The team of Academician Guo Guangcan of the University of Science and Technology of China has made important progress in the research of integrated photonic chip quantum devices. The research group of Changling Zou and Li Ming proposed a general method for synthesizing optical nonlinear processes, and experimentally observed the synthesis of high-order nonlinear processes with high efficiency in integrated chip microcavity, and demonstrated its application potential in cross-band quantum entangled light sources. The relevant results are presented as "Synthetic five-wave mixing in an integrated microcavity for visible-telecom entanglement. The title "generation" was published online in the international academic journal Nature Communications on October 20.

Since the advent of laser, nonlinear optical effects have been widely used in optical imaging, optical sensing, frequency conversion and precision spectroscopy. For the emerging quantum information processing, it is also the core element to realize the operation of quantum entangled light source and quantum logic gate. However, due to the intrinsic property that the nonlinear polarizability of materials decreases exponentially with order, the application of optical nonlinearity is mainly limited to the second and third order processes, and the higher-order processes involving multiple photons at the same time are rarely studied. On the one hand, low-order processes limit the performance of traditional nonlinear and optical quantum devices, such as the scalability of quantum light sources; On the other hand, people are also curious about novel nonlinearities and quantum physical phenomena implied by higher-order nonlinear processes.

The nonlinear interaction between photons can be enhanced by using micro-nano optical structures on integrated photonic chips, which has become a research hotspot in integrated optics and nonlinear optics in the world. Li Ming et al. from Zou Changling's research group have been committed to the research of integrated photonic chip quantum devices for a long time, developed the nonlinear photonics enhanced by microcavities, and proposed and confirmed the synergistic effect of multiple nonlinear processes in microcavities [PRL 126, 133601 (2021); PRA 98, 013854 (2018)], opens up new avenues for low-photon, even single-photon quantum devices at room temperature [PRL 129, 043601 (2022); PRApplied 13, 044013 (2020)]. At present, the team has been able to increase the rate of decay of nonlinear interaction strength with order from 10-10 to 10-5. Even so, the experimental observation of efficiency rate nonlinear effects of order greater than three on integrated photonic chips remains a challenge.

To solve this problem, Li Ming et al. proposed a novel synthetic theory of nonlinear processes, that is, using the inherent strong second-order, third-order and other low-order effects of materials, to realize the nonlinear photon interaction of any form and order by artificially regulating the nonlinear optical network formed by the cascade of multiple low-order processes. This method avoids modifying the nonlinear response of the material at the atomic scale, and only needs to control the geometry of the micro and nano devices to realize high order nonlinear processes with high efficiency and reconfiguration.

Using an integrated aluminum nitride optical microcavity, the team experimentally manipulated both a second-order summing process and a third-order four-wave mixing process to synthesize a higher-order fourth-order nonlinear process. Experiments show that the synthetic process is more than 500 times stronger than the fourth-order nonlinear effect inherent in the material. If the quality factor of the microcavity is further improved, the enhancement factor can reach more than 10 million.

Figure 1. Schematic diagram of synthetic nonlinear five-wave mixing

The team applied synthetic fourth-order nonlinearities to generate quantum-entangled light sources across the vision-communication band. The coherence of the synthetic process was verified by measuring the time-energy entanglement between the photons in the cross-band. Compared with the traditional generation method of cross-band quantum entangled light source, this work greatly reduces the difficulty of phase matching, and only requires a single pump laser in the communication band, showing the advantages and application potential of synthetic nonlinear process. The reviewers highly recognized the innovative Nature of the work (" it should be published in Nature communication for its innovation qualities).

Wang Jiaqi and Yang Yuanhao, PhD candidates at the Key Laboratory of Quantum Information of the Chinese Academy of Sciences, are co-first authors of the paper, and Li Ming, associate researcher and Professor Zou Changling are corresponding authors of the paper. This research was supported by the key research and development Plan of the Ministry of Science and Technology, the National Natural Science Foundation of China, the Natural Science Foundation of Anhui Province and the basic research funds of central universities.

Paper link:

https://www.nature.com/articles/s41467-022-33914-5

(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)