干涉仪差分提取原子对两束近共振探测光产生的相移，以实现基态原子数的非破坏探测。在项目执行阶段，我们首先搭建了一套完整的冷原子物理系统，完成了399nm激光和556nm激光的锁频和实验所需各频率分量的获取，并实现了蓝绿两级MOT的装载。蓝MOT原子数大于5E6，绿MOT原子数约1E6。之后，我们完成了双色Mach-Zehnder干涉仪和非破坏探测光路的搭建，制作了相位差探测电路，并最终观察到了绿MOT中的基态原子导致的探测光相位移动，相移量约为100mrad。.实验表明利用双色Mach-Zehnder干涉仪进行原子数的非破坏探测是完全可行的。不过目前的结果还有不足之处：观测到原子导致的相移量显著小于理论预期值，相移探测的信噪比也很低。后期可通过优化探测光路和相差探测电路加以改进。本项目研究的这种非破坏探测原子的方法，从原理上具有很高的可靠性和鲁棒性，它不仅在镱光钟或其它种类光钟中具有应用价值，事实上在冷原子物理领域的其它地方都具有广阔的应用前景。.此外，在本项目的支持下，我们还进行了如下研究：（1）研究了镱光钟的BBR频移；（2）为提高实验中所需各种激光的频率稳定性，我们提出了一种四通道超稳腔的设计，并分析了它的振动及温度敏感度。