Fabrication, testing and assembly of high-finesse optical fiber microcavity for molecule cavity QED experiment

Research background

Thanks to their favorable isolation and coherence, rich energy level structures and permanent electric dipole interaction, ultracold molecules has not only shown unique advantage in the research of quantum chemistry, but also developed novel methods for quantum simulation, quantum computation and precision measurement beyond cold and ultracold atom systems. Current direct synthesis methods of cold molecules, however, are confined to photoassociation and magnetoassociation whose efficiency of molecular state preparation is far from perfect, due to complex energy level structures of molecules. With destructive detection of the synthesized molecules, these methods impose severe restrictions on the application of ultracold molecules. In order to further increase the state preparation efficiency of ultracold molecules, to study its reaction dynamics and to realize nondestructive detection of ultracold molecules, ultracold molecule cavity QED has been proposed and studied in experiments. Optical resonators are required to selectively enhance the coupling strength of matter and certain electromagnetic modes.

In particular, hybrid systems of high finesse optical cavity and cold atoms have been widely applied in quantum optics and quantum information experiments, and cavity QED research based on large organic molecules has made great progress in theories and experiments. For ultracold molecules, optical fiber Fabry-Pérot microcavities with small mode volume and open optical access can greatly enhance the coupling strength to overcome the drawback set by small Franck-Condon coefficients, which enables the enhancement of certain spontaneous emission channels to deterministically control the final state in the molecule synthesis process, followed by a highly-efficient conversion of molecular excitation to cavity photon. Moreover, the inbuilt fiber-coupled readout channel of cavity photon allows heralding of successful preparation of ultracold molecules. The non-destructive measurement of quantum state of molecules including rovibrational levels and reaction products can be further realized by the detection of the strong coupling between molecules and the cavity field. This will pave the way for quantum chemistry research and application in quantum optics and quantum information based on ultracold molecules.

The research group led by Prof. Guangcan Guo and Prof. Chuangfeng Li, Dr. Jian Wang from the University of Science and Technology of China, has conducted research on a high-finesse optical fiber Fabry-Pérot microcavity for the study of molecule cavity QED, which is published in Chinese Optics Letters, Volume 20, Issue 12, 2022 (Y. Pan, et al., Fabrication, testing and assembly of high-finesse optical fiber microcavity for molecule cavity QED experiment).

The fabrication and testing method of high-finesse optical fiber microcavity is described in detail in this work, including the preparation and profile characterization of cavity mirror by a home-made CO₂ laser ablation system and white light profilometer, assembly of high precision apparatus for fiber microcavity alignment and spectroscopy observation, and finally the setup of fiber microcavity assembly suitable for experiments of ultracold molecules. The assembly of optical fiber Fabry-Pérot microcavity includes V grooves, shear piezo, spacers and baseplate, as shown in figure 1.

The working wavelength can be tuned by adjusting cavity length to be resonant with different transitions of ultracold molecules. The finesse of the fiber microcavity has reached 24000 at the transition wavelength of Rb₂ molecule, and it been experimentally confirmed that the mirror annealing procedure plays a key role in achieving such a high finesse. In order to compatible with the condition of ultracold molecule experiments, the fiber microcavity assembly, keeping a high finesse constantly, has shown great stability under the test of ultrahigh vacuum environment. Theoretically, this fiber microcavity can greatly overcome the restriction on strong coupling set by the small Franck-Condon coefficients in molecular transition. The cooperativity between a single molecule and the cavity mode can surpass unity even with a Franck-Condon coefficient of 0.01, which allows the system to reach strong coupling regime of molecule cavity QED. And this cooperativity between single molecule and the cavity can even reach 18 for certain molecular transition.

Dr. Jian Wang, the corresponding author of this work considers this as a foundation of the study of ultracold molecule cavity QED, which has a groundbreaking impact on the experimental research of quantum chemistry, quantum optics and quantum information based on ultracold molecules.In the future, the research group will work on the deterministic quantum state preparation and nondestructive measurement of ultracold molecules, and realize the strong coupling of ultracold molecules and quantized light field, to develop quantum simulation, quantum computation and precision measurement technologies based on molecule cavity QED.

