[论文解读] Optical Atomic Clock Interrogation Via an Integrated Spiral Cavity Laser

William Loh, David Reens+19 more|arXiv (Cornell University)|Mar 19, 2024

Atomic and Subatomic Physics Research被引用 4

一句话总结

该论文展示了一种芯片集成螺旋腔激光器（ISCL），实现了 $7.5 \times 10^{-14}$ 的频率不稳定度，使 $^{88}$Sr$^+$ 离子的高稳定性探测成为可能，适用于光学原子钟。ISCL 实现了全集成化、便携式钟系统，其短期不稳定度可平均降低至 $3.9 \times 10^{-14}/\sqrt{\tau}$，标志着在利用集成光子学实现可现场部署、可量产的光学钟方面迈出了关键一步。

ABSTRACT

Optical atomic clocks have demonstrated revolutionary advances in precision timekeeping, but their applicability to the real world is critically dependent on whether such clocks can operate outside a laboratory setting. The challenge to clock portability stems from the many obstacles not only in miniaturizing the underlying components of the clock $-$ namely the ultrastable laser, the frequency comb, and the atomic reference itself $-$ but also in making the clock resilient to environmental fluctuations. Photonic integration offers one compelling solution to simultaneously address the problems of miniaturization and ruggedization, but brings with it a new set of challenges in recreating the functionality of an optical clock using chip-scale building blocks. The clock laser used for atom interrogation is one particular point of uncertainty, as the performance of the meticulously-engineered bulk-cavity stabilized lasers would be exceptionally difficult to transfer to chip. Here we demonstrate that a chip-integrated ultrahigh quality factor (Q) spiral cavity, when interfaced with a 1348 nm seed laser, reaches a fractional frequency instability of $7.5 imes 10^{-14}$, meeting the stability requirements for interrogating the narrow-linewidth transition of $^{88}$Sr$^+$ upon frequency doubling to 674 nm. In addition to achieving the record for laser stability on chip, we use this laser to showcase the operation of a Sr-ion clock with short-term instability averaging down as $3.9 imes 10^{-14} / \sqrtτ$, where $τ$ is the averaging time. Our demonstration of an optical atomic clock interrogated by an integrated spiral cavity laser opens the door for future advanced clock systems to be entirely constructed using lightweight, portable, and mass-manufacturable integrated optics and electronics.

研究动机与目标

• 开发一种适用于芯片级光学原子钟运行的紧凑、稳定激光源。
• 克服将超稳激光器微型化以实现现场部署光学钟的挑战。
• 实现满足 $^{88}$Sr$^+$ 离子窄线宽跃迁探测严格要求的芯片级激光稳定性。
• 展示一种使用芯片级组件实现全集成光学钟系统的方案，以提升便携性和抗振性。
• 在真实钟协议中验证 ISCL 的性能，实现低短期不稳定度。

提出的方法

• 采用基于氮化硅波导制造的高品质因子（Q）谐振器的芯片集成螺旋腔激光器（ISCL）。
• 将 ISCL 与波长为 1348 nm 的种子激光器耦合，并采用比例-积分（PI）锁定的反馈控制系统稳定激光频率。
• 实施双环锁定方案并引入积分项，以增强对缓慢激光漂移的鲁棒性。
• 对囚禁的 $^{88}$Sr$^+$ 离子进行拉比光谱和拉姆齐光谱测量，以验证激光性能。
• 采用基于 FPGA 的实时控制，以最小化启动延迟并确保快速锁定至原子共振。
• 对光纤传输系统应用振动和噪声抑制技术，以减少环境干扰。

Figure 1: ISCL laser design. a , Illustration of an eventual fully integrated clock laser comprising an external cavity seed laser probing an ultrahigh quality factor spiral resonator, a phase modulator and photodetector to enable Pound-Drever-Hall (PDH) Drever et al. ( 1983 ) locking of the seed to

实验结果

研究问题

• RQ1 芯片集成螺旋腔激光器能否实现对 $^{88}$Sr$^+$ 钟跃迁所需的频率稳定性？
• RQ2 在短期不稳定度方面，ISCL 的性能与块状激光系统相比如何？
• RQ3 ISCL 能否在包含实时反馈和原子态探测的完整光学钟协议中有效应用？
• RQ4 ISCL 中实现低噪声与高稳定性的最佳芯片光学功率是多少？
• RQ5 ISCL 在多大程度上能够实现全集成化、便携式光学钟系统？

主要发现

• ISCL 实现了 $7.5 \times 10^{-14}$ 的频率不稳定度，达到 $^{88}$Sr$^+$ 钟探测的稳定性阈值。
• 系统展示了 $3.9 \times 10^{-14}/\sqrt{\tau}$ 的短期不稳定度，表明其在实时钟运行中表现出色。
• 实现稳定性的最佳芯片光学功率约为 4.1 mW，实现了噪声降低与带内不稳定度之间的平衡。
• 反馈控制中引入积分项显著提升了锁定的鲁棒性，尤其在缓慢漂移条件下表现更优。
• 启动序列在 10 ms 内成功锁定至原子共振，首次尝试成功率超过 90%。
• 采用 2.5 ms 探测脉冲的窄线宽光谱测量实现了 380 Hz 的傅里叶受限线宽，证实了高光谱分辨率。

Figure 2: Spiral waveguide modes. a , Spiral waveguide device layers comprising a Si 3 N 4 core and SiO 2 cladding. The combination of layers forms a dilute mode with ultralow loss. b , Spiral waveguide transverse modes supported by the device structure. The modes expand outside the nitride core to