镱光钟跃迁几率的无破坏测量

Ytterbium lattice clock is currently one of optical clocks with the highest precision. In contrast with an optical ion clock, optical lattice clocks based on neutral atoms can interrogate many atoms simultaneously, and thus have much lower quantum projection noises. However, the long-term frequency instability of a lattice clock has so far been limited by the Dick effect, which is still well above the limit set by the quantum projection noise. The Dick effect can be significantly suppressed by reducing the time spent on the preparation of the cold atoms, and hence increasing the duty cycle which is defined by ratio of the interrogation time to the cycle time Tc. Usually, the transition probability of the clock transition is detected by the resonant fluorescence method which also leads to the total loss of the detected atoms due to the heating effects. Enough time must be spent to repeat the cooling process in order to trap new atoms for the subsequent cycle. ..This project is aimed to realize the so-called nondestructive measurement of the transition probability for an ytterbium clock. The basic strategy is the measurement of the phase shift of the probe light induced by the atoms in the ground state, by employing a scheme of Mach-Zehnder interferometer. The novelty of our design lies in the weak inter-combination transition which is chosen for the phase detection. Compared with a strong transition, the detection based on this weak transition has not only larger phase shift signals, but also lower photon scattering rates, and hence negligible atomic losses. Nearly all atoms can thus remain in the optical lattice. Our nondestructive measurement technique, once applied to a Yb clock in the future, will significantly reduce the preparation time of the atomic sample, enable a larger duty cycle (possibly higher than 0.8), and eventually make the averaging time an order of magnitude shorter to reach a stability of the order of E-18.

镱原子光钟是目前精度最高的光钟之一。原子光钟可以同时探询多个原子，其量子投影噪声远低于离子光钟。然而，原子光钟的这一优势还远未发挥出来，长期频率稳定度主要受限于Dick效应。抑制Dick效应的一个有效途径是尽量减少冷原子的制备时间，从而提高原子探询的占空比。关于钟跃迁的跃迁几率的探测，传统的共振荧光法是一种加热原子并使其丢失的破坏性测量；光钟必须花费一定的时间重新制备足量的冷原子。本项目针对Yb光钟提出并将实现跃迁几率的无破坏测量。基本思路是以Mach-Zender干涉仪的方式测量基态原子的相移。项目的特色在于选择了弱互合跃迁进行探测。相对于强跃迁，弱互合跃迁不仅有更强的相移信号，而且对原子的加热效应非常微弱，从而使几乎所有原子得以保存在光晶格中。该无破坏测量技术未来应用于Yb光钟，将大幅度减少冷原子制备时间，使占空比达到0.8以上，进而使稳定度达到E-18所需的平均时间缩短一个数量级。

光晶格原子光钟具有极低的量子投影噪声，但其频率稳定度仍受限于相对较大的Dick噪声。该项目研究组试图实现钟跃迁几率的无原子损耗测量方案，旨在缩短运行周期的“死时间”以充分抑制Dick噪声。该项目围绕镱原子光钟跃迁几率的无破坏测量问题，系统研究了晶格光场对弱互合跃迁线的光致频移、光晶格囚禁原子的色散特性、基于Mach-Zehnder (MZ)干涉仪的脉冲式相位探测等一系列问题和技术，在基态镱原子的无损耗探测方面取得了一系列重要研究成果，主要包括： .1）弱互合跃迁光致频移的测量：此项光频移的测量对判定相移信号的强度和整个探测方案的可行性至关重要。项目组测量了典型实验参数下频移量，获得了与理论估算值一致的结果，从而确证了无破坏测量方案的可行性。2）用于基态镱原子测量的MZ干涉仪：成功搭建了可测量镱原子的MZ干涉仪，实现了两臂相位差的快速锁定，探测光载波抑制效果超过30dB，探测灵敏度达到8.6 V/rad，信噪比优于预期指标。项目探索的无破坏测量技术应用于镱原子晶格光钟，将有助于充分抑制Dick噪声，从而提升镱原子光钟的精度指标。3）镱光钟的研制取得重要进展：突破了冷原子的光晶格装载、光纤噪声抑制、超窄线宽激光、自旋极化、钟跃迁的探询、归一化探测等多项关键技术，最终实现了的光钟闭环运转和交互运转模式。环内稳定度达到3E-17；交互运转模式下的测量精度达到2E-16。