Prof. Ren Xifeng from Academician Guo Guangcan's team of our University, together with Prof. Qiu Chengwei and Dr. Guo Qiangbing from National University of Singapore, have made a major breakthrough in the study of nonlinear quantum light sources in two-dimensional materials. The results were published on January 4 in Nature under the title "Ultrathin quantum light source with van der Waals NbOCl2 crystal."
Miniaturization and integration are ideal solutions to the problems of poor stability and unscalability of spatial optical quantum systems, and are also the only way for optical quantum computing and quantum communication to become large-scale and practical. As an indispensable part of quantum optical system, the miniaturization of quantum light source has always been the focus of research. In his previous cooperation with Nanjing University and other institutions, Professor Ren Xifeng introduced the superstructure surface into the field of quantum information, integrated the superlens array and nonlinear optical crystal, realized the 100-path parameter downconversion, and prepared ultra-high-dimensional quantum entangled states and multi-photon sources [Science 368, 1487 (2020)].
In order to further improve the degree of integration of quantum light sources, Professor Ren Xifeng, together with collaborators from the National University of Singapore and other units, has for the first time used the nonlinear process of a new two-dimensional material, NbOCl2, to achieve an ultra-thin quantum light source, with a thickness as low as 46nm.
Figure 1: Structural testing of NbOCl2 crystals with monolayer thickness of about 0.65nm.
The crystal structure of two-dimensional materials is stable, but the interaction between atomic layers is much weaker. Based on this characteristic, the single-layer two-dimensional material can maintain the stability of the physical properties while maintaining the thickness of the atomic scale, so that the two-dimensional material can be stably and flexibly directly coupled with a variety of micro and nano scale optical devices, so it is widely used in various important components of integrated photonic chips. Common two-dimensional materials (WS2, WSe2, etc.) have a large second-order nonlinear coefficient, but the thickness of the single layer is too thin (<1nm), resulting in a very low overall nonlinear signal strength. If the number of layers of the material is increased, the second-order nonlinear process will weaken or even disappear due to the spatial symmetry caused by multi-layer stacking.
In this study, the collaborators used a novel NbOCl2 material, which not only has the high second-order nonlinear coefficient characteristic of common single-layer two-dimensional materials, but more importantly, its interlayer electron coupling is weak and its spatial structure is asymmetric. This characteristic makes its second-order nonlinear signal strength increase with the increase of the number of layers of two-dimensional materials, which can exceed the WS2 frequency doubling strength of single-layer two-dimensional materials by more than two orders of magnitude.
图二：NbOCl2二维材料的倍频二阶非线性过程测试。
Figure 3: Quantum light source based on NbOCl2 2D material.
The collaborators further tested the spontaneous parametric downconversion process of multi-layer NbOCl2 2D materials. In the experiment, a continuous laser with wavelength of 404 nm (maximum pumping power of 59 mW) was used to pump the two-dimensional material, and the wavelength parametric light near 808 nm generated by the down-conversion process was collected. The second order correlation function g (2) is far more than 2, which proves that the process produces non-classical correlated photon pairs. The authors also measured the relationship between the intensity of the parametric optical signal and the thickness of the two-dimensional material, and the experimental results were in good agreement with the theoretical expectations. It is worth noting that the experiment confirmed that the thickness of the material as low as 46 nm can also prepare a quantum light source, which is the thinnest nonlinear quantum light source reported internationally at present. This research not only provides an integrated quantum light source for the study of optical quantum information, but also opens up a new direction for the study of nonlinear two-dimensional materials.
Dr. Qiangbing Guo (postdoctoral fellow, National University of Singapore), Xiaozhuo Qi (PhD student, University of Science and Technology of China, now working at Tianjin Polytechnic University), Lishu Zhang (National University of Singapore), and Meng Gao (University of Chinese Academy of Sciences) are joint first authors of this work. Dr. Guo Qiangbing from the National University of Singapore, Professor Ren Xifeng from the University of Science and Technology of China, Professor Qiu Chengwei from the National University of Singapore, Professor Stephen J. Pennycook and Professor Andrew T. S.Ee are joint corresponding authors of this work. Academician Guo Guangcan and Dr. Wu Yunkun are the co-authors of this paper. The work was funded by the Ministry of Science and Technology, the National Foundation of China, the Chinese Academy of Sciences, Anhui Province, and the University of Science and Technology of China.
Paper link：https://www.nature.com/articles/s41586-022-05393-7
Guo, Q., Qi, XZ., Zhang, L. et al. Ultrathin quantum light source with van der Waals NbOCl2 crystal. Nature 613, 53–59 (2023). https://doi.org/10.1038/s41586-022-05393-7
(Key Laboratory of Quantum Information, Chinese Academy of Sciences; Institute of Quantum Information and Quantum Technology Innovation, Chinese Academy of
Sciences; School of Physics; Department of Scientific Research)
Source: China University of Science and Technology News