光纤微腔，超冷分子实验新装置

超冷分子的制备与探测

得益于良好的孤立性与相干性、丰富的能级结构以及稳定的电偶极相互作用，超冷分子不仅在量子化学研究中展现了独特优势，更在冷原子与超冷原子技术的基础上开辟了实现量子模拟、量子计算和量子精密测量的新道路。然而，目前直接合成冷分子的实验手段局限于自由空间超冷原子的光缔合或磁缔合，由于内部能级复杂，这两种方法的分子态制备效率不高，而且对所合成分子的探测都是破坏性的，极大限制了超冷分子的应用。

为进一步提高超冷分子态制备的效率、研究超冷分子的反应动力学以及实现非破坏的超冷分子探测，超冷分子腔量子电动力学被提出。在超冷分子腔量子电动力学实验中通常需要用光学谐振腔来选择性地增强物质与特定模式光场的耦合强度，其中，高精细度光学腔与冷原子的混合系统已广泛应用于量子光学和量子信息实验中，基于有机大分子的腔量子电动力学研究也有了系列的理论和实验进展。

对超冷分子而言，光纤微腔模式体积小、可操纵性强并具有腔模光子的光纤耦合输出通道，因而能够克服分子弗兰克-康登系数的限制，极大地提高耦合强度，这使得腔场可以通过增强分子制备过程中特定的自发辐射通道来高效控制分子的终态。同时，分子的激发被高效地转化为腔模光子，预报超冷分子的制备。通过探测分子与腔场的强耦合，可以实现分子的量子态、超冷分子振转能级光谱和反应产物的非破坏测量，这为后续超冷分子在量子化学、量子光学和量子信息等相关领域的研究与应用奠定了基础。

高精细度光纤微腔

中国科学技术大学的郭光灿院士团队的李传锋教授、王健副研究员研究组制作、测试了一种适用于分子腔量子电动力学研究的高精细度光纤法布里-珀罗微腔，为超冷分子的腔量子电动力学研究奠定了基础，相关成果发表在Chinese Optics Letters, 2022年第20卷第12期上(Y. Pan, et al., Fabrication, testing and assembly of high-finesse optical fiber microcavity for molecule cavity QED experiment)，并被选为当期封面。

封面展示了用于光与超冷分子强耦合研究的高精细度光纤法布里-珀罗微腔，外界操控与探测光场可以从侧向或者从光纤直接输入腔内，在这种新型微腔的帮助下能实现可控的超冷分子合成与分子量子态的非破坏性探测，促进分子腔量子电动力学的研究。

研究团队利用实验室自行搭建的CO₂激光烧蚀装置与白光轮廓仪进行腔镜制备与形貌表征，组装高精度的光纤微腔对准与光谱扫描装置，最终搭建适用于超冷分子实验的光纤微腔组装体。光纤法布里-珀罗微腔包含了V型槽、剪切压电陶瓷、连接件和基板，如图1 所示。

谐振腔的工作波长可以通过调节腔长的方式去适配超冷分子中不同跃迁能级的需要。在Rb₂分子跃迁波长上，光纤微腔的精细度能达到24000，且腔镜的退火工艺对高精细度获得的重要作用被实验验证。此外，光纤微腔的组装体在超高真空环境下的测试中展现出良好的稳定性，其高精细度可以被长期保持，满足了超冷分子所需的实验条件。

该光纤微腔理论上可以极大地克服分子跃迁过程中小弗兰克-康登系数对强耦合的限制，即使弗兰克-康登系数只有0.01，腔模与Rb₂分子的协同因子也能达到大于1的水平，使系统进入分子腔量子电动力学中的强耦合区域。而对于特定的分子跃迁谱线，单分子与腔的协同因子可以达到18。

未来展望

该项工作为超冷分子的腔量子电动力学研究打下了基础，这对基于超冷分子的量子化学、量子光学与量子信息技术的实验研究具有开创性的意义。未来，研究组将使用该装置实现超冷分子量子态的确定性制备与非破坏测量，实现超冷分子与量子化光场的强耦合，由此发展基于分子腔量子电动力学的量子模拟、量子计算与精密测量技术。